| Message ID | 9873b8bd7d14ff8cd2a5782b434b39f076679eeb.1587531463.git.josh@joshtriplett.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v5,1/3] fs: Support setting a minimum fd for "lowest available fd" allocation | expand |
[CC += linux-api] On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote: > > Inspired by the X protocol's handling of XIDs, allow userspace to select > the file descriptor opened by openat2, so that it can use the resulting > file descriptor in subsequent system calls without waiting for the > response to openat2. > > In io_uring, this allows sequences like openat2/read/close without > waiting for the openat2 to complete. Multiple such sequences can > overlap, as long as each uses a distinct file descriptor. > > Add a new O_SPECIFIC_FD open flag to enable this behavior, only accepted > by openat2 for now (ignored by open/openat like all unknown flags). Add > an fd field to struct open_how (along with appropriate padding, and > verify that the padding is 0 to allow replacing the padding with a field > in the future). > > The file table has a corresponding new function > get_specific_unused_fd_flags, which gets the specified file descriptor > if O_SPECIFIC_FD is set (and the fd isn't -1); otherwise it falls back > to get_unused_fd_flags, to simplify callers. > > The specified file descriptor must not already be open; if it is, > get_specific_unused_fd_flags will fail with -EBUSY. This helps catch > userspace errors. > > When O_SPECIFIC_FD is set, and fd is not -1, openat2 will use the > specified file descriptor rather than finding the lowest available one. > > Test program: > > #include <err.h> > #include <fcntl.h> > #include <stdio.h> > #include <unistd.h> > > int main(void) > { > struct open_how how = { > .flags = O_RDONLY | O_SPECIFIC_FD, > .fd = 42 > }; > int fd = openat2(AT_FDCWD, "/dev/null", &how, sizeof(how)); > if (fd < 0) > err(1, "openat2"); > printf("fd=%d\n", fd); // prints fd=42 > return 0; > } > > Signed-off-by: Josh Triplett <josh@joshtriplett.org> > --- > fs/fcntl.c | 2 +- > fs/file.c | 39 +++++++++++++++++++ > fs/io_uring.c | 3 +- > fs/open.c | 8 +++- > include/linux/fcntl.h | 5 ++- > include/linux/file.h | 3 ++ > include/uapi/asm-generic/fcntl.h | 4 ++ > include/uapi/linux/openat2.h | 3 ++ > tools/testing/selftests/openat2/helpers.c | 2 +- > tools/testing/selftests/openat2/helpers.h | 21 +++++++--- > .../testing/selftests/openat2/openat2_test.c | 35 ++++++++++++++++- > 11 files changed, 111 insertions(+), 14 deletions(-) > > diff --git a/fs/fcntl.c b/fs/fcntl.c > index 2e4c0fa2074b..0357ad667563 100644 > --- a/fs/fcntl.c > +++ b/fs/fcntl.c > @@ -1033,7 +1033,7 @@ static int __init fcntl_init(void) > * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY > * is defined as O_NONBLOCK on some platforms and not on others. > */ > - BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ != > + BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ != > HWEIGHT32( > (VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) | > __FMODE_EXEC | __FMODE_NONOTIFY)); > diff --git a/fs/file.c b/fs/file.c > index ba06140d89af..0674c3a2d3a5 100644 > --- a/fs/file.c > +++ b/fs/file.c > @@ -567,6 +567,45 @@ void put_unused_fd(unsigned int fd) > > EXPORT_SYMBOL(put_unused_fd); > > +int __get_specific_unused_fd_flags(unsigned int fd, unsigned int flags, > + unsigned long nofile) > +{ > + int ret; > + struct fdtable *fdt; > + struct files_struct *files = current->files; > + > + if (!(flags & O_SPECIFIC_FD) || fd == UINT_MAX) > + return __get_unused_fd_flags(flags, nofile); > + > + if (fd >= nofile) > + return -EBADF; > + > + spin_lock(&files->file_lock); > + ret = expand_files(files, fd); > + if (unlikely(ret < 0)) > + goto out_unlock; > + fdt = files_fdtable(files); > + if (fdt->fd[fd]) { > + ret = -EBUSY; > + goto out_unlock; > + } > + __set_open_fd(fd, fdt); > + if (flags & O_CLOEXEC) > + __set_close_on_exec(fd, fdt); > + else > + __clear_close_on_exec(fd, fdt); > + ret = fd; > + > +out_unlock: > + spin_unlock(&files->file_lock); > + return ret; > +} > + > +int get_specific_unused_fd_flags(unsigned int fd, unsigned int flags) > +{ > + return __get_specific_unused_fd_flags(fd, flags, rlimit(RLIMIT_NOFILE)); > +} > + > /* > * Install a file pointer in the fd array. > * > diff --git a/fs/io_uring.c b/fs/io_uring.c > index 358f97be9c7b..4a69e1daf3fe 100644 > --- a/fs/io_uring.c > +++ b/fs/io_uring.c > @@ -2997,7 +2997,8 @@ static int io_openat2(struct io_kiocb *req, bool force_nonblock) > if (ret) > goto err; > > - ret = __get_unused_fd_flags(req->open.how.flags, req->open.nofile); > + ret = __get_specific_unused_fd_flags(req->open.how.fd, > + req->open.how.flags, req->open.nofile); > if (ret < 0) > goto err; > > diff --git a/fs/open.c b/fs/open.c > index 719b320ede52..54b2782dfa7a 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -962,6 +962,8 @@ inline struct open_how build_open_how(int flags, umode_t mode) > .mode = mode & S_IALLUGO, > }; > > + /* O_SPECIFIC_FD doesn't work in open calls that use build_open_how. */ > + how.flags &= ~O_SPECIFIC_FD; > /* O_PATH beats everything else. */ > if (how.flags & O_PATH) > how.flags &= O_PATH_FLAGS; > @@ -989,6 +991,10 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op) > return -EINVAL; > if (how->resolve & ~VALID_RESOLVE_FLAGS) > return -EINVAL; > + if (how->__padding != 0) > + return -EINVAL; > + if (!(flags & O_SPECIFIC_FD) && how->fd != 0) > + return -EINVAL; > > /* Deal with the mode. */ > if (WILL_CREATE(flags)) { > @@ -1143,7 +1149,7 @@ static long do_sys_openat2(int dfd, const char __user *filename, > if (IS_ERR(tmp)) > return PTR_ERR(tmp); > > - fd = get_unused_fd_flags(how->flags); > + fd = get_specific_unused_fd_flags(how->fd, how->flags); > if (fd >= 0) { > struct file *f = do_filp_open(dfd, tmp, &op); > if (IS_ERR(f)) { > diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h > index 7bcdcf4f6ab2..728849bcd8fa 100644 > --- a/include/linux/fcntl.h > +++ b/include/linux/fcntl.h > @@ -10,7 +10,7 @@ > (O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \ > O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \ > FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \ > - O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE) > + O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_SPECIFIC_FD) > > /* List of all valid flags for the how->upgrade_mask argument: */ > #define VALID_UPGRADE_FLAGS \ > @@ -23,7 +23,8 @@ > > /* List of all open_how "versions". */ > #define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */ > -#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0 > +#define OPEN_HOW_SIZE_VER1 32 /* added fd and pad */ > +#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER1 > > #ifndef force_o_largefile > #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T)) > diff --git a/include/linux/file.h b/include/linux/file.h > index b67986f818d2..a63301864a36 100644 > --- a/include/linux/file.h > +++ b/include/linux/file.h > @@ -87,6 +87,9 @@ extern void set_close_on_exec(unsigned int fd, int flag); > extern bool get_close_on_exec(unsigned int fd); > extern int __get_unused_fd_flags(unsigned flags, unsigned long nofile); > extern int get_unused_fd_flags(unsigned flags); > +extern int __get_specific_unused_fd_flags(unsigned int fd, unsigned int flags, > + unsigned long nofile); > +extern int get_specific_unused_fd_flags(unsigned int fd, unsigned int flags); > extern void put_unused_fd(unsigned int fd); > extern unsigned int increase_min_fd(unsigned int num); > > diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h > index 9dc0bf0c5a6e..d3de5b8b3955 100644 > --- a/include/uapi/asm-generic/fcntl.h > +++ b/include/uapi/asm-generic/fcntl.h > @@ -89,6 +89,10 @@ > #define __O_TMPFILE 020000000 > #endif > > +#ifndef O_SPECIFIC_FD > +#define O_SPECIFIC_FD 01000000000 /* open as specified fd */ > +#endif > + > /* a horrid kludge trying to make sure that this will fail on old kernels */ > #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY) > #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT) > diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h > index 58b1eb711360..dcbf9dc333b5 100644 > --- a/include/uapi/linux/openat2.h > +++ b/include/uapi/linux/openat2.h > @@ -15,11 +15,14 @@ > * @flags: O_* flags. > * @mode: O_CREAT/O_TMPFILE file mode. > * @resolve: RESOLVE_* flags. > + * @fd: Specific file descriptor to use, for O_SPECIFIC_FD. > */ > struct open_how { > __u64 flags; > __u64 mode; > __u64 resolve; > + __u32 fd; > + __u32 __padding; /* Must be zeroed */ > }; > > /* how->resolve flags for openat2(2). */ > diff --git a/tools/testing/selftests/openat2/helpers.c b/tools/testing/selftests/openat2/helpers.c > index 5074681ffdc9..b6533f0b1124 100644 > --- a/tools/testing/selftests/openat2/helpers.c > +++ b/tools/testing/selftests/openat2/helpers.c > @@ -98,7 +98,7 @@ void __attribute__((constructor)) init(void) > struct open_how how = {}; > int fd; > > - BUILD_BUG_ON(sizeof(struct open_how) != OPEN_HOW_SIZE_VER0); > + BUILD_BUG_ON(sizeof(struct open_how) != OPEN_HOW_SIZE_VER1); > > /* Check openat2(2) support. */ > fd = sys_openat2(AT_FDCWD, ".", &how); > diff --git a/tools/testing/selftests/openat2/helpers.h b/tools/testing/selftests/openat2/helpers.h > index a6ea27344db2..d38818033b45 100644 > --- a/tools/testing/selftests/openat2/helpers.h > +++ b/tools/testing/selftests/openat2/helpers.h > @@ -24,28 +24,37 @@ > #endif /* SYS_openat2 */ > > /* > - * Arguments for how openat2(2) should open the target path. If @resolve is > - * zero, then openat2(2) operates very similarly to openat(2). > + * Arguments for how openat2(2) should open the target path. If only @flags and > + * @mode are non-zero, then openat2(2) operates very similarly to openat(2). > * > - * However, unlike openat(2), unknown bits in @flags result in -EINVAL rather > - * than being silently ignored. @mode must be zero unless one of {O_CREAT, > - * O_TMPFILE} are set. > + * However, unlike openat(2), unknown or invalid bits in @flags result in > + * -EINVAL rather than being silently ignored. @mode must be zero unless one of > + * {O_CREAT, O_TMPFILE} are set. > * > * @flags: O_* flags. > * @mode: O_CREAT/O_TMPFILE file mode. > * @resolve: RESOLVE_* flags. > + * @fd: Specific file descriptor to use, for O_SPECIFIC_FD. > */ > struct open_how { > __u64 flags; > __u64 mode; > __u64 resolve; > + __u32 fd; > + __u32 __padding; /* Must be zeroed */ > }; > > +/* List of all open_how "versions". */ > #define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */ > -#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0 > +#define OPEN_HOW_SIZE_VER1 32 /* added fd and pad */ > +#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER1 > > bool needs_openat2(const struct open_how *how); > > +#ifndef O_SPECIFIC_FD > +#define O_SPECIFIC_FD 01000000000 > +#endif > + > #ifndef RESOLVE_IN_ROOT > /* how->resolve flags for openat2(2). */ > #define RESOLVE_NO_XDEV 0x01 /* Block mount-point crossings > diff --git a/tools/testing/selftests/openat2/openat2_test.c b/tools/testing/selftests/openat2/openat2_test.c > index b386367c606b..f999b4bb0dc2 100644 > --- a/tools/testing/selftests/openat2/openat2_test.c > +++ b/tools/testing/selftests/openat2/openat2_test.c > @@ -40,7 +40,7 @@ struct struct_test { > int err; > }; > > -#define NUM_OPENAT2_STRUCT_TESTS 7 > +#define NUM_OPENAT2_STRUCT_TESTS 8 > #define NUM_OPENAT2_STRUCT_VARIATIONS 13 > > void test_openat2_struct(void) > @@ -52,6 +52,9 @@ void test_openat2_struct(void) > { .name = "normal struct", > .arg.inner.flags = O_RDONLY, > .size = sizeof(struct open_how) }, > + { .name = "v0 struct", > + .arg.inner.flags = O_RDONLY, > + .size = OPEN_HOW_SIZE_VER0 }, > /* Bigger struct, with zeroed out end. */ > { .name = "bigger struct (zeroed out)", > .arg.inner.flags = O_RDONLY, > @@ -155,7 +158,7 @@ struct flag_test { > int err; > }; > > -#define NUM_OPENAT2_FLAG_TESTS 23 > +#define NUM_OPENAT2_FLAG_TESTS 31 > > void test_openat2_flags(void) > { > @@ -223,6 +226,30 @@ void test_openat2_flags(void) > { .name = "invalid how.resolve and O_PATH", > .how.flags = O_PATH, > .how.resolve = 0x1337, .err = -EINVAL }, > + > + /* O_SPECIFIC_FD tests */ > + { .name = "O_SPECIFIC_FD", > + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = 42 }, > + { .name = "O_SPECIFIC_FD if fd exists", > + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = 2, > + .err = -EBUSY }, > + { .name = "O_SPECIFIC_FD with fd -1", > + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = -1 }, > + { .name = "fd without O_SPECIFIC_FD", > + .how.flags = O_RDONLY, .how.fd = 42, > + .err = -EINVAL }, > + { .name = "fd -1 without O_SPECIFIC_FD", > + .how.flags = O_RDONLY, .how.fd = -1, > + .err = -EINVAL }, > + { .name = "existing fd without O_SPECIFIC_FD", > + .how.flags = O_RDONLY, .how.fd = 2, > + .err = -EINVAL }, > + { .name = "Non-zero padding with O_SPECIFIC_FD", > + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = 42, > + .how.__padding = 42, .err = -EINVAL }, > + { .name = "Non-zero padding without O_SPECIFIC_FD", > + .how.flags = O_RDONLY, > + .how.__padding = 42, .err = -EINVAL }, > }; > > BUILD_BUG_ON(ARRAY_LEN(tests) != NUM_OPENAT2_FLAG_TESTS); > @@ -268,6 +295,10 @@ void test_openat2_flags(void) > if (!(test->how.flags & O_LARGEFILE)) > fdflags &= ~O_LARGEFILE; > failed |= (fdflags != test->how.flags); > + > + if (test->how.flags & O_SPECIFIC_FD > + && test->how.fd != -1) > + failed |= (fd != test->how.fd); > } > > if (failed) { > -- > 2.26.2 >
On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote: > > [CC += linux-api] > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote: > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > the file descriptor opened by openat2, so that it can use the resulting > > file descriptor in subsequent system calls without waiting for the > > response to openat2. > > > > In io_uring, this allows sequences like openat2/read/close without > > waiting for the openat2 to complete. Multiple such sequences can > > overlap, as long as each uses a distinct file descriptor. If this is primarily an io_uring feature, then why burden the normal openat2 API with this? Add this flag to the io_uring API, by all means. This would also allow Implementing a private fd table for io_uring. I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including openat2 and freely use the private fd space without having to worry about interactions with other parts of the system. Thanks, Miklos
On Wed, Apr 22, 2020 at 09:55:56AM +0200, Miklos Szeredi wrote: > On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages) > <mtk.manpages@gmail.com> wrote: > > > > [CC += linux-api] > > > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote: > > > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > > the file descriptor opened by openat2, so that it can use the resulting > > > file descriptor in subsequent system calls without waiting for the > > > response to openat2. > > > > > > In io_uring, this allows sequences like openat2/read/close without > > > waiting for the openat2 to complete. Multiple such sequences can > > > overlap, as long as each uses a distinct file descriptor. > > If this is primarily an io_uring feature, then why burden the normal > openat2 API with this? This feature was inspired by io_uring; it isn't exclusively of value with io_uring. (And io_uring doesn't normally change the semantics of syscalls.) > This would also allow Implementing a private fd table for io_uring. > I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including > openat2 and freely use the private fd space without having to worry > about interactions with other parts of the system. I definitely don't want to add a special kind of file descriptor that doesn't work in normal syscalls taking file descriptors. A file descriptor allocated via O_SPECIFIC_FD is an entirely normal file descriptor, and works anywhere a file descriptor normally works.
On Thu, Apr 23, 2020 at 2:48 AM Josh Triplett <josh@joshtriplett.org> wrote: > > On Wed, Apr 22, 2020 at 09:55:56AM +0200, Miklos Szeredi wrote: > > On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages) > > <mtk.manpages@gmail.com> wrote: > > > > > > [CC += linux-api] > > > > > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote: > > > > > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > > > the file descriptor opened by openat2, so that it can use the resulting > > > > file descriptor in subsequent system calls without waiting for the > > > > response to openat2. > > > > > > > > In io_uring, this allows sequences like openat2/read/close without > > > > waiting for the openat2 to complete. Multiple such sequences can > > > > overlap, as long as each uses a distinct file descriptor. > > > > If this is primarily an io_uring feature, then why burden the normal > > openat2 API with this? > > This feature was inspired by io_uring; it isn't exclusively of value > with io_uring. (And io_uring doesn't normally change the semantics of > syscalls.) What's the use case of O_SPECIFIC_FD beyond io_uring? > > > This would also allow Implementing a private fd table for io_uring. > > I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including > > openat2 and freely use the private fd space without having to worry > > about interactions with other parts of the system. > > I definitely don't want to add a special kind of file descriptor that > doesn't work in normal syscalls taking file descriptors. A file > descriptor allocated via O_SPECIFIC_FD is an entirely normal file > descriptor, and works anywhere a file descriptor normally works. What's the use case of allocating a file descriptor within io_uring and using it outside of io_uring? Thanks, Miklos
On Thu, Apr 23, 2020 at 06:24:14AM +0200, Miklos Szeredi wrote: > On Thu, Apr 23, 2020 at 2:48 AM Josh Triplett <josh@joshtriplett.org> wrote: > > On Wed, Apr 22, 2020 at 09:55:56AM +0200, Miklos Szeredi wrote: > > > On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages) > > > <mtk.manpages@gmail.com> wrote: > > > > > > > > [CC += linux-api] > > > > > > > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote: > > > > > > > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > > > > the file descriptor opened by openat2, so that it can use the resulting > > > > > file descriptor in subsequent system calls without waiting for the > > > > > response to openat2. > > > > > > > > > > In io_uring, this allows sequences like openat2/read/close without > > > > > waiting for the openat2 to complete. Multiple such sequences can > > > > > overlap, as long as each uses a distinct file descriptor. > > > > > > If this is primarily an io_uring feature, then why burden the normal > > > openat2 API with this? > > > > This feature was inspired by io_uring; it isn't exclusively of value > > with io_uring. (And io_uring doesn't normally change the semantics of > > syscalls.) > > What's the use case of O_SPECIFIC_FD beyond io_uring? Avoiding a call to dup2 and close, if you need something as a specific file descriptor, such as when setting up to exec something, or when debugging a program. I don't expect it to be as widely used as with io_uring, but I also don't want io_uring versions of syscalls to diverge from the underlying syscalls, and this would be a heavy divergence. > > > This would also allow Implementing a private fd table for io_uring. > > > I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including > > > openat2 and freely use the private fd space without having to worry > > > about interactions with other parts of the system. > > > > I definitely don't want to add a special kind of file descriptor that > > doesn't work in normal syscalls taking file descriptors. A file > > descriptor allocated via O_SPECIFIC_FD is an entirely normal file > > descriptor, and works anywhere a file descriptor normally works. > > What's the use case of allocating a file descriptor within io_uring > and using it outside of io_uring? Calling a syscall not provided via io_uring. Calling a library that doesn't use io_uring. Passing the file descriptor via UNIX socket to another program. Passing the file descriptor via exec to another program. Userspace is modular, and file descriptors are widely used.
On Thu, Apr 23, 2020 at 6:42 AM Josh Triplett <josh@joshtriplett.org> wrote: > > On Thu, Apr 23, 2020 at 06:24:14AM +0200, Miklos Szeredi wrote: > > On Thu, Apr 23, 2020 at 2:48 AM Josh Triplett <josh@joshtriplett.org> wrote: > > > On Wed, Apr 22, 2020 at 09:55:56AM +0200, Miklos Szeredi wrote: > > > > On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages) > > > > <mtk.manpages@gmail.com> wrote: > > > > > > > > > > [CC += linux-api] > > > > > > > > > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote: > > > > > > > > > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > > > > > the file descriptor opened by openat2, so that it can use the resulting > > > > > > file descriptor in subsequent system calls without waiting for the > > > > > > response to openat2. > > > > > > > > > > > > In io_uring, this allows sequences like openat2/read/close without > > > > > > waiting for the openat2 to complete. Multiple such sequences can > > > > > > overlap, as long as each uses a distinct file descriptor. > > > > > > > > If this is primarily an io_uring feature, then why burden the normal > > > > openat2 API with this? > > > > > > This feature was inspired by io_uring; it isn't exclusively of value > > > with io_uring. (And io_uring doesn't normally change the semantics of > > > syscalls.) > > > > What's the use case of O_SPECIFIC_FD beyond io_uring? > > Avoiding a call to dup2 and close, if you need something as a specific > file descriptor, such as when setting up to exec something, or when > debugging a program. > > I don't expect it to be as widely used as with io_uring, but I also > don't want io_uring versions of syscalls to diverge from the underlying > syscalls, and this would be a heavy divergence. What are the plans for those syscalls that don't easily lend themselves to this modification (such as accept(2))? Do we want to introduce another variant of these? Is that really worth it? If not, we are faced with the same divergence. Compared to that, having a common flag for file ops to enable the use of fixed and private file descriptors is a clean and well contained interface. > > > > This would also allow Implementing a private fd table for io_uring. > > > > I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including > > > > openat2 and freely use the private fd space without having to worry > > > > about interactions with other parts of the system. > > > > > > I definitely don't want to add a special kind of file descriptor that > > > doesn't work in normal syscalls taking file descriptors. A file > > > descriptor allocated via O_SPECIFIC_FD is an entirely normal file > > > descriptor, and works anywhere a file descriptor normally works. > > > > What's the use case of allocating a file descriptor within io_uring > > and using it outside of io_uring? > > Calling a syscall not provided via io_uring. Calling a library that > doesn't use io_uring. Passing the file descriptor via UNIX socket to > another program. Passing the file descriptor via exec to another > program. Userspace is modular, and file descriptors are widely used. I mean, you could open the file descriptor outside of io_uring in such cases, no? The point of O_SPECIFIC_FD is to be able to perform short sequences of open/dosomething/close without having to block and having to issue separate syscalls. If you're going to issue separate syscalls anyway, then I see no point in doing the open within io_uring. Or? Thanks, Miklos
On Thu, Apr 23, 2020 at 08:04:25AM +0200, Miklos Szeredi wrote: > On Thu, Apr 23, 2020 at 6:42 AM Josh Triplett <josh@joshtriplett.org> wrote: > > > > On Thu, Apr 23, 2020 at 06:24:14AM +0200, Miklos Szeredi wrote: > > > On Thu, Apr 23, 2020 at 2:48 AM Josh Triplett <josh@joshtriplett.org> wrote: > > > > On Wed, Apr 22, 2020 at 09:55:56AM +0200, Miklos Szeredi wrote: > > > > > On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages) > > > > > <mtk.manpages@gmail.com> wrote: > > > > > > > > > > > > [CC += linux-api] > > > > > > > > > > > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote: > > > > > > > > > > > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > > > > > > the file descriptor opened by openat2, so that it can use the resulting > > > > > > > file descriptor in subsequent system calls without waiting for the > > > > > > > response to openat2. > > > > > > > > > > > > > > In io_uring, this allows sequences like openat2/read/close without > > > > > > > waiting for the openat2 to complete. Multiple such sequences can > > > > > > > overlap, as long as each uses a distinct file descriptor. > > > > > > > > > > If this is primarily an io_uring feature, then why burden the normal > > > > > openat2 API with this? > > > > > > > > This feature was inspired by io_uring; it isn't exclusively of value > > > > with io_uring. (And io_uring doesn't normally change the semantics of > > > > syscalls.) > > > > > > What's the use case of O_SPECIFIC_FD beyond io_uring? > > > > Avoiding a call to dup2 and close, if you need something as a specific > > file descriptor, such as when setting up to exec something, or when > > debugging a program. > > > > I don't expect it to be as widely used as with io_uring, but I also > > don't want io_uring versions of syscalls to diverge from the underlying > > syscalls, and this would be a heavy divergence. > > What are the plans for those syscalls that don't easily lend > themselves to this modification (such as accept(2))? accept4 has a flags argument with more flags available, so it'd be entirely possible to cleanly extend it further without introducing a new version. The same goes for other fd-producing syscalls that still have flag space available. This may or may not provide sufficient motivation on its own to introduce a new syscall variant of a syscall that isn't currently extensible. > Compared to that, having a common flag for file ops to enable the use > of fixed and private file descriptors is a clean and well contained > interface. "private" is not a desirable property here. See below. Even if the userspace-specified fd mechanism were to become something only accessible via io_uring (which I'd prefer to avoid), that's not a reason to avoid generating real file descriptors that work anywhere a file descriptor works. > > > > > This would also allow Implementing a private fd table for io_uring. > > > > > I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including > > > > > openat2 and freely use the private fd space without having to worry > > > > > about interactions with other parts of the system. > > > > > > > > I definitely don't want to add a special kind of file descriptor that > > > > doesn't work in normal syscalls taking file descriptors. A file > > > > descriptor allocated via O_SPECIFIC_FD is an entirely normal file > > > > descriptor, and works anywhere a file descriptor normally works. > > > > > > What's the use case of allocating a file descriptor within io_uring > > > and using it outside of io_uring? > > > > Calling a syscall not provided via io_uring. Calling a library that > > doesn't use io_uring. Passing the file descriptor via UNIX socket to > > another program. Passing the file descriptor via exec to another > > program. Userspace is modular, and file descriptors are widely used. > > I mean, you could open the file descriptor outside of io_uring in such > cases, no? I would prefer to not introduce that limitation in the first place, and instead open normal file descriptors. > The point of O_SPECIFIC_FD is to be able to perform short > sequences of open/dosomething/close without having to block and having > to issue separate syscalls. "close" is not a required component. It's entirely possible to use io_uring to open a file descriptor, do various things with it, and then leave it open for subsequent usage via either other io_uring chains or standalone syscalls. > If you're going to issue separate > syscalls anyway, then I see no point in doing the open within > io_uring. Or? io_uring is not an all-or-nothing proposition. There's value in using io_uring for some operations without converting an entire program (and every library it might potentially use on a file descriptor) entirely to io_uring. Userspace is modular, and file descriptors are a common element used by many different bits of userspace.
On Thu, Apr 23, 2020 at 9:33 AM Josh Triplett <josh@joshtriplett.org> wrote: > > What are the plans for those syscalls that don't easily lend > > themselves to this modification (such as accept(2))? > > accept4 has a flags argument with more flags available, so it'd be > entirely possible to cleanly extend it further without introducing a new > version. Variable argument syscalls, you are thinking? > > I mean, you could open the file descriptor outside of io_uring in such > > cases, no? > > I would prefer to not introduce that limitation in the first place, and > instead open normal file descriptors. > > > The point of O_SPECIFIC_FD is to be able to perform short > > sequences of open/dosomething/close without having to block and having > > to issue separate syscalls. > > "close" is not a required component. It's entirely possible to use > io_uring to open a file descriptor, do various things with it, and then > leave it open for subsequent usage via either other io_uring chains or > standalone syscalls. If this use case arraises, we could add an op to dup/move a private descriptor to a public one. io_uring can return values, right? Still not convinced... Thanks, Miklos
On Thu, Apr 23, 2020 at 9:45 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > > I would prefer to not introduce that limitation in the first place, and > > instead open normal file descriptors. > > > > > The point of O_SPECIFIC_FD is to be able to perform short > > > sequences of open/dosomething/close without having to block and having > > > to issue separate syscalls. > > > > "close" is not a required component. It's entirely possible to use > > io_uring to open a file descriptor, do various things with it, and then > > leave it open for subsequent usage via either other io_uring chains or > > standalone syscalls. > > If this use case arraises, we could add an op to dup/move a private > descriptor to a public one. io_uring can return values, right? > > Still not convinced... Oh, and we haven't even touched on the biggest advantage of a private fd table: not having to dirty a cacheline on fdget/fdput due to the possibility of concurrent close() in a MT application. I believe this is a sticking point in some big enterprise apps and it may even be a driving force for io_uring. Thanks, Miklos
On Thu, Apr 23, 2020 at 09:45:45AM +0200, Miklos Szeredi wrote: > On Thu, Apr 23, 2020 at 9:33 AM Josh Triplett <josh@joshtriplett.org> wrote: > > > What are the plans for those syscalls that don't easily lend > > > themselves to this modification (such as accept(2))? > > > > accept4 has a flags argument with more flags available, so it'd be > > entirely possible to cleanly extend it further without introducing a new > > version. > > Variable argument syscalls, you are thinking? That or repurposing an existing pointer-sized argument as an open_how-style struct, yes. But in any case, I'm not proposing that; I'm proposing changes to the existing highly extensible openat2 syscall. > > > I mean, you could open the file descriptor outside of io_uring in such > > > cases, no? > > > > I would prefer to not introduce that limitation in the first place, and > > instead open normal file descriptors. > > > > > The point of O_SPECIFIC_FD is to be able to perform short > > > sequences of open/dosomething/close without having to block and having > > > to issue separate syscalls. > > > > "close" is not a required component. It's entirely possible to use > > io_uring to open a file descriptor, do various things with it, and then > > leave it open for subsequent usage via either other io_uring chains or > > standalone syscalls. > > If this use case arraises, This wasn't a hypothetical "someone might want this". I'm stating that this is a requirement I'm seeking to meet with this patch series, and one I intend to use. The primary use case is interoperability with other code using file descriptors and not using io_uring.
On Thu, Apr 23, 2020 at 9:57 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > > On Thu, Apr 23, 2020 at 9:45 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > > > > I would prefer to not introduce that limitation in the first place, and > > > instead open normal file descriptors. > > > > > > > The point of O_SPECIFIC_FD is to be able to perform short > > > > sequences of open/dosomething/close without having to block and having > > > > to issue separate syscalls. > > > > > > "close" is not a required component. It's entirely possible to use > > > io_uring to open a file descriptor, do various things with it, and then > > > leave it open for subsequent usage via either other io_uring chains or > > > standalone syscalls. > > > > If this use case arraises, we could add an op to dup/move a private > > descriptor to a public one. io_uring can return values, right? > > > > Still not convinced... > > Oh, and we haven't even touched on the biggest advantage of a private > fd table: not having to dirty a cacheline on fdget/fdput due to the > possibility of concurrent close() in a MT application. > > I believe this is a sticking point in some big enterprise apps and it > may even be a driving force for io_uring. https://lwn.net/Articles/787473/ And an interesting (very old) article referenced from above, that gives yet a new angle on fd allocation issues: https://lwn.net/Articles/236843/ A private fd space would be perfect for libraries such as glibc. Thanks, Miklos
On Thu, Apr 23, 2020 at 11:20 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > > On Thu, Apr 23, 2020 at 9:57 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > > > > On Thu, Apr 23, 2020 at 9:45 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > > > > > > I would prefer to not introduce that limitation in the first place, and > > > > instead open normal file descriptors. > > > > > > > > > The point of O_SPECIFIC_FD is to be able to perform short > > > > > sequences of open/dosomething/close without having to block and having > > > > > to issue separate syscalls. > > > > > > > > "close" is not a required component. It's entirely possible to use > > > > io_uring to open a file descriptor, do various things with it, and then > > > > leave it open for subsequent usage via either other io_uring chains or > > > > standalone syscalls. > > > > > > If this use case arraises, we could add an op to dup/move a private > > > descriptor to a public one. io_uring can return values, right? > > > > > > Still not convinced... > > > > Oh, and we haven't even touched on the biggest advantage of a private > > fd table: not having to dirty a cacheline on fdget/fdput due to the > > possibility of concurrent close() in a MT application. > > > > I believe this is a sticking point in some big enterprise apps and it > > may even be a driving force for io_uring. > > https://lwn.net/Articles/787473/ > > And an interesting (very old) article referenced from above, that > gives yet a new angle on fd allocation issues: > > https://lwn.net/Articles/236843/ > > A private fd space would be perfect for libraries such as glibc. Ah, io_uring already implements a fixed private fd table via io_uring_register(IORING_REGISTER_FILES,...), we just need a way to wire up open, socket, accept, etc. to fill a slot in that table instead of, or in addition to allocating a slot in the fd_table. Thanks, Miklos
diff --git a/fs/fcntl.c b/fs/fcntl.c index 2e4c0fa2074b..0357ad667563 100644 --- a/fs/fcntl.c +++ b/fs/fcntl.c @@ -1033,7 +1033,7 @@ static int __init fcntl_init(void) * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY * is defined as O_NONBLOCK on some platforms and not on others. */ - BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ != + BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ != HWEIGHT32( (VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) | __FMODE_EXEC | __FMODE_NONOTIFY)); diff --git a/fs/file.c b/fs/file.c index ba06140d89af..0674c3a2d3a5 100644 --- a/fs/file.c +++ b/fs/file.c @@ -567,6 +567,45 @@ void put_unused_fd(unsigned int fd) EXPORT_SYMBOL(put_unused_fd); +int __get_specific_unused_fd_flags(unsigned int fd, unsigned int flags, + unsigned long nofile) +{ + int ret; + struct fdtable *fdt; + struct files_struct *files = current->files; + + if (!(flags & O_SPECIFIC_FD) || fd == UINT_MAX) + return __get_unused_fd_flags(flags, nofile); + + if (fd >= nofile) + return -EBADF; + + spin_lock(&files->file_lock); + ret = expand_files(files, fd); + if (unlikely(ret < 0)) + goto out_unlock; + fdt = files_fdtable(files); + if (fdt->fd[fd]) { + ret = -EBUSY; + goto out_unlock; + } + __set_open_fd(fd, fdt); + if (flags & O_CLOEXEC) + __set_close_on_exec(fd, fdt); + else + __clear_close_on_exec(fd, fdt); + ret = fd; + +out_unlock: + spin_unlock(&files->file_lock); + return ret; +} + +int get_specific_unused_fd_flags(unsigned int fd, unsigned int flags) +{ + return __get_specific_unused_fd_flags(fd, flags, rlimit(RLIMIT_NOFILE)); +} + /* * Install a file pointer in the fd array. * diff --git a/fs/io_uring.c b/fs/io_uring.c index 358f97be9c7b..4a69e1daf3fe 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -2997,7 +2997,8 @@ static int io_openat2(struct io_kiocb *req, bool force_nonblock) if (ret) goto err; - ret = __get_unused_fd_flags(req->open.how.flags, req->open.nofile); + ret = __get_specific_unused_fd_flags(req->open.how.fd, + req->open.how.flags, req->open.nofile); if (ret < 0) goto err; diff --git a/fs/open.c b/fs/open.c index 719b320ede52..54b2782dfa7a 100644 --- a/fs/open.c +++ b/fs/open.c @@ -962,6 +962,8 @@ inline struct open_how build_open_how(int flags, umode_t mode) .mode = mode & S_IALLUGO, }; + /* O_SPECIFIC_FD doesn't work in open calls that use build_open_how. */ + how.flags &= ~O_SPECIFIC_FD; /* O_PATH beats everything else. */ if (how.flags & O_PATH) how.flags &= O_PATH_FLAGS; @@ -989,6 +991,10 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op) return -EINVAL; if (how->resolve & ~VALID_RESOLVE_FLAGS) return -EINVAL; + if (how->__padding != 0) + return -EINVAL; + if (!(flags & O_SPECIFIC_FD) && how->fd != 0) + return -EINVAL; /* Deal with the mode. */ if (WILL_CREATE(flags)) { @@ -1143,7 +1149,7 @@ static long do_sys_openat2(int dfd, const char __user *filename, if (IS_ERR(tmp)) return PTR_ERR(tmp); - fd = get_unused_fd_flags(how->flags); + fd = get_specific_unused_fd_flags(how->fd, how->flags); if (fd >= 0) { struct file *f = do_filp_open(dfd, tmp, &op); if (IS_ERR(f)) { diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h index 7bcdcf4f6ab2..728849bcd8fa 100644 --- a/include/linux/fcntl.h +++ b/include/linux/fcntl.h @@ -10,7 +10,7 @@ (O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \ O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \ FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \ - O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE) + O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_SPECIFIC_FD) /* List of all valid flags for the how->upgrade_mask argument: */ #define VALID_UPGRADE_FLAGS \ @@ -23,7 +23,8 @@ /* List of all open_how "versions". */ #define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */ -#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0 +#define OPEN_HOW_SIZE_VER1 32 /* added fd and pad */ +#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER1 #ifndef force_o_largefile #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T)) diff --git a/include/linux/file.h b/include/linux/file.h index b67986f818d2..a63301864a36 100644 --- a/include/linux/file.h +++ b/include/linux/file.h @@ -87,6 +87,9 @@ extern void set_close_on_exec(unsigned int fd, int flag); extern bool get_close_on_exec(unsigned int fd); extern int __get_unused_fd_flags(unsigned flags, unsigned long nofile); extern int get_unused_fd_flags(unsigned flags); +extern int __get_specific_unused_fd_flags(unsigned int fd, unsigned int flags, + unsigned long nofile); +extern int get_specific_unused_fd_flags(unsigned int fd, unsigned int flags); extern void put_unused_fd(unsigned int fd); extern unsigned int increase_min_fd(unsigned int num); diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h index 9dc0bf0c5a6e..d3de5b8b3955 100644 --- a/include/uapi/asm-generic/fcntl.h +++ b/include/uapi/asm-generic/fcntl.h @@ -89,6 +89,10 @@ #define __O_TMPFILE 020000000 #endif +#ifndef O_SPECIFIC_FD +#define O_SPECIFIC_FD 01000000000 /* open as specified fd */ +#endif + /* a horrid kludge trying to make sure that this will fail on old kernels */ #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY) #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT) diff --git a/include/uapi/linux/openat2.h b/include/uapi/linux/openat2.h index 58b1eb711360..dcbf9dc333b5 100644 --- a/include/uapi/linux/openat2.h +++ b/include/uapi/linux/openat2.h @@ -15,11 +15,14 @@ * @flags: O_* flags. * @mode: O_CREAT/O_TMPFILE file mode. * @resolve: RESOLVE_* flags. + * @fd: Specific file descriptor to use, for O_SPECIFIC_FD. */ struct open_how { __u64 flags; __u64 mode; __u64 resolve; + __u32 fd; + __u32 __padding; /* Must be zeroed */ }; /* how->resolve flags for openat2(2). */ diff --git a/tools/testing/selftests/openat2/helpers.c b/tools/testing/selftests/openat2/helpers.c index 5074681ffdc9..b6533f0b1124 100644 --- a/tools/testing/selftests/openat2/helpers.c +++ b/tools/testing/selftests/openat2/helpers.c @@ -98,7 +98,7 @@ void __attribute__((constructor)) init(void) struct open_how how = {}; int fd; - BUILD_BUG_ON(sizeof(struct open_how) != OPEN_HOW_SIZE_VER0); + BUILD_BUG_ON(sizeof(struct open_how) != OPEN_HOW_SIZE_VER1); /* Check openat2(2) support. */ fd = sys_openat2(AT_FDCWD, ".", &how); diff --git a/tools/testing/selftests/openat2/helpers.h b/tools/testing/selftests/openat2/helpers.h index a6ea27344db2..d38818033b45 100644 --- a/tools/testing/selftests/openat2/helpers.h +++ b/tools/testing/selftests/openat2/helpers.h @@ -24,28 +24,37 @@ #endif /* SYS_openat2 */ /* - * Arguments for how openat2(2) should open the target path. If @resolve is - * zero, then openat2(2) operates very similarly to openat(2). + * Arguments for how openat2(2) should open the target path. If only @flags and + * @mode are non-zero, then openat2(2) operates very similarly to openat(2). * - * However, unlike openat(2), unknown bits in @flags result in -EINVAL rather - * than being silently ignored. @mode must be zero unless one of {O_CREAT, - * O_TMPFILE} are set. + * However, unlike openat(2), unknown or invalid bits in @flags result in + * -EINVAL rather than being silently ignored. @mode must be zero unless one of + * {O_CREAT, O_TMPFILE} are set. * * @flags: O_* flags. * @mode: O_CREAT/O_TMPFILE file mode. * @resolve: RESOLVE_* flags. + * @fd: Specific file descriptor to use, for O_SPECIFIC_FD. */ struct open_how { __u64 flags; __u64 mode; __u64 resolve; + __u32 fd; + __u32 __padding; /* Must be zeroed */ }; +/* List of all open_how "versions". */ #define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */ -#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0 +#define OPEN_HOW_SIZE_VER1 32 /* added fd and pad */ +#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER1 bool needs_openat2(const struct open_how *how); +#ifndef O_SPECIFIC_FD +#define O_SPECIFIC_FD 01000000000 +#endif + #ifndef RESOLVE_IN_ROOT /* how->resolve flags for openat2(2). */ #define RESOLVE_NO_XDEV 0x01 /* Block mount-point crossings diff --git a/tools/testing/selftests/openat2/openat2_test.c b/tools/testing/selftests/openat2/openat2_test.c index b386367c606b..f999b4bb0dc2 100644 --- a/tools/testing/selftests/openat2/openat2_test.c +++ b/tools/testing/selftests/openat2/openat2_test.c @@ -40,7 +40,7 @@ struct struct_test { int err; }; -#define NUM_OPENAT2_STRUCT_TESTS 7 +#define NUM_OPENAT2_STRUCT_TESTS 8 #define NUM_OPENAT2_STRUCT_VARIATIONS 13 void test_openat2_struct(void) @@ -52,6 +52,9 @@ void test_openat2_struct(void) { .name = "normal struct", .arg.inner.flags = O_RDONLY, .size = sizeof(struct open_how) }, + { .name = "v0 struct", + .arg.inner.flags = O_RDONLY, + .size = OPEN_HOW_SIZE_VER0 }, /* Bigger struct, with zeroed out end. */ { .name = "bigger struct (zeroed out)", .arg.inner.flags = O_RDONLY, @@ -155,7 +158,7 @@ struct flag_test { int err; }; -#define NUM_OPENAT2_FLAG_TESTS 23 +#define NUM_OPENAT2_FLAG_TESTS 31 void test_openat2_flags(void) { @@ -223,6 +226,30 @@ void test_openat2_flags(void) { .name = "invalid how.resolve and O_PATH", .how.flags = O_PATH, .how.resolve = 0x1337, .err = -EINVAL }, + + /* O_SPECIFIC_FD tests */ + { .name = "O_SPECIFIC_FD", + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = 42 }, + { .name = "O_SPECIFIC_FD if fd exists", + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = 2, + .err = -EBUSY }, + { .name = "O_SPECIFIC_FD with fd -1", + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = -1 }, + { .name = "fd without O_SPECIFIC_FD", + .how.flags = O_RDONLY, .how.fd = 42, + .err = -EINVAL }, + { .name = "fd -1 without O_SPECIFIC_FD", + .how.flags = O_RDONLY, .how.fd = -1, + .err = -EINVAL }, + { .name = "existing fd without O_SPECIFIC_FD", + .how.flags = O_RDONLY, .how.fd = 2, + .err = -EINVAL }, + { .name = "Non-zero padding with O_SPECIFIC_FD", + .how.flags = O_RDONLY | O_SPECIFIC_FD, .how.fd = 42, + .how.__padding = 42, .err = -EINVAL }, + { .name = "Non-zero padding without O_SPECIFIC_FD", + .how.flags = O_RDONLY, + .how.__padding = 42, .err = -EINVAL }, }; BUILD_BUG_ON(ARRAY_LEN(tests) != NUM_OPENAT2_FLAG_TESTS); @@ -268,6 +295,10 @@ void test_openat2_flags(void) if (!(test->how.flags & O_LARGEFILE)) fdflags &= ~O_LARGEFILE; failed |= (fdflags != test->how.flags); + + if (test->how.flags & O_SPECIFIC_FD + && test->how.fd != -1) + failed |= (fd != test->how.fd); } if (failed) {
Inspired by the X protocol's handling of XIDs, allow userspace to select the file descriptor opened by openat2, so that it can use the resulting file descriptor in subsequent system calls without waiting for the response to openat2. In io_uring, this allows sequences like openat2/read/close without waiting for the openat2 to complete. Multiple such sequences can overlap, as long as each uses a distinct file descriptor. Add a new O_SPECIFIC_FD open flag to enable this behavior, only accepted by openat2 for now (ignored by open/openat like all unknown flags). Add an fd field to struct open_how (along with appropriate padding, and verify that the padding is 0 to allow replacing the padding with a field in the future). The file table has a corresponding new function get_specific_unused_fd_flags, which gets the specified file descriptor if O_SPECIFIC_FD is set (and the fd isn't -1); otherwise it falls back to get_unused_fd_flags, to simplify callers. The specified file descriptor must not already be open; if it is, get_specific_unused_fd_flags will fail with -EBUSY. This helps catch userspace errors. When O_SPECIFIC_FD is set, and fd is not -1, openat2 will use the specified file descriptor rather than finding the lowest available one. Test program: #include <err.h> #include <fcntl.h> #include <stdio.h> #include <unistd.h> int main(void) { struct open_how how = { .flags = O_RDONLY | O_SPECIFIC_FD, .fd = 42 }; int fd = openat2(AT_FDCWD, "/dev/null", &how, sizeof(how)); if (fd < 0) err(1, "openat2"); printf("fd=%d\n", fd); // prints fd=42 return 0; } Signed-off-by: Josh Triplett <josh@joshtriplett.org> --- fs/fcntl.c | 2 +- fs/file.c | 39 +++++++++++++++++++ fs/io_uring.c | 3 +- fs/open.c | 8 +++- include/linux/fcntl.h | 5 ++- include/linux/file.h | 3 ++ include/uapi/asm-generic/fcntl.h | 4 ++ include/uapi/linux/openat2.h | 3 ++ tools/testing/selftests/openat2/helpers.c | 2 +- tools/testing/selftests/openat2/helpers.h | 21 +++++++--- .../testing/selftests/openat2/openat2_test.c | 35 ++++++++++++++++- 11 files changed, 111 insertions(+), 14 deletions(-)