| Message ID | 20190521113448.20654-1-christian@brauner.io (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | [1/2] open: add close_range() | expand |
* Christian Brauner: > +/** > + * __close_range() - Close all file descriptors in a given range. > + * > + * @fd: starting file descriptor to close > + * @max_fd: last file descriptor to close > + * > + * This closes a range of file descriptors. All file descriptors > + * from @fd up to and including @max_fd are closed. > + */ > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd) > +{ > + unsigned int cur_max; > + > + if (fd > max_fd) > + return -EINVAL; > + > + rcu_read_lock(); > + cur_max = files_fdtable(files)->max_fds; > + rcu_read_unlock(); > + > + /* cap to last valid index into fdtable */ > + if (max_fd >= cur_max) > + max_fd = cur_max - 1; > + > + while (fd <= max_fd) > + __close_fd(files, fd++); > + > + return 0; > +} This seems rather drastic. How long does this block in kernel mode? Maybe it's okay as long as the maximum possible value for cur_max stays around 4 million or so. Solaris has an fdwalk function: <https://docs.oracle.com/cd/E88353_01/html/E37843/closefrom-3c.html> So a different way to implement this would expose a nextfd system call to userspace, so that we can use that to implement both fdwalk and closefrom. But maybe fdwalk is just too obscure, given the existence of /proc. I'll happily implement closefrom on top of close_range in glibc (plus fallback for older kernels based on /proc—with an abort in case that doesn't work because the RLIMIT_NOFILE hack is unreliable unfortunately). Thanks, Florian
On Tue, May 21, 2019 at 02:09:29PM +0200, Florian Weimer wrote: > * Christian Brauner: > > > +/** > > + * __close_range() - Close all file descriptors in a given range. > > + * > > + * @fd: starting file descriptor to close > > + * @max_fd: last file descriptor to close > > + * > > + * This closes a range of file descriptors. All file descriptors > > + * from @fd up to and including @max_fd are closed. > > + */ > > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd) > > +{ > > + unsigned int cur_max; > > + > > + if (fd > max_fd) > > + return -EINVAL; > > + > > + rcu_read_lock(); > > + cur_max = files_fdtable(files)->max_fds; > > + rcu_read_unlock(); > > + > > + /* cap to last valid index into fdtable */ > > + if (max_fd >= cur_max) > > + max_fd = cur_max - 1; > > + > > + while (fd <= max_fd) > > + __close_fd(files, fd++); > > + > > + return 0; > > +} > > This seems rather drastic. How long does this block in kernel mode? > Maybe it's okay as long as the maximum possible value for cur_max stays > around 4 million or so. That's probably valid concern when you reach very high numbers though I wonder how relevant this is in practice. Also, you would only be blocking yourself I imagine, i.e. you can't DOS another task with this unless your multi-threaded. > > Solaris has an fdwalk function: > > <https://docs.oracle.com/cd/E88353_01/html/E37843/closefrom-3c.html> > > So a different way to implement this would expose a nextfd system call Meh. If nextfd() then I would like it to be able to: - get the nextfd(fd) >= fd - get highest open fd e.g. nextfd(-1) But then I wonder if nextfd() needs to be a syscall and isn't just either: fcntl(fd, F_GET_NEXT)? or prctl(PR_GET_NEXT)? Technically, one could also do: fd_range(unsigned fd, unsigend end_fd, unsigned flags); fd_range(3, 50, FD_RANGE_CLOSE); /* return highest fd within the range [3, 50] */ fd_range(3, 50, FD_RANGE_NEXT); /* return highest fd */ fd_range(3, UINT_MAX, FD_RANGE_NEXT); This syscall could also reasonably be extended. > to userspace, so that we can use that to implement both fdwalk and > closefrom. But maybe fdwalk is just too obscure, given the existence of > /proc. Yeah we probably don't need fdwalk. > > I'll happily implement closefrom on top of close_range in glibc (plus > fallback for older kernels based on /proc—with an abort in case that > doesn't work because the RLIMIT_NOFILE hack is unreliable > unfortunately). > > Thanks, > Florian
* Christian Brauner: >> Solaris has an fdwalk function: >> >> <https://docs.oracle.com/cd/E88353_01/html/E37843/closefrom-3c.html> >> >> So a different way to implement this would expose a nextfd system call > > Meh. If nextfd() then I would like it to be able to: > - get the nextfd(fd) >= fd > - get highest open fd e.g. nextfd(-1) The highest open descriptor isn't istering for fdwalk because nextfd would just fail. > But then I wonder if nextfd() needs to be a syscall and isn't just > either: > fcntl(fd, F_GET_NEXT)? > or > prctl(PR_GET_NEXT)? I think the fcntl route is a bit iffy because you might need it to get the *first* valid descriptor. >> to userspace, so that we can use that to implement both fdwalk and >> closefrom. But maybe fdwalk is just too obscure, given the existence of >> /proc. > > Yeah we probably don't need fdwalk. Agreed. Just wanted to bring it up for completeness. I certainly don't want to derail the implementation of close_range. Thanks, Florian
On Tue, May 21, 2019 at 03:10:11PM +0200, Florian Weimer wrote: > * Christian Brauner: > > >> Solaris has an fdwalk function: > >> > >> <https://docs.oracle.com/cd/E88353_01/html/E37843/closefrom-3c.html> > >> > >> So a different way to implement this would expose a nextfd system call > > > > Meh. If nextfd() then I would like it to be able to: > > - get the nextfd(fd) >= fd > > - get highest open fd e.g. nextfd(-1) > > The highest open descriptor isn't istering for fdwalk because nextfd > would just fail. > > > But then I wonder if nextfd() needs to be a syscall and isn't just > > either: > > fcntl(fd, F_GET_NEXT)? > > or > > prctl(PR_GET_NEXT)? > > I think the fcntl route is a bit iffy because you might need it to get > the *first* valid descriptor. Oh, how would that be difficult? Maybe I'm missing context. Couldn't you just do fcntl(0, F_GET_NEXT) > > >> to userspace, so that we can use that to implement both fdwalk and > >> closefrom. But maybe fdwalk is just too obscure, given the existence of > >> /proc. > > > > Yeah we probably don't need fdwalk. > > Agreed. Just wanted to bring it up for completeness. I certainly don't > want to derail the implementation of close_range. No, that's perfectly fine. I mean, you clearly need this and are one of the major stakeholders. For example, Rust (probably also Python) will call down into libc and not use the syscall directly. They kinda do this with getfdtable<sm> rn already. So what you say makes sense for libc has some relevance for the other tools as well. Christian
On Tue, May 21, 2019 at 03:10:11PM +0200, Florian Weimer wrote: > * Christian Brauner: > > >> Solaris has an fdwalk function: > >> > >> <https://docs.oracle.com/cd/E88353_01/html/E37843/closefrom-3c.html> > >> > >> So a different way to implement this would expose a nextfd system call > > > > Meh. If nextfd() then I would like it to be able to: > > - get the nextfd(fd) >= fd > > - get highest open fd e.g. nextfd(-1) > > The highest open descriptor isn't istering for fdwalk because nextfd > would just fail. Sure. I was thinking about other usecases. For example, sometimes in userspace you want to do the following: save_fd = dup(fd, <well-known-number-at-the-end-of-the-range); close_range(3, (save_fd - 1)); Which brings me to another point. So even if we don't do close_range() I would like libc to maybe give us something like close_range() for such scenarios. > > > But then I wonder if nextfd() needs to be a syscall and isn't just > > either: > > fcntl(fd, F_GET_NEXT)? > > or > > prctl(PR_GET_NEXT)? > > I think the fcntl route is a bit iffy because you might need it to get > the *first* valid descriptor. > > >> to userspace, so that we can use that to implement both fdwalk and > >> closefrom. But maybe fdwalk is just too obscure, given the existence of > >> /proc. > > > > Yeah we probably don't need fdwalk. > > Agreed. Just wanted to bring it up for completeness. I certainly don't > want to derail the implementation of close_range. > > Thanks, > Florian
On Tue, May 21, 2019 at 01:34:47PM +0200, Christian Brauner wrote: > This adds the close_range() syscall. It allows to efficiently close a range > of file descriptors up to all file descriptors of a calling task. > > The syscall came up in a recent discussion around the new mount API and > making new file descriptor types cloexec by default. During this > discussion, Al suggested the close_range() syscall (cf. [1]). Note, a > syscall in this manner has been requested by various people over time. > > First, it helps to close all file descriptors of an exec()ing task. This > can be done safely via (quoting Al's example from [1] verbatim): > > /* that exec is sensitive */ > unshare(CLONE_FILES); > /* we don't want anything past stderr here */ > close_range(3, ~0U); > execve(....); > > The code snippet above is one way of working around the problem that file > descriptors are not cloexec by default. This is aggravated by the fact that > we can't just switch them over without massively regressing userspace. For > a whole class of programs having an in-kernel method of closing all file > descriptors is very helpful (e.g. demons, service managers, programming > language standard libraries, container managers etc.). > (Please note, unshare(CLONE_FILES) should only be needed if the calling > task is multi-threaded and shares the file descriptor table with another > thread in which case two threads could race with one thread allocating > file descriptors and the other one closing them via close_range(). For the > general case close_range() before the execve() is sufficient.) > > Second, it allows userspace to avoid implementing closing all file > descriptors by parsing through /proc/<pid>/fd/* and calling close() on each > file descriptor. From looking at various large(ish) userspace code bases > this or similar patterns are very common in: > - service managers (cf. [4]) > - libcs (cf. [6]) > - container runtimes (cf. [5]) > - programming language runtimes/standard libraries > - Python (cf. [2]) > - Rust (cf. [7], [8]) > As Dmitry pointed out there's even a long-standing glibc bug about missing > kernel support for this task (cf. [3]). > In addition, the syscall will also work for tasks that do not have procfs > mounted and on kernels that do not have procfs support compiled in. In such > situations the only way to make sure that all file descriptors are closed > is to call close() on each file descriptor up to UINT_MAX or RLIMIT_NOFILE, > OPEN_MAX trickery (cf. comment [8] on Rust). > > The performance is striking. For good measure, comparing the following > simple close_all_fds() userspace implementation that is essentially just > glibc's version in [6]: > > static int close_all_fds(void) > { > DIR *dir; > struct dirent *direntp; > > dir = opendir("/proc/self/fd"); > if (!dir) > return -1; > > while ((direntp = readdir(dir))) { > int fd; > if (strcmp(direntp->d_name, ".") == 0) > continue; > if (strcmp(direntp->d_name, "..") == 0) > continue; > fd = atoi(direntp->d_name); > if (fd == 0 || fd == 1 || fd == 2) > continue; > close(fd); > } > > closedir(dir); /* cannot fail */ > return 0; > } > > to close_range() yields: > 1. closing 4 open files: > - close_all_fds(): ~280 us > - close_range(): ~24 us > > 2. closing 1000 open files: > - close_all_fds(): ~5000 us > - close_range(): ~800 us > > close_range() is designed to allow for some flexibility. Specifically, it > does not simply always close all open file descriptors of a task. Instead, > callers can specify an upper bound. > This is e.g. useful for scenarios where specific file descriptors are > created with well-known numbers that are supposed to be excluded from > getting closed. > For extra paranoia close_range() comes with a flags argument. This can e.g. > be used to implement extension. Once can imagine userspace wanting to stop > at the first error instead of ignoring errors under certain circumstances. > There might be other valid ideas in the future. In any case, a flag > argument doesn't hurt and keeps us on the safe side. > > >From an implementation side this is kept rather dumb. It saw some input > from David and Jann but all nonsense is obviously my own! > - Errors to close file descriptors are currently ignored. (Could be changed > by setting a flag in the future if needed.) > - __close_range() is a rather simplistic wrapper around __close_fd(). > My reasoning behind this is based on the nature of how __close_fd() needs > to release an fd. But maybe I misunderstood specifics: > We take the files_lock and rcu-dereference the fdtable of the calling > task, we find the entry in the fdtable, get the file and need to release > files_lock before calling filp_close(). > In the meantime the fdtable might have been altered so we can't just > retake the spinlock and keep the old rcu-reference of the fdtable > around. Instead we need to grab a fresh reference to the fdtable. > If my reasoning is correct then there's really no point in fancyfying > __close_range(): We just need to rcu-dereference the fdtable of the > calling task once to cap the max_fd value correctly and then go on > calling __close_fd() in a loop. > +/** > + * __close_range() - Close all file descriptors in a given range. > + * > + * @fd: starting file descriptor to close > + * @max_fd: last file descriptor to close > + * > + * This closes a range of file descriptors. All file descriptors > + * from @fd up to and including @max_fd are closed. > + */ > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd) > +{ > + unsigned int cur_max; > + > + if (fd > max_fd) > + return -EINVAL; > + > + rcu_read_lock(); > + cur_max = files_fdtable(files)->max_fds; > + rcu_read_unlock(); > + > + /* cap to last valid index into fdtable */ > + if (max_fd >= cur_max) > + max_fd = cur_max - 1; > + > + while (fd <= max_fd) > + __close_fd(files, fd++); > + > + return 0; > +} Umm... That's going to be very painful if you dup2() something to MAX_INT and then run that; roughly 2G iterations of bouncing ->file_lock up and down, without anything that would yield CPU in process. If anything, I would suggest something like fd = *start_fd; grab the lock fdt = files_fdtable(files); more: look for the next eviction candidate in ->open_fds, starting at fd if there's none up to max_fd drop the lock return NULL *start_fd = fd + 1; if the fscker is really opened and not just reserved rcu_assign_pointer(fdt->fd[fd], NULL); __put_unused_fd(files, fd); drop the lock return the file we'd got if (unlikely(need_resched())) drop lock cond_resched(); grab lock fdt = files_fdtable(files); goto more; with the main loop being basically while ((file = pick_next(files, &start_fd, max_fd)) != NULL) filp_close(file, files);
Al Viro <viro@zeniv.linux.org.uk> wrote: > Umm... That's going to be very painful if you dup2() something to MAX_INT and > then run that; roughly 2G iterations of bouncing ->file_lock up and down, > without anything that would yield CPU in process. > > If anything, I would suggest something like > > fd = *start_fd; > grab the lock > fdt = files_fdtable(files); > more: > look for the next eviction candidate in ->open_fds, starting at fd > if there's none up to max_fd > drop the lock > return NULL > *start_fd = fd + 1; > if the fscker is really opened and not just reserved > rcu_assign_pointer(fdt->fd[fd], NULL); > __put_unused_fd(files, fd); > drop the lock > return the file we'd got > if (unlikely(need_resched())) > drop lock > cond_resched(); > grab lock > fdt = files_fdtable(files); > goto more; > > with the main loop being basically > while ((file = pick_next(files, &start_fd, max_fd)) != NULL) > filp_close(file, files); If we can live with close_from(int first) rather than close_range(), then this can perhaps be done a lot more efficiently by: new = alloc_fdtable(first); spin_lock(&files->file_lock); old = files_fdtable(files); copy_fds(new, old, 0, first - 1); rcu_assign_pointer(files->fdt, new); spin_unlock(&files->file_lock); clear_fds(old, 0, first - 1); close_fdt_from(old, first); kfree_rcu(old); David
On Tue, May 21, 2019 at 05:30:27PM +0100, David Howells wrote: > Al Viro <viro@zeniv.linux.org.uk> wrote: > > > Umm... That's going to be very painful if you dup2() something to MAX_INT and > > then run that; roughly 2G iterations of bouncing ->file_lock up and down, > > without anything that would yield CPU in process. > > > > If anything, I would suggest something like > > > > fd = *start_fd; > > grab the lock > > fdt = files_fdtable(files); > > more: > > look for the next eviction candidate in ->open_fds, starting at fd > > if there's none up to max_fd > > drop the lock > > return NULL > > *start_fd = fd + 1; > > if the fscker is really opened and not just reserved > > rcu_assign_pointer(fdt->fd[fd], NULL); > > __put_unused_fd(files, fd); > > drop the lock > > return the file we'd got > > if (unlikely(need_resched())) > > drop lock > > cond_resched(); > > grab lock > > fdt = files_fdtable(files); > > goto more; > > > > with the main loop being basically > > while ((file = pick_next(files, &start_fd, max_fd)) != NULL) > > filp_close(file, files); > > If we can live with close_from(int first) rather than close_range(), then this > can perhaps be done a lot more efficiently by: Yeah, you mentioned this before. I do like being able to specify an upper bound to have the ability to place fds strategically after said upper bound. I have used this quite a few times where I know that given task may have inherited up to m fds and I want to inherit a specific pipe who's fd I know. Then I'd dup2(pipe_fd, <upper_bound + 1>) and then close all other fds. Is that too much of a corner case? Christian
On Tue, May 21, 2019 at 04:00:06PM +0100, Al Viro wrote: > On Tue, May 21, 2019 at 01:34:47PM +0200, Christian Brauner wrote: > > > This adds the close_range() syscall. It allows to efficiently close a range > > of file descriptors up to all file descriptors of a calling task. > > > > The syscall came up in a recent discussion around the new mount API and > > making new file descriptor types cloexec by default. During this > > discussion, Al suggested the close_range() syscall (cf. [1]). Note, a > > syscall in this manner has been requested by various people over time. > > > > First, it helps to close all file descriptors of an exec()ing task. This > > can be done safely via (quoting Al's example from [1] verbatim): > > > > /* that exec is sensitive */ > > unshare(CLONE_FILES); > > /* we don't want anything past stderr here */ > > close_range(3, ~0U); > > execve(....); > > > > The code snippet above is one way of working around the problem that file > > descriptors are not cloexec by default. This is aggravated by the fact that > > we can't just switch them over without massively regressing userspace. For > > a whole class of programs having an in-kernel method of closing all file > > descriptors is very helpful (e.g. demons, service managers, programming > > language standard libraries, container managers etc.). > > (Please note, unshare(CLONE_FILES) should only be needed if the calling > > task is multi-threaded and shares the file descriptor table with another > > thread in which case two threads could race with one thread allocating > > file descriptors and the other one closing them via close_range(). For the > > general case close_range() before the execve() is sufficient.) > > > > Second, it allows userspace to avoid implementing closing all file > > descriptors by parsing through /proc/<pid>/fd/* and calling close() on each > > file descriptor. From looking at various large(ish) userspace code bases > > this or similar patterns are very common in: > > - service managers (cf. [4]) > > - libcs (cf. [6]) > > - container runtimes (cf. [5]) > > - programming language runtimes/standard libraries > > - Python (cf. [2]) > > - Rust (cf. [7], [8]) > > As Dmitry pointed out there's even a long-standing glibc bug about missing > > kernel support for this task (cf. [3]). > > In addition, the syscall will also work for tasks that do not have procfs > > mounted and on kernels that do not have procfs support compiled in. In such > > situations the only way to make sure that all file descriptors are closed > > is to call close() on each file descriptor up to UINT_MAX or RLIMIT_NOFILE, > > OPEN_MAX trickery (cf. comment [8] on Rust). > > > > The performance is striking. For good measure, comparing the following > > simple close_all_fds() userspace implementation that is essentially just > > glibc's version in [6]: > > > > static int close_all_fds(void) > > { > > DIR *dir; > > struct dirent *direntp; > > > > dir = opendir("/proc/self/fd"); > > if (!dir) > > return -1; > > > > while ((direntp = readdir(dir))) { > > int fd; > > if (strcmp(direntp->d_name, ".") == 0) > > continue; > > if (strcmp(direntp->d_name, "..") == 0) > > continue; > > fd = atoi(direntp->d_name); > > if (fd == 0 || fd == 1 || fd == 2) > > continue; > > close(fd); > > } > > > > closedir(dir); /* cannot fail */ > > return 0; > > } > > > > to close_range() yields: > > 1. closing 4 open files: > > - close_all_fds(): ~280 us > > - close_range(): ~24 us > > > > 2. closing 1000 open files: > > - close_all_fds(): ~5000 us > > - close_range(): ~800 us > > > > close_range() is designed to allow for some flexibility. Specifically, it > > does not simply always close all open file descriptors of a task. Instead, > > callers can specify an upper bound. > > This is e.g. useful for scenarios where specific file descriptors are > > created with well-known numbers that are supposed to be excluded from > > getting closed. > > For extra paranoia close_range() comes with a flags argument. This can e.g. > > be used to implement extension. Once can imagine userspace wanting to stop > > at the first error instead of ignoring errors under certain circumstances. > > There might be other valid ideas in the future. In any case, a flag > > argument doesn't hurt and keeps us on the safe side. > > > > >From an implementation side this is kept rather dumb. It saw some input > > from David and Jann but all nonsense is obviously my own! > > - Errors to close file descriptors are currently ignored. (Could be changed > > by setting a flag in the future if needed.) > > - __close_range() is a rather simplistic wrapper around __close_fd(). > > My reasoning behind this is based on the nature of how __close_fd() needs > > to release an fd. But maybe I misunderstood specifics: > > We take the files_lock and rcu-dereference the fdtable of the calling > > task, we find the entry in the fdtable, get the file and need to release > > files_lock before calling filp_close(). > > In the meantime the fdtable might have been altered so we can't just > > retake the spinlock and keep the old rcu-reference of the fdtable > > around. Instead we need to grab a fresh reference to the fdtable. > > If my reasoning is correct then there's really no point in fancyfying > > __close_range(): We just need to rcu-dereference the fdtable of the > > calling task once to cap the max_fd value correctly and then go on > > calling __close_fd() in a loop. > > > +/** > > + * __close_range() - Close all file descriptors in a given range. > > + * > > + * @fd: starting file descriptor to close > > + * @max_fd: last file descriptor to close > > + * > > + * This closes a range of file descriptors. All file descriptors > > + * from @fd up to and including @max_fd are closed. > > + */ > > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd) > > +{ > > + unsigned int cur_max; > > + > > + if (fd > max_fd) > > + return -EINVAL; > > + > > + rcu_read_lock(); > > + cur_max = files_fdtable(files)->max_fds; > > + rcu_read_unlock(); > > + > > + /* cap to last valid index into fdtable */ > > + if (max_fd >= cur_max) > > + max_fd = cur_max - 1; > > + > > + while (fd <= max_fd) > > + __close_fd(files, fd++); > > + > > + return 0; > > +} > > Umm... That's going to be very painful if you dup2() something to MAX_INT and > then run that; roughly 2G iterations of bouncing ->file_lock up and down, > without anything that would yield CPU in process. > > If anything, I would suggest something like > > fd = *start_fd; > grab the lock > fdt = files_fdtable(files); > more: > look for the next eviction candidate in ->open_fds, starting at fd > if there's none up to max_fd > drop the lock > return NULL > *start_fd = fd + 1; > if the fscker is really opened and not just reserved > rcu_assign_pointer(fdt->fd[fd], NULL); > __put_unused_fd(files, fd); > drop the lock > return the file we'd got > if (unlikely(need_resched())) > drop lock > cond_resched(); > grab lock > fdt = files_fdtable(files); > goto more; > > with the main loop being basically > while ((file = pick_next(files, &start_fd, max_fd)) != NULL) > filp_close(file, files); That's obviously much more clever than what I had. I honestly have never thought about using open_fds before this. Seemed extremely localized to file.c Thanks for the pointers! Christian
On Tue, May 21, 2019 at 05:30:27PM +0100, David Howells wrote: > If we can live with close_from(int first) rather than close_range(), then this > can perhaps be done a lot more efficiently by: > > new = alloc_fdtable(first); > spin_lock(&files->file_lock); > old = files_fdtable(files); > copy_fds(new, old, 0, first - 1); > rcu_assign_pointer(files->fdt, new); > spin_unlock(&files->file_lock); > clear_fds(old, 0, first - 1); > close_fdt_from(old, first); > kfree_rcu(old); I really hate to think how that would interact with POSIX locks...
On Tue, May 21, 2019 at 08:20:09PM +0100, Al Viro wrote: > On Tue, May 21, 2019 at 05:30:27PM +0100, David Howells wrote: > > > If we can live with close_from(int first) rather than close_range(), then this > > can perhaps be done a lot more efficiently by: > > > > new = alloc_fdtable(first); > > spin_lock(&files->file_lock); > > old = files_fdtable(files); > > copy_fds(new, old, 0, first - 1); > > rcu_assign_pointer(files->fdt, new); > > spin_unlock(&files->file_lock); > > clear_fds(old, 0, first - 1); > > close_fdt_from(old, first); > > kfree_rcu(old); > > I really hate to think how that would interact with POSIX locks... POSIX locks store current->files in fl_owner; David's resizing the underlying files->fdt, just like growing from 64 to 256 fds.
On Tue, May 21, 2019 at 9:41 AM Christian Brauner <christian@brauner.io> wrote: > > Yeah, you mentioned this before. I do like being able to specify an > upper bound to have the ability to place fds strategically after said > upper bound. I suspect that's the case. And if somebody really wants to just close everything and uses a large upper bound, we can - if we really want to - just compare the upper bound to the file table size, and do an optimized case for that. We do that upper bound comparison anyway to limit the size of the walk, so *if* it's a big deal, that case could then do the whole "shrink fdtable" case too. But I don't believe it's worth optimizing for unless somebody really has a load where that is shown to be a big deal. Just do the silly and simple loop, and add a cond_resched() in the loop, like close_files() does for the "we have a _lot_ of files open" case. Linus
On Tue, May 21, 2019 at 10:23 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Tue, May 21, 2019 at 9:41 AM Christian Brauner <christian@brauner.io> wrote: > > > > Yeah, you mentioned this before. I do like being able to specify an > > upper bound to have the ability to place fds strategically after said > > upper bound. > > I suspect that's the case. > > And if somebody really wants to just close everything and uses a large > upper bound, we can - if we really want to - just compare the upper > bound to the file table size, and do an optimized case for that. We do > that upper bound comparison anyway to limit the size of the walk, so > *if* it's a big deal, that case could then do the whole "shrink > fdtable" case too. Makes sense. > > But I don't believe it's worth optimizing for unless somebody really > has a load where that is shown to be a big deal. Just do the silly > and simple loop, and add a cond_resched() in the loop, like > close_files() does for the "we have a _lot_ of files open" case. Ok. I will resend a v1 later with the cond_resched() logic you and Al suggested added. Thanks! Christian
diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl index 9e7704e44f6d..b55d93af8096 100644 --- a/arch/alpha/kernel/syscalls/syscall.tbl +++ b/arch/alpha/kernel/syscalls/syscall.tbl @@ -473,3 +473,4 @@ 541 common fsconfig sys_fsconfig 542 common fsmount sys_fsmount 543 common fspick sys_fspick +545 common close_range sys_close_range diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl index aaf479a9e92d..0125c97c75dd 100644 --- a/arch/arm/tools/syscall.tbl +++ b/arch/arm/tools/syscall.tbl @@ -447,3 +447,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h index c39e90600bb3..9a3270d29b42 100644 --- a/arch/arm64/include/asm/unistd32.h +++ b/arch/arm64/include/asm/unistd32.h @@ -886,6 +886,8 @@ __SYSCALL(__NR_fsconfig, sys_fsconfig) __SYSCALL(__NR_fsmount, sys_fsmount) #define __NR_fspick 433 __SYSCALL(__NR_fspick, sys_fspick) +#define __NR_close_range 435 +__SYSCALL(__NR_close_range, sys_close_range) /* * Please add new compat syscalls above this comment and update diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl index e01df3f2f80d..1a90b464e96f 100644 --- a/arch/ia64/kernel/syscalls/syscall.tbl +++ b/arch/ia64/kernel/syscalls/syscall.tbl @@ -354,3 +354,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl index 7e3d0734b2f3..2dee2050f9ef 100644 --- a/arch/m68k/kernel/syscalls/syscall.tbl +++ b/arch/m68k/kernel/syscalls/syscall.tbl @@ -433,3 +433,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl index 26339e417695..923ef69e5a76 100644 --- a/arch/microblaze/kernel/syscalls/syscall.tbl +++ b/arch/microblaze/kernel/syscalls/syscall.tbl @@ -439,3 +439,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl index 0e2dd68ade57..967ed9de51cd 100644 --- a/arch/mips/kernel/syscalls/syscall_n32.tbl +++ b/arch/mips/kernel/syscalls/syscall_n32.tbl @@ -372,3 +372,4 @@ 431 n32 fsconfig sys_fsconfig 432 n32 fsmount sys_fsmount 433 n32 fspick sys_fspick +435 n32 close_range sys_close_range diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl index 5eebfa0d155c..71de731102b1 100644 --- a/arch/mips/kernel/syscalls/syscall_n64.tbl +++ b/arch/mips/kernel/syscalls/syscall_n64.tbl @@ -348,3 +348,4 @@ 431 n64 fsconfig sys_fsconfig 432 n64 fsmount sys_fsmount 433 n64 fspick sys_fspick +435 n64 close_range sys_close_range diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl index 3cc1374e02d0..5a325ab29f88 100644 --- a/arch/mips/kernel/syscalls/syscall_o32.tbl +++ b/arch/mips/kernel/syscalls/syscall_o32.tbl @@ -421,3 +421,4 @@ 431 o32 fsconfig sys_fsconfig 432 o32 fsmount sys_fsmount 433 o32 fspick sys_fspick +435 o32 close_range sys_close_range diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl index c9e377d59232..dcc0a0879139 100644 --- a/arch/parisc/kernel/syscalls/syscall.tbl +++ b/arch/parisc/kernel/syscalls/syscall.tbl @@ -430,3 +430,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl index 103655d84b4b..ba2c1f078cbd 100644 --- a/arch/powerpc/kernel/syscalls/syscall.tbl +++ b/arch/powerpc/kernel/syscalls/syscall.tbl @@ -515,3 +515,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl index e822b2964a83..d7c9043d2902 100644 --- a/arch/s390/kernel/syscalls/syscall.tbl +++ b/arch/s390/kernel/syscalls/syscall.tbl @@ -436,3 +436,4 @@ 431 common fsconfig sys_fsconfig sys_fsconfig 432 common fsmount sys_fsmount sys_fsmount 433 common fspick sys_fspick sys_fspick +435 common close_range sys_close_range sys_close_range diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl index 016a727d4357..9b5e6bf0ce32 100644 --- a/arch/sh/kernel/syscalls/syscall.tbl +++ b/arch/sh/kernel/syscalls/syscall.tbl @@ -436,3 +436,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl index e047480b1605..8c674a1e0072 100644 --- a/arch/sparc/kernel/syscalls/syscall.tbl +++ b/arch/sparc/kernel/syscalls/syscall.tbl @@ -479,3 +479,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl index ad968b7bac72..7f7a89a96707 100644 --- a/arch/x86/entry/syscalls/syscall_32.tbl +++ b/arch/x86/entry/syscalls/syscall_32.tbl @@ -438,3 +438,4 @@ 431 i386 fsconfig sys_fsconfig __ia32_sys_fsconfig 432 i386 fsmount sys_fsmount __ia32_sys_fsmount 433 i386 fspick sys_fspick __ia32_sys_fspick +435 i386 close_range sys_close_range __ia32_sys_close_range diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl index b4e6f9e6204a..0f7d47ae921c 100644 --- a/arch/x86/entry/syscalls/syscall_64.tbl +++ b/arch/x86/entry/syscalls/syscall_64.tbl @@ -355,6 +355,7 @@ 431 common fsconfig __x64_sys_fsconfig 432 common fsmount __x64_sys_fsmount 433 common fspick __x64_sys_fspick +435 common close_range __x64_sys_close_range # # x32-specific system call numbers start at 512 to avoid cache impact diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl index 5fa0ee1c8e00..b489532265d0 100644 --- a/arch/xtensa/kernel/syscalls/syscall.tbl +++ b/arch/xtensa/kernel/syscalls/syscall.tbl @@ -404,3 +404,4 @@ 431 common fsconfig sys_fsconfig 432 common fsmount sys_fsmount 433 common fspick sys_fspick +435 common close_range sys_close_range diff --git a/fs/file.c b/fs/file.c index 3da91a112bab..3680977a685a 100644 --- a/fs/file.c +++ b/fs/file.c @@ -641,6 +641,36 @@ int __close_fd(struct files_struct *files, unsigned fd) } EXPORT_SYMBOL(__close_fd); /* for ksys_close() */ +/** + * __close_range() - Close all file descriptors in a given range. + * + * @fd: starting file descriptor to close + * @max_fd: last file descriptor to close + * + * This closes a range of file descriptors. All file descriptors + * from @fd up to and including @max_fd are closed. + */ +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd) +{ + unsigned int cur_max; + + if (fd > max_fd) + return -EINVAL; + + rcu_read_lock(); + cur_max = files_fdtable(files)->max_fds; + rcu_read_unlock(); + + /* cap to last valid index into fdtable */ + if (max_fd >= cur_max) + max_fd = cur_max - 1; + + while (fd <= max_fd) + __close_fd(files, fd++); + + return 0; +} + /* * variant of __close_fd that gets a ref on the file for later fput */ diff --git a/fs/open.c b/fs/open.c index 9c7d724a6f67..c7baaee7aa47 100644 --- a/fs/open.c +++ b/fs/open.c @@ -1174,6 +1174,26 @@ SYSCALL_DEFINE1(close, unsigned int, fd) return retval; } +/** + * close_range() - Close all file descriptors in a given range. + * + * @fd: starting file descriptor to close + * @max_fd: last file descriptor to close + * @flags: reserved for future extensions + * + * This closes a range of file descriptors. All file descriptors + * from @fd up to and including @max_fd are closed. + * Currently, errors to close a given file descriptor are ignored. + */ +SYSCALL_DEFINE3(close_range, unsigned int, fd, unsigned int, max_fd, + unsigned int, flags) +{ + if (flags) + return -EINVAL; + + return __close_range(current->files, fd, max_fd); +} + /* * This routine simulates a hangup on the tty, to arrange that users * are given clean terminals at login time. diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h index f07c55ea0c22..fcd07181a365 100644 --- a/include/linux/fdtable.h +++ b/include/linux/fdtable.h @@ -121,6 +121,8 @@ extern void __fd_install(struct files_struct *files, unsigned int fd, struct file *file); extern int __close_fd(struct files_struct *files, unsigned int fd); +extern int __close_range(struct files_struct *files, unsigned int fd, + unsigned int max_fd); extern int __close_fd_get_file(unsigned int fd, struct file **res); extern struct kmem_cache *files_cachep; diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h index e2870fe1be5b..c0189e223255 100644 --- a/include/linux/syscalls.h +++ b/include/linux/syscalls.h @@ -441,6 +441,8 @@ asmlinkage long sys_fchown(unsigned int fd, uid_t user, gid_t group); asmlinkage long sys_openat(int dfd, const char __user *filename, int flags, umode_t mode); asmlinkage long sys_close(unsigned int fd); +asmlinkage long sys_close_range(unsigned int fd, unsigned int max_fd, + unsigned int flags); asmlinkage long sys_vhangup(void); /* fs/pipe.c */ diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h index a87904daf103..3f36c8745d24 100644 --- a/include/uapi/asm-generic/unistd.h +++ b/include/uapi/asm-generic/unistd.h @@ -844,9 +844,11 @@ __SYSCALL(__NR_fsconfig, sys_fsconfig) __SYSCALL(__NR_fsmount, sys_fsmount) #define __NR_fspick 433 __SYSCALL(__NR_fspick, sys_fspick) +#define __NR_close_range 435 +__SYSCALL(__NR_close_range, sys_close_range) #undef __NR_syscalls -#define __NR_syscalls 434 +#define __NR_syscalls 436 /* * 32 bit systems traditionally used different
This adds the close_range() syscall. It allows to efficiently close a range of file descriptors up to all file descriptors of a calling task. The syscall came up in a recent discussion around the new mount API and making new file descriptor types cloexec by default. During this discussion, Al suggested the close_range() syscall (cf. [1]). Note, a syscall in this manner has been requested by various people over time. First, it helps to close all file descriptors of an exec()ing task. This can be done safely via (quoting Al's example from [1] verbatim): /* that exec is sensitive */ unshare(CLONE_FILES); /* we don't want anything past stderr here */ close_range(3, ~0U); execve(....); The code snippet above is one way of working around the problem that file descriptors are not cloexec by default. This is aggravated by the fact that we can't just switch them over without massively regressing userspace. For a whole class of programs having an in-kernel method of closing all file descriptors is very helpful (e.g. demons, service managers, programming language standard libraries, container managers etc.). (Please note, unshare(CLONE_FILES) should only be needed if the calling task is multi-threaded and shares the file descriptor table with another thread in which case two threads could race with one thread allocating file descriptors and the other one closing them via close_range(). For the general case close_range() before the execve() is sufficient.) Second, it allows userspace to avoid implementing closing all file descriptors by parsing through /proc/<pid>/fd/* and calling close() on each file descriptor. From looking at various large(ish) userspace code bases this or similar patterns are very common in: - service managers (cf. [4]) - libcs (cf. [6]) - container runtimes (cf. [5]) - programming language runtimes/standard libraries - Python (cf. [2]) - Rust (cf. [7], [8]) As Dmitry pointed out there's even a long-standing glibc bug about missing kernel support for this task (cf. [3]). In addition, the syscall will also work for tasks that do not have procfs mounted and on kernels that do not have procfs support compiled in. In such situations the only way to make sure that all file descriptors are closed is to call close() on each file descriptor up to UINT_MAX or RLIMIT_NOFILE, OPEN_MAX trickery (cf. comment [8] on Rust). The performance is striking. For good measure, comparing the following simple close_all_fds() userspace implementation that is essentially just glibc's version in [6]: static int close_all_fds(void) { DIR *dir; struct dirent *direntp; dir = opendir("/proc/self/fd"); if (!dir) return -1; while ((direntp = readdir(dir))) { int fd; if (strcmp(direntp->d_name, ".") == 0) continue; if (strcmp(direntp->d_name, "..") == 0) continue; fd = atoi(direntp->d_name); if (fd == 0 || fd == 1 || fd == 2) continue; close(fd); } closedir(dir); /* cannot fail */ return 0; } to close_range() yields: 1. closing 4 open files: - close_all_fds(): ~280 us - close_range(): ~24 us 2. closing 1000 open files: - close_all_fds(): ~5000 us - close_range(): ~800 us close_range() is designed to allow for some flexibility. Specifically, it does not simply always close all open file descriptors of a task. Instead, callers can specify an upper bound. This is e.g. useful for scenarios where specific file descriptors are created with well-known numbers that are supposed to be excluded from getting closed. For extra paranoia close_range() comes with a flags argument. This can e.g. be used to implement extension. Once can imagine userspace wanting to stop at the first error instead of ignoring errors under certain circumstances. There might be other valid ideas in the future. In any case, a flag argument doesn't hurt and keeps us on the safe side. From an implementation side this is kept rather dumb. It saw some input from David and Jann but all nonsense is obviously my own! - Errors to close file descriptors are currently ignored. (Could be changed by setting a flag in the future if needed.) - __close_range() is a rather simplistic wrapper around __close_fd(). My reasoning behind this is based on the nature of how __close_fd() needs to release an fd. But maybe I misunderstood specifics: We take the files_lock and rcu-dereference the fdtable of the calling task, we find the entry in the fdtable, get the file and need to release files_lock before calling filp_close(). In the meantime the fdtable might have been altered so we can't just retake the spinlock and keep the old rcu-reference of the fdtable around. Instead we need to grab a fresh reference to the fdtable. If my reasoning is correct then there's really no point in fancyfying __close_range(): We just need to rcu-dereference the fdtable of the calling task once to cap the max_fd value correctly and then go on calling __close_fd() in a loop. /* References */ [1]: https://lore.kernel.org/lkml/20190516165021.GD17978@ZenIV.linux.org.uk/ [2]: https://github.com/python/cpython/blob/9e4f2f3a6b8ee995c365e86d976937c141d867f8/Modules/_posixsubprocess.c#L220 [3]: https://sourceware.org/bugzilla/show_bug.cgi?id=10353#c7 [4]: https://github.com/systemd/systemd/blob/5238e9575906297608ff802a27e2ff9effa3b338/src/basic/fd-util.c#L217 [5]: https://github.com/lxc/lxc/blob/ddf4b77e11a4d08f09b7b9cd13e593f8c047edc5/src/lxc/start.c#L236 [6]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/grantpt.c;h=2030e07fa6e652aac32c775b8c6e005844c3c4eb;hb=HEAD#l17 Note that this is an internal implementation that is not exported. Currently, libc seems to not provide an exported version of this because of missing kernel support to do this. [7]: https://github.com/rust-lang/rust/issues/12148 [8]: https://github.com/rust-lang/rust/blob/5f47c0613ed4eb46fca3633c1297364c09e5e451/src/libstd/sys/unix/process2.rs#L303-L308 Rust's solution is slightly different but is equally unperformant. Rust calls getdtablesize() which is a glibc library function that simply returns the current RLIMIT_NOFILE or OPEN_MAX values. Rust then goes on to call close() on each fd. That's obviously overkill for most tasks. Rarely, tasks - especially non-demons - hit RLIMIT_NOFILE or OPEN_MAX. Let's be nice and assume an unprivileged user with RLIMIT_NOFILE set to 1024. Even in this case, there's a very high chance that in the common case Rust is calling the close() syscall 1021 times pointlessly if the task just has 0, 1, and 2 open. Suggested-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Christian Brauner <christian@brauner.io> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Jann Horn <jannh@google.com> Cc: David Howells <dhowells@redhat.com> Cc: Dmitry V. Levin <ldv@altlinux.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Florian Weimer <fweimer@redhat.com> Cc: linux-api@vger.kernel.org --- arch/alpha/kernel/syscalls/syscall.tbl | 1 + arch/arm/tools/syscall.tbl | 1 + arch/arm64/include/asm/unistd32.h | 2 ++ arch/ia64/kernel/syscalls/syscall.tbl | 1 + arch/m68k/kernel/syscalls/syscall.tbl | 1 + arch/microblaze/kernel/syscalls/syscall.tbl | 1 + arch/mips/kernel/syscalls/syscall_n32.tbl | 1 + arch/mips/kernel/syscalls/syscall_n64.tbl | 1 + arch/mips/kernel/syscalls/syscall_o32.tbl | 1 + arch/parisc/kernel/syscalls/syscall.tbl | 1 + arch/powerpc/kernel/syscalls/syscall.tbl | 1 + arch/s390/kernel/syscalls/syscall.tbl | 1 + arch/sh/kernel/syscalls/syscall.tbl | 1 + arch/sparc/kernel/syscalls/syscall.tbl | 1 + arch/x86/entry/syscalls/syscall_32.tbl | 1 + arch/x86/entry/syscalls/syscall_64.tbl | 1 + arch/xtensa/kernel/syscalls/syscall.tbl | 1 + fs/file.c | 30 +++++++++++++++++++++ fs/open.c | 20 ++++++++++++++ include/linux/fdtable.h | 2 ++ include/linux/syscalls.h | 2 ++ include/uapi/asm-generic/unistd.h | 4 ++- 22 files changed, 75 insertions(+), 1 deletion(-)