[03/12] futex: Flag conversion

Commit Message

Jens Axboe July 28, 2023, 4:42 p.m. UTC

From: Peter Zijlstra <[e-mail]>

Futex has 3 sets of flags:

 - legacy futex op bits
 - futex2 flags
 - internal flags

Add a few helpers to convert from the API flags into the internal
flags.

Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>
---
 kernel/futex/futex.h    | 64 +++++++++++++++++++++++++++++++++++++++--
 kernel/futex/syscalls.c | 24 ++++++----------
 kernel/futex/waitwake.c |  4 +--
 3 files changed, 72 insertions(+), 20 deletions(-)

Comments

Gabriel Krisman Bertazi Aug. 9, 2023, 6:04 p.m. UTC | #1

Jens Axboe <[e-mail]> writes:

> From: Peter Zijlstra <[e-mail]>
>
> Futex has 3 sets of flags:
>
>  - legacy futex op bits
>  - futex2 flags
>  - internal flags
>
> Add a few helpers to convert from the API flags into the internal
> flags.
>
> Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
> Signed-off-by: Jens Axboe <[e-mail]>
> ---
>  kernel/futex/futex.h    | 64 +++++++++++++++++++++++++++++++++++++++--
>  kernel/futex/syscalls.c | 24 ++++++----------
>  kernel/futex/waitwake.c |  4 +--
>  3 files changed, 72 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
> index b5379c0e6d6d..c0e04599904a 100644
> --- a/kernel/futex/futex.h
> +++ b/kernel/futex/futex.h
> @@ -5,6 +5,7 @@
>  #include <linux/futex.h>
>  #include <linux/rtmutex.h>
>  #include <linux/sched/wake_q.h>
> +#include <linux/compat.h>
>  
>  #ifdef CONFIG_PREEMPT_RT
>  #include <linux/rcuwait.h>
> @@ -16,8 +17,15 @@
>   * Futex flags used to encode options to functions and preserve them across
>   * restarts.
>   */
> +#define FLAGS_SIZE_8		0x00
> +#define FLAGS_SIZE_16		0x01
> +#define FLAGS_SIZE_32		0x02
> +#define FLAGS_SIZE_64		0x03
> +
> +#define FLAGS_SIZE_MASK		0x03
> +
>  #ifdef CONFIG_MMU
> -# define FLAGS_SHARED		0x01
> +# define FLAGS_SHARED		0x10
>  #else
>  /*
>   * NOMMU does not have per process address space. Let the compiler optimize
> @@ -25,8 +33,58 @@
>   */
>  # define FLAGS_SHARED		0x00
>  #endif
> -#define FLAGS_CLOCKRT		0x02
> -#define FLAGS_HAS_TIMEOUT	0x04
> +#define FLAGS_CLOCKRT		0x20
> +#define FLAGS_HAS_TIMEOUT	0x40
> +#define FLAGS_NUMA		0x80
> +
> +/* FUTEX_ to FLAGS_ */
> +static inline unsigned int futex_to_flags(unsigned int op)
> +{
> +	unsigned int flags = FLAGS_SIZE_32;
> +
> +	if (!(op & FUTEX_PRIVATE_FLAG))
> +		flags |= FLAGS_SHARED;
> +
> +	if (op & FUTEX_CLOCK_REALTIME)
> +		flags |= FLAGS_CLOCKRT;
> +
> +	return flags;
> +}
> +
> +/* FUTEX2_ to FLAGS_ */
> +static inline unsigned int futex2_to_flags(unsigned int flags2)
> +{
> +	unsigned int flags = flags2 & FUTEX2_64;

FUTEX2_64 -> FLAGS_SIZE_MASK

> +
> +	if (!(flags2 & FUTEX2_PRIVATE))
> +		flags |= FLAGS_SHARED;
> +
> +	if (flags2 & FUTEX2_NUMA)
> +		flags |= FLAGS_NUMA;
> +
> +	return flags;
> +}
> +
> +static inline bool futex_flags_valid(unsigned int flags)
> +{
> +	/* Only 64bit futexes for 64bit code */
> +	if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()) {
> +		if ((flags & FLAGS_SIZE_MASK) == FLAGS_SIZE_64)
> +			return false;
> +	}

I read the comment above as '64bit code can only have 64bit futexes',
which is obviously wrong and not what the code is checking.
Something like this would be better:

/* Reject 64bit futexes on !64bit code. */

Or Perhaps make it generic:

/* Don't allow futexes larger than the word size */
  if (futex_size(flags) > (__WORDSIZE/8) || in_compat_syscall())

Patch

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index b5379c0e6d6d..c0e04599904a 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -5,6 +5,7 @@ 
 #include <linux/futex.h>
 #include <linux/rtmutex.h>
 #include <linux/sched/wake_q.h>
+#include <linux/compat.h>
 
 #ifdef CONFIG_PREEMPT_RT
 #include <linux/rcuwait.h>
@@ -16,8 +17,15 @@ 
  * Futex flags used to encode options to functions and preserve them across
  * restarts.
  */
+#define FLAGS_SIZE_8		0x00
+#define FLAGS_SIZE_16		0x01
+#define FLAGS_SIZE_32		0x02
+#define FLAGS_SIZE_64		0x03
+
+#define FLAGS_SIZE_MASK		0x03
+
 #ifdef CONFIG_MMU
-# define FLAGS_SHARED		0x01
+# define FLAGS_SHARED		0x10
 #else
 /*
  * NOMMU does not have per process address space. Let the compiler optimize
@@ -25,8 +33,58 @@ 
  */
 # define FLAGS_SHARED		0x00
 #endif
-#define FLAGS_CLOCKRT		0x02
-#define FLAGS_HAS_TIMEOUT	0x04
+#define FLAGS_CLOCKRT		0x20
+#define FLAGS_HAS_TIMEOUT	0x40
+#define FLAGS_NUMA		0x80
+
+/* FUTEX_ to FLAGS_ */
+static inline unsigned int futex_to_flags(unsigned int op)
+{
+	unsigned int flags = FLAGS_SIZE_32;
+
+	if (!(op & FUTEX_PRIVATE_FLAG))
+		flags |= FLAGS_SHARED;
+
+	if (op & FUTEX_CLOCK_REALTIME)
+		flags |= FLAGS_CLOCKRT;
+
+	return flags;
+}
+
+/* FUTEX2_ to FLAGS_ */
+static inline unsigned int futex2_to_flags(unsigned int flags2)
+{
+	unsigned int flags = flags2 & FUTEX2_64;
+
+	if (!(flags2 & FUTEX2_PRIVATE))
+		flags |= FLAGS_SHARED;
+
+	if (flags2 & FUTEX2_NUMA)
+		flags |= FLAGS_NUMA;
+
+	return flags;
+}
+
+static inline bool futex_flags_valid(unsigned int flags)
+{
+	/* Only 64bit futexes for 64bit code */
+	if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()) {
+		if ((flags & FLAGS_SIZE_MASK) == FLAGS_SIZE_64)
+			return false;
+	}
+
+	/* Only 32bit futexes are implemented -- for now */
+	if ((flags & FLAGS_SIZE_MASK) != FLAGS_SIZE_32)
+		return false;
+
+	return true;
+}
+
+static inline unsigned int futex_size(unsigned int flags)
+{
+	unsigned int size = flags & FLAGS_SIZE_MASK;
+	return 1 << size; /* {0,1,2,3} -> {1,2,4,8} */
+}
 
 #ifdef CONFIG_FAIL_FUTEX
 extern bool should_fail_futex(bool fshared);
diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index cfc8001e88ea..36824b64219a 100644
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -1,6 +1,5 @@ 
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-#include <linux/compat.h>
 #include <linux/syscalls.h>
 #include <linux/time_namespace.h>
 
@@ -85,15 +84,12 @@  SYSCALL_DEFINE3(get_robust_list, int, pid,
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 		u32 __user *uaddr2, u32 val2, u32 val3)
 {
+	unsigned int flags = futex_to_flags(op);
 	int cmd = op & FUTEX_CMD_MASK;
-	unsigned int flags = 0;
 
-	if (!(op & FUTEX_PRIVATE_FLAG))
-		flags |= FLAGS_SHARED;
-
-	if (op & FUTEX_CLOCK_REALTIME) {
-		flags |= FLAGS_CLOCKRT;
-		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
+	if (flags & FLAGS_CLOCKRT) {
+		if (cmd != FUTEX_WAIT_BITSET &&
+		    cmd != FUTEX_WAIT_REQUEUE_PI &&
 		    cmd != FUTEX_LOCK_PI2)
 			return -ENOSYS;
 	}
@@ -201,21 +197,19 @@  static int futex_parse_waitv(struct futex_vector *futexv,
 	unsigned int i;
 
 	for (i = 0; i < nr_futexes; i++) {
+		unsigned int flags;
+
 		if (copy_from_user(&aux, &uwaitv[i], sizeof(aux)))
 			return -EFAULT;
 
 		if ((aux.flags & ~FUTEX2_MASK) || aux.__reserved)
 			return -EINVAL;
 
-		if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()) {
-			if ((aux.flags & FUTEX2_64) == FUTEX2_64)
-				return -EINVAL;
-		}
-
-		if ((aux.flags & FUTEX2_64) != FUTEX2_32)
+		flags = futex2_to_flags(aux.flags);
+		if (!futex_flags_valid(flags))
 			return -EINVAL;
 
-		futexv[i].w.flags = aux.flags;
+		futexv[i].w.flags = flags;
 		futexv[i].w.val = aux.val;
 		futexv[i].w.uaddr = aux.uaddr;
 		futexv[i].q = futex_q_init;
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index ba01b9408203..fa9757766103 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -419,11 +419,11 @@  static int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *wo
 	 */
 retry:
 	for (i = 0; i < count; i++) {
-		if ((vs[i].w.flags & FUTEX_PRIVATE_FLAG) && retry)
+		if (!(vs[i].w.flags & FLAGS_SHARED) && retry)
 			continue;
 
 		ret = get_futex_key(u64_to_user_ptr(vs[i].w.uaddr),
-				    !(vs[i].w.flags & FUTEX_PRIVATE_FLAG),
+				    vs[i].w.flags & FLAGS_SHARED,
 				    &vs[i].q.key, FUTEX_READ);
 
 		if (unlikely(ret))