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futex: FLAGS_STRICT

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The current semantics for futex_wake() are a bit loose, specifically
asking for 0 futexes to be woken actually gets you 1.

Adding a !nr check to sys_futex_wake() makes that it would return 0
for unaligned futex words, because that check comes in the shared
futex_wake() function. Adding the !nr check there, would affect the
legacy sys_futex() semantics.

Hence frob a flag :-(

Suggested-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230921105248.048643656@noisy.programming.kicks-ass.net
This commit is contained in:
peterz@infradead.org 2023-09-21 12:45:11 +02:00 committed by Peter Zijlstra
parent 9f6c532f59
commit 43adf84495
3 changed files with 15 additions and 11 deletions
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21
kernel/futex/futex.h
View File

@ -17,25 +17,26 @@
* 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_8 0x0000
#define FLAGS_SIZE_16 0x0001
#define FLAGS_SIZE_32 0x0002
#define FLAGS_SIZE_64 0x0003
#define FLAGS_SIZE_MASK 0x03
#define FLAGS_SIZE_MASK 0x0003
#ifdef CONFIG_MMU
# define FLAGS_SHARED 0x10
# define FLAGS_SHARED 0x0010
#else
/*
* NOMMU does not have per process address space. Let the compiler optimize
* code away.
*/
# define FLAGS_SHARED 0x00
# define FLAGS_SHARED 0x0000
#endif
#define FLAGS_CLOCKRT 0x20
#define FLAGS_HAS_TIMEOUT 0x40
#define FLAGS_NUMA 0x80
#define FLAGS_CLOCKRT 0x0020
#define FLAGS_HAS_TIMEOUT 0x0040
#define FLAGS_NUMA 0x0080
#define FLAGS_STRICT 0x0100
/* FUTEX_ to FLAGS_ */
static inline unsigned int futex_to_flags(unsigned int op)

2
kernel/futex/syscalls.c
View File

@ -333,7 +333,7 @@ SYSCALL_DEFINE4(futex_wake,
if (!futex_validate_input(flags, mask))
return -EINVAL;
return futex_wake(uaddr, flags, nr, mask);
return futex_wake(uaddr, FLAGS_STRICT | flags, nr, mask);
}
#ifdef CONFIG_COMPAT

3
kernel/futex/waitwake.c
View File

@ -155,6 +155,9 @@ int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
if (unlikely(ret != 0))
return ret;
if ((flags & FLAGS_STRICT) && !nr_wake)
return 0;
hb = futex_hash(&key);
/* Make sure we really have tasks to wakeup */