From: Thomas Gleixner Date: Tue, 2 Jun 2026 09:09:59 +0000 (+0200) Subject: futex: Add robust futex unlock IP range X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=042df0c1d48609a85580dcbaff498c95ced20a5f;p=thirdparty%2Flinux.git futex: Add robust futex unlock IP range There will be a VDSO function to unlock robust futexes in user space. The unlock sequence is racy vs. clearing the list_pending_op pointer in the tasks robust list head. To plug this race the kernel needs to know the instruction window. As the VDSO is per MM the addresses are stored in mm_struct::futex. Architectures which implement support for this have to update these addresses when the VDSO is (re)mapped and indicate the pending op pointer size which is matching the IP. Arguably this could be resolved by chasing mm->context->vdso->image, but that's architecture specific and requires to touch quite some cache lines. Having it in mm::futex reduces the cache line impact and avoids having yet another set of architecture specific functionality. To support multi size robust list applications (gaming) this provides two ranges when COMPAT is enabled. Signed-off-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: André Almeida Link: https://patch.msgid.link/20260602090535.718926819@kernel.org --- diff --git a/include/linux/futex.h b/include/linux/futex.h index 9e6218c2be664..cb2a182547dd9 100644 --- a/include/linux/futex.h +++ b/include/linux/futex.h @@ -81,11 +81,9 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4) #ifdef CONFIG_FUTEX_PRIVATE_HASH int futex_hash_allocate_default(void); void futex_hash_free(struct mm_struct *mm); -void futex_mm_init(struct mm_struct *mm); #else /* CONFIG_FUTEX_PRIVATE_HASH */ static inline int futex_hash_allocate_default(void) { return 0; } static inline int futex_hash_free(struct mm_struct *mm) { return 0; } -static inline void futex_mm_init(struct mm_struct *mm) { } #endif /* !CONFIG_FUTEX_PRIVATE_HASH */ #else /* CONFIG_FUTEX */ @@ -104,7 +102,24 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsig } static inline int futex_hash_allocate_default(void) { return 0; } static inline int futex_hash_free(struct mm_struct *mm) { return 0; } -static inline void futex_mm_init(struct mm_struct *mm) { } #endif /* !CONFIG_FUTEX */ +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +void futex_reset_cs_ranges(struct futex_mm_data *fd); + +static inline void futex_set_vdso_cs_range(struct futex_mm_data *fd, unsigned int idx, + unsigned long start, unsigned long end, bool sz32) +{ + fd->unlock.cs_ranges[idx].start_ip = start; + fd->unlock.cs_ranges[idx].len = end - start; + fd->unlock.cs_ranges[idx].pop_size32 = sz32; +} +#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */ + +#if defined(CONFIG_FUTEX_PRIVATE_HASH) || defined(CONFIG_FUTEX_ROBUST_UNLOCK) +void futex_mm_init(struct mm_struct *mm); +#else +static inline void futex_mm_init(struct mm_struct *mm) { } +#endif + #endif /* _LINUX_FUTEX_H */ diff --git a/include/linux/futex_types.h b/include/linux/futex_types.h index d41557d2790d5..d320c0571f0c8 100644 --- a/include/linux/futex_types.h +++ b/include/linux/futex_types.h @@ -55,12 +55,40 @@ struct futex_mm_phash { struct futex_mm_phash { }; #endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +/** + * struct futex_unlock_cs_range - Range for the VDSO unlock critical section + * @start_ip: The start IP of the robust futex unlock critical section (inclusive) + * @len: The length of the robust futex unlock critical section + * @pop_size32: Pending OP pointer size indicator. 0 == 64-bit, 1 == 32-bit + */ +struct futex_unlock_cs_range { + unsigned long start_ip; + unsigned int len; + unsigned int pop_size32; +}; + +#define FUTEX_ROBUST_MAX_CS_RANGES (1 + IS_ENABLED(CONFIG_COMPAT)) + +/** + * struct futex_unlock_cs_ranges - Futex unlock VSDO critical sections + * @cs_ranges: Array of critical section ranges + */ +struct futex_unlock_cs_ranges { + struct futex_unlock_cs_range cs_ranges[FUTEX_ROBUST_MAX_CS_RANGES]; +}; +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +struct futex_unlock_cs_ranges { }; +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ + /** * struct futex_mm_data - Futex related per MM data * @phash: Futex private hash related data + * @unlock: Futex unlock VDSO critical sections */ struct futex_mm_data { struct futex_mm_phash phash; + struct futex_unlock_cs_ranges unlock; }; #else /* CONFIG_FUTEX */ struct futex_sched_data { }; diff --git a/init/Kconfig b/init/Kconfig index 2937c4d308aec..165b08e336a85 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1842,6 +1842,12 @@ config FUTEX_MPOL depends on FUTEX && NUMA default y +config HAVE_FUTEX_ROBUST_UNLOCK + bool + +config FUTEX_ROBUST_UNLOCK + def_bool FUTEX && HAVE_GENERIC_VDSO && GENERIC_IRQ_ENTRY && RSEQ && HAVE_FUTEX_ROBUST_UNLOCK + config EPOLL bool "Enable eventpoll support" if EXPERT default y diff --git a/kernel/futex/core.c b/kernel/futex/core.c index 77ccb7787c349..aad6e501fabd3 100644 --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -1761,11 +1761,11 @@ static bool futex_ref_is_dead(struct futex_private_hash *fph) return atomic_long_read(&mm->futex.phash.atomic) == 0; } -void futex_mm_init(struct mm_struct *mm) +static void futex_hash_init_mm(struct futex_mm_data *fd) { - memset(&mm->futex, 0, sizeof(mm->futex)); - mutex_init(&mm->futex.phash.lock); - mm->futex.phash.batches = get_state_synchronize_rcu(); + memset(&fd->phash, 0, sizeof(fd->phash)); + mutex_init(&fd->phash.lock); + fd->phash.batches = get_state_synchronize_rcu(); } void futex_hash_free(struct mm_struct *mm) @@ -1969,19 +1969,47 @@ static int futex_hash_get_slots(void) return fph->hash_mask + 1; return 0; } +#else /* CONFIG_FUTEX_PRIVATE_HASH */ +static inline int futex_hash_allocate(unsigned int hslots, unsigned int flags) { return -EINVAL; } +static inline int futex_hash_get_slots(void) { return 0; } +static inline void futex_hash_init_mm(struct futex_mm_data *fd) { } +#endif /* !CONFIG_FUTEX_PRIVATE_HASH */ -#else +#ifdef CONFIG_FUTEX_ROBUST_UNLOCK +static void futex_invalidate_cs_ranges(struct futex_mm_data *fd) +{ + /* + * Invalidate start_ip so that the quick check fails for ip >= start_ip + * if VDSO is not mapped or the second slot is not available for compat + * tasks as they use VDSO32 which does not provide the 64-bit pointer + * variant. + */ + for (int i = 0; i < FUTEX_ROBUST_MAX_CS_RANGES; i++) + fd->unlock.cs_ranges[i].start_ip = ~0UL; +} -static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags) +void futex_reset_cs_ranges(struct futex_mm_data *fd) { - return -EINVAL; + memset(fd->unlock.cs_ranges, 0, sizeof(fd->unlock.cs_ranges)); + futex_invalidate_cs_ranges(fd); } -static int futex_hash_get_slots(void) +static void futex_robust_unlock_init_mm(struct futex_mm_data *fd) { - return 0; + /* mm_dup() preserves the range, mm_alloc() clears it */ + if (!fd->unlock.cs_ranges[0].start_ip) + futex_invalidate_cs_ranges(fd); } +#else /* CONFIG_FUTEX_ROBUST_UNLOCK */ +static inline void futex_robust_unlock_init_mm(struct futex_mm_data *fd) { } +#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */ +#if defined(CONFIG_FUTEX_PRIVATE_HASH) || defined(CONFIG_FUTEX_ROBUST_UNLOCK) +void futex_mm_init(struct mm_struct *mm) +{ + futex_hash_init_mm(&mm->futex); + futex_robust_unlock_init_mm(&mm->futex); +} #endif int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)