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 <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.718926819@kernel.org

2 files changed, 46 insertions, 3 deletions

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 9e6218c2be66..cb2a182547dd 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 d41557d2790d..d320c0571f0c 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 { };