futex: FLAGS_STRICT

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

kernel/futex/futex.h

@@ -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)

kernel/futex/syscalls.c

@@ -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

kernel/futex/waitwake.c

@@ -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 */