Re: [PATCH v2 05/14] futex: Add sys_futex_wake()

On Wed, Aug 09, 2023 at 07:25:19PM -0300, André Almeida wrote:

Hi Peter,

Em 07/08/2023 09:18, Peter Zijlstra escreveu:

quoted

To complement sys_futex_waitv() add sys_futex_wake(). This syscall
implements what was previously known as FUTEX_WAKE_BITSET except it
uses 'unsigned long' for the bitmask and takes FUTEX2 flags.

The 'unsigned long' allows FUTEX2_SIZE_U64 on 64bit platforms.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
---

[...]

quoted

+/*
+ * sys_futex_wake - Wake a number of futexes
+ * @uaddr:	Address of the futex(es) to wake
+ * @mask:	bitmask
+ * @nr:		Number of the futexes to wake
+ * @flags:	FUTEX2 flags
+ *
+ * Identical to the traditional FUTEX_WAKE_BITSET op, except it is part of the
+ * futex2 family of calls.
+ */
+
+SYSCALL_DEFINE4(futex_wake,
+		void __user *, uaddr,
+		unsigned long, mask,
+		int, nr,
+		unsigned int, flags)
+{

Do you think we could have a

	if (!nr)
		return 0;

here? Otherwise, calling futex_wake(&f, 0, flags) will wake 1 futex (if
available), which is a strange undocumented behavior in my opinion.

Oh 'cute' that.. yeah, but how about I put it ...

quoted

+	if (flags & ~FUTEX2_VALID_MASK)
+		return -EINVAL;
+
+	flags = futex2_to_flags(flags);
+	if (!futex_flags_valid(flags))
+		return -EINVAL;
+
+	if (!futex_validate_input(flags, mask))
+		return -EINVAL;

here, because otherwise we get:

	sys_futex_wake(&f, 0xFFFF, 0, FUTEX2_SIZE_U8)

to return 0, even though that is 'obviously' nonsensical and should
return -EINVAL. Or even garbage flags would be 'accepted'.

(because 0xFFFF is larger than U8 can accomodate)

quoted

+
+	return futex_wake(uaddr, flags, nr, mask);
+}