linux-kernel - [PATCH 4/6] futex: Enable FUTEX2

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Message-Id: <20241025093944.707639534@infradead.org>
Date: Fri, 25 Oct 2024 11:03:51 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: tglx@...utronix.de
Cc: linux-kernel@...r.kernel.org,
 peterz@...radead.org,
 mingo@...hat.com,
 dvhart@...radead.org,
 dave@...olabs.net,
 andrealmeid@...lia.com,
 Andrew Morton <akpm@...ux-foundation.org>,
 urezki@...il.com,
 hch@...radead.org,
 lstoakes@...il.com,
 Arnd Bergmann <arnd@...db.de>,
 linux-api@...r.kernel.org,
 linux-mm@...ck.org,
 linux-arch@...r.kernel.org,
 malteskarupke@....de,
 cl@...ux.com,
 llong@...hat.com
Subject: [PATCH 4/6] futex: Enable FUTEX2_{8,16}

When futexes are no longer u32 aligned, the lower offset bits are no
longer available to put type info in. However, since offset is the
offset within a page, there are plenty bits available on the top end.

After that, pass flags into futex_get_value_locked() for WAIT and
disallow FUTEX2_SIZE_U64 instead of mandating FUTEX2_SIZE_U32.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
---
 include/linux/futex.h   |   11 ++++++-----
 kernel/futex/core.c     |    9 +++++++++
 kernel/futex/futex.h    |    4 ++--
 kernel/futex/waitwake.c |    5 +++--
 4 files changed, 20 insertions(+), 9 deletions(-)

--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -16,18 +16,19 @@ struct task_struct;
  * The key type depends on whether it's a shared or private mapping.
  * Don't rearrange members without looking at hash_futex().
  *
- * offset is aligned to a multiple of sizeof(u32) (== 4) by definition.
- * We use the two low order bits of offset to tell what is the kind of key :
+ * offset is the position within a page and is in the range [0, PAGE_SIZE).
+ * The high bits of the offset indicate what kind of key this is:
  *  00 : Private process futex (PTHREAD_PROCESS_PRIVATE)
  *       (no reference on an inode or mm)
  *  01 : Shared futex (PTHREAD_PROCESS_SHARED)
  *	mapped on a file (reference on the underlying inode)
  *  10 : Shared futex (PTHREAD_PROCESS_SHARED)
  *       (but private mapping on an mm, and reference taken on it)
-*/
+ */
 
-#define FUT_OFF_INODE    1 /* We set bit 0 if key has a reference on inode */
-#define FUT_OFF_MMSHARED 2 /* We set bit 1 if key has a reference on mm */
+#define FUT_OFF_INODE    (PAGE_SIZE << 0)
+#define FUT_OFF_MMSHARED (PAGE_SIZE << 1)
+#define FUT_OFF_SIZE	 (PAGE_SIZE << 2)
 
 union futex_key {
 	struct {
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -313,6 +313,15 @@ int get_futex_key(void __user *uaddr, un
 	}
 
 	/*
+	 * Encode the futex size in the offset. This makes cross-size
+	 * wake-wait fail -- see futex_match().
+	 *
+	 * NOTE that cross-size wake-wait is fundamentally broken wrt
+	 * FLAGS_NUMA.
+	 */
+	key->both.offset |= FUT_OFF_SIZE * (flags & FLAGS_SIZE_MASK);
+
+	/*
 	 * PROCESS_PRIVATE futexes are fast.
 	 * As the mm cannot disappear under us and the 'key' only needs
 	 * virtual address, we dont even have to find the underlying vma.
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -81,8 +81,8 @@ static inline bool futex_flags_valid(uns
 			return false;
 	}
 
-	/* Only 32bit futexes are implemented -- for now */
-	if ((flags & FLAGS_SIZE_MASK) != FLAGS_SIZE_32)
+	/* 64bit futexes aren't implemented -- yet */
+	if ((flags & FLAGS_SIZE_MASK) == FLAGS_SIZE_64)
 		return false;
 
 	/*
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -449,11 +449,12 @@ int futex_wait_multiple_setup(struct fut
 
 	for (i = 0; i < count; i++) {
 		u32 __user *uaddr = (u32 __user *)(unsigned long)vs[i].w.uaddr;
+		unsigned int flags = vs[i].w.flags;
 		struct futex_q *q = &vs[i].q;
 		u32 val = vs[i].w.val;
 
 		hb = futex_q_lock(q);
-		ret = futex_get_value_locked(&uval, uaddr, FLAGS_SIZE_32);
+		ret = futex_get_value_locked(&uval, uaddr, flags);
 
 		if (!ret && uval == val) {
 			/*
@@ -621,7 +622,7 @@ int futex_wait_setup(u32 __user *uaddr,
 retry_private:
 	*hb = futex_q_lock(q);
 
-	ret = futex_get_value_locked(&uval, uaddr, FLAGS_SIZE_32);
+	ret = futex_get_value_locked(&uval, uaddr, flags);
 
 	if (ret) {
 		futex_q_unlock(*hb);