+ fork-honor-task_structs-declared-alignment.patch added to mm-nonmm-unstable branch

+ fork-honor-task_structs-declared-alignment.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: fork: honor task_struct's declared alignment
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     fork-honor-task_structs-declared-alignment.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fork-honor-task_structs-declared-alignment.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Karl Mehltretter <kmehltretter@gmail.com>
Subject: fork: honor task_struct's declared alignment
Date: Fri, 10 Jul 2026 14:39:57 +0200

Since commit cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant
size"), struct task_struct is declared __attribute__((aligned(64))) on all
architectures.

But fork_init() still sets the task_struct slab cache's alignment to

	align = max(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN)

which is smaller than 64 on architectures whose cache lines are below 64
bytes: e.g.  32 on ARMv5.

In practice plain SLUB happens to hand out 64-byte-aligned objects anyway.
With CONFIG_SLUB_DEBUG_ON the red-zone padding shifts objects to the
requested alignment.

With CONFIG_UBSAN_ALIGNMENT=y a boot on QEMU versatilepb (ARM926EJ-S,
v7.2-rc2, gcc 13.3) floods the console with reports like:

  UBSAN: misaligned-access in include/linux/sched.h:2087:9
  member access within misaligned address c295d7e0 for type 'struct task_struct'
  which requires 64 byte alignment
  CPU: 0 UID: 0 PID: 15 Comm: pr/ttyAMA-1 Not tainted 7.2.0-rc2 #1 VOLUNTARY

Set the slab alignment to at least the type's declared alignment.

Link: https://lore.kernel.org/20260710123957.31774-1-kmehltretter@gmail.com
Fixes: cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant size")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Bradley Morgan <include@grrlz.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Karl Mehltretter <kmehltretter@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Assisted-by: Claude:claude-fable-5
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/fork.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/kernel/fork.c~fork-honor-task_structs-declared-alignment
+++ a/kernel/fork.c
@@ -858,7 +858,8 @@ void __init fork_init(void)
 #ifndef ARCH_MIN_TASKALIGN
 #define ARCH_MIN_TASKALIGN	0
 #endif
-	int align = max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN);
+	int align = max3(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN,
+			 __alignof__(struct task_struct));
 	unsigned long useroffset, usersize;
 
 	/* create a slab on which task_structs can be allocated */
_

Patches currently in -mm which might be from kmehltretter@gmail.com are

klist-avoid-accesses-after-waking-klist_remove.patch
squashfs-fix-fragment-index-table-sizing-overflow-on-32-bit.patch
squashfs-make-the-fragment-index-table-bounds-check-overflow-safe.patch
fork-honor-task_structs-declared-alignment.patch