linux-kernel - Re: [PATCH] slab: Fix too strict alignment check in create

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Message-ID: <4f70f8d3-4ba5-43dc-af1c-f8e207d27e9f@suse.cz>
Date: Wed, 20 Nov 2024 16:03:04 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Jens Axboe <axboe@...nel.dk>,
 Pavel Begunkov <asml.silence@...il.com>, Mike Rapoport <rppt@...nel.org>,
 Christian Brauner <brauner@...nel.org>, Guenter Roeck <linux@...ck-us.net>,
 Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>
Cc: linux-mm@...ck.org, io-uring@...r.kernel.org, linux-m68k@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slab: Fix too strict alignment check in create_cache()

On 11/20/24 13:49, Geert Uytterhoeven wrote:
> On m68k, where the minimum alignment of unsigned long is 2 bytes:
> 
>     Kernel panic - not syncing: __kmem_cache_create_args: Failed to create slab 'io_kiocb'. Error -22
>     CPU: 0 UID: 0 PID: 1 Comm: swapper Not tainted 6.12.0-atari-03776-g7eaa1f99261a #1783
>     Stack from 0102fe5c:
> 	    0102fe5c 00514a2b 00514a2b ffffff00 00000001 0051f5ed 00425e78 00514a2b
> 	    0041eb74 ffffffea 00000310 0051f5ed ffffffea ffffffea 00601f60 00000044
> 	    0102ff20 000e7a68 0051ab8e 004383b8 0051f5ed ffffffea 000000b8 00000007
> 	    01020c00 00000000 000e77f0 0041e5f0 005f67c0 0051f5ed 000000b6 0102fef4
> 	    00000310 0102fef4 00000000 00000016 005f676c 0060a34c 00000010 00000004
> 	    00000038 0000009a 01000000 000000b8 005f668e 0102e000 00001372 0102ff88
>     Call Trace: [<00425e78>] dump_stack+0xc/0x10
>      [<0041eb74>] panic+0xd8/0x26c
>      [<000e7a68>] __kmem_cache_create_args+0x278/0x2e8
>      [<000e77f0>] __kmem_cache_create_args+0x0/0x2e8
>      [<0041e5f0>] memset+0x0/0x8c
>      [<005f67c0>] io_uring_init+0x54/0xd2
> 
> The minimal alignment of an integral type may differ from its size,
> hence is not safe to assume that an arbitrary freeptr_t (which is
> basically an unsigned long) is always aligned to 4 or 8 bytes.
> 
> As nothing seems to require the additional alignment, it is safe to fix
> this by relaxing the check to the actual minimum alignment of freeptr_t.
> 
> Fixes: aaa736b186239b7d ("io_uring: specify freeptr usage for SLAB_TYPESAFE_BY_RCU io_kiocb cache")
> Fixes: d345bd2e9834e2da ("mm: add kmem_cache_create_rcu()")
> Reported-by: Guenter Roeck <linux@...ck-us.net>
> Closes: https://lore.kernel.org/37c588d4-2c32-4aad-a19e-642961f200d7@roeck-us.net
> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>

Thanks, will add it to slab pull for 6.13.

> ---
>  mm/slab_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 893d320599151845..f2f201d865c108bd 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -230,7 +230,7 @@ static struct kmem_cache *create_cache(const char *name,
>  	if (args->use_freeptr_offset &&
>  	    (args->freeptr_offset >= object_size ||
>  	     !(flags & SLAB_TYPESAFE_BY_RCU) ||
> -	     !IS_ALIGNED(args->freeptr_offset, sizeof(freeptr_t))))
> +	     !IS_ALIGNED(args->freeptr_offset, __alignof(freeptr_t))))

Seems only bunch of places uses __alignof but many use __alignoff__ and this
also is what seems to be documented?

>  		goto out;
>  
>  	err = -ENOMEM;