[PATCH] slub: print just the function name in /sys/kernel/slab/*/ctor

[PATCH] slub: print just the function name in /sys/kernel/slab/*/ctor

[Alexey Dobriyan]

Sysfs "ctor" file prints offset/length of the cache's ctor function

	$ sudo cat /sys/kernel/slab/bdev_cache/ctor
	init_once+0x0/0x10

This is not useful:

Offset will always be 0 because ctor is a function.

I'm not sure what ctor function size is doing here, it should be in
/proc/kallsyms

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 mm/slub.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -9149,7 +9149,7 @@ static ssize_t ctor_show(struct kmem_cache *s, char *buf)
 {
 	if (!s->ctor)
 		return 0;
-	return sysfs_emit(buf, "%pS\n", s->ctor);
+	return sysfs_emit(buf, "%ps\n", s->ctor);
 }
 SLAB_ATTR_RO(ctor);
 

Re: [PATCH] slub: print just the function name in /sys/kernel/slab/*/ctor

[Harry Yoo]

[-- Attachment #1.1: Type: text/plain, Size: 984 bytes --]



On 7/25/26 10:28 PM, Alexey Dobriyan wrote:
> Sysfs "ctor" file prints offset/length of the cache's ctor function
> 
> 	$ sudo cat /sys/kernel/slab/bdev_cache/ctor
> 	init_once+0x0/0x10
> 
> This is not useful:
> 
> Offset will always be 0 because ctor is a function.
> 
> I'm not sure what ctor function size is doing here, it should be in
> /proc/kallsyms
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---

I'm not convinced that changing this (without strong justification)
after exposing it to sysfs for 10+ years is worth the trouble.

>  mm/slub.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -9149,7 +9149,7 @@ static ssize_t ctor_show(struct kmem_cache *s, char *buf)
>  {
>  	if (!s->ctor)
>  		return 0;
> -	return sysfs_emit(buf, "%pS\n", s->ctor);
> +	return sysfs_emit(buf, "%ps\n", s->ctor);
>  }
>  SLAB_ATTR_RO(ctor);
>  

-- 
Cheers,
Harry / Hyeonggon


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

Re: [PATCH] slub: print just the function name in /sys/kernel/slab/*/ctor

[Christoph Lameter (Ampere)]

Reviewed-by: Christoph Lameter (Ampere) <cl@gentwo.org>

Re: [PATCH] slub: print just the function name in /sys/kernel/slab/*/ctor

[Vlastimil Babka (SUSE)]

On 7/27/26 07:27, Harry Yoo wrote:
> 
> 
> On 7/25/26 10:28 PM, Alexey Dobriyan wrote:
>> Sysfs "ctor" file prints offset/length of the cache's ctor function
>> 
>> 	$ sudo cat /sys/kernel/slab/bdev_cache/ctor
>> 	init_once+0x0/0x10
>> 
>> This is not useful:
>> 
>> Offset will always be 0 because ctor is a function.
>> 
>> I'm not sure what ctor function size is doing here, it should be in
>> /proc/kallsyms
>> 
>> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
>> ---
> 
> I'm not convinced that changing this (without strong justification)
> after exposing it to sysfs for 10+ years is worth the trouble.

Agreed. It's a pity this was exported in the first place. I can't see a
benefit for anyone knowing what the function is called. Should have been at
most a flag whether there's ctor or not.
But possibly a justification is not to leak the function size, which might
be theoretically (although unlikely) a hint to some attack.
Well at least if somebody complains about getting broken, it's trivial to
revert and we can hear about their usecase.
In-tree tools (slabinfo etc) were checked to work properly?

>>  mm/slub.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -9149,7 +9149,7 @@ static ssize_t ctor_show(struct kmem_cache *s, char *buf)
>>  {
>>  	if (!s->ctor)
>>  		return 0;
>> -	return sysfs_emit(buf, "%pS\n", s->ctor);
>> +	return sysfs_emit(buf, "%ps\n", s->ctor);
>>  }
>>  SLAB_ATTR_RO(ctor);
>>  
> 

Re: [PATCH] slub: print just the function name in /sys/kernel/slab/*/ctor

[Harry Yoo]

On Thu, Jul 30, 2026 at 04:15:26PM +0200, Vlastimil Babka (SUSE) wrote:
> On 7/27/26 07:27, Harry Yoo wrote:
> > 
> > 
> > On 7/25/26 10:28 PM, Alexey Dobriyan wrote:
> >> Sysfs "ctor" file prints offset/length of the cache's ctor function
> >> 
> >> 	$ sudo cat /sys/kernel/slab/bdev_cache/ctor
> >> 	init_once+0x0/0x10
> >> 
> >> This is not useful:
> >> 
> >> Offset will always be 0 because ctor is a function.
> >> 
> >> I'm not sure what ctor function size is doing here, it should be in
> >> /proc/kallsyms
> >> 
> >> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> >> ---
> > 
> > I'm not convinced that changing this (without strong justification)
> > after exposing it to sysfs for 10+ years is worth the trouble.
> 
> Agreed. It's a pity this was exported in the first place. I can't see a
> benefit for anyone knowing what the function is called. Should have been at
> most a flag whether there's ctor or not.

Agreed.

> But possibly a justification is not to leak the function size, which might
> be theoretically (although unlikely) a hint to some attack.

...but you need to be the owner (root) to read this :P 

> Well at least if somebody complains about getting broken, it's trivial to
> revert and we can hear about their usecase.

If we were to experiment with this, we'll change it to print a flag
rather than the ctor's name, right?

> In-tree tools (slabinfo etc) were checked to work properly?

Looks like it doesn't read ctor at all.

Commit a87615b8f9e2 ("SLUB: slabinfo upgrade") added -o/--ops option
that should "Display of ctor / dtor etc.", but it reads "ops" attribute
that has never existed? Bit puzzled.

> >>  mm/slub.c |    2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> --- a/mm/slub.c
> >> +++ b/mm/slub.c
> >> @@ -9149,7 +9149,7 @@ static ssize_t ctor_show(struct kmem_cache *s, char *buf)
> >>  {
> >>  	if (!s->ctor)
> >>  		return 0;
> >> -	return sysfs_emit(buf, "%pS\n", s->ctor);
> >> +	return sysfs_emit(buf, "%ps\n", s->ctor);
> >>  }
> >>  SLAB_ATTR_RO(ctor);

-- 
Cheers,
Harry / Hyeonggon