[PATCH] mm: slub: fix conversion of freelist_corrupted()

[PATCH] mm: slub: fix conversion of freelist_corrupted()

[Eugeniu Rosca]

Commit 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in
deactivate_slab()") suffered an update when picked up from LKML [1].

Specifically, relocating 'freelist = NULL' into 'freelist_corrupted()'
created a no-op statement. Fix it by sticking to the behavior intended
in the original patch [1]. Prefer the lowest-line-count solution.

[1] https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/

Fixes: 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in deactivate_slab()")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dongli Zhang <dongli.zhang@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
---
 mm/slub.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 68c02b2eecd9..9a3e963b02a3 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -677,7 +677,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
 	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
 	    !check_valid_pointer(s, page, nextfree)) {
 		object_err(s, page, freelist, "Freechain corrupt");
-		freelist = NULL;
 		slab_fix(s, "Isolate corrupted freechain");
 		return true;
 	}
@@ -2184,8 +2183,10 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
 		 * 'freelist' is already corrupted.  So isolate all objects
 		 * starting at 'freelist'.
 		 */
-		if (freelist_corrupted(s, page, freelist, nextfree))
+		if (freelist_corrupted(s, page, freelist, nextfree)) {
+			freelist = NULL;
 			break;
+		}
 
 		do {
 			prior = page->freelist;
-- 
2.28.0

Re: [PATCH] mm: slub: fix conversion of freelist_corrupted()

[Andrew Morton]

On Tue, 11 Aug 2020 14:46:56 +0200 Eugeniu Rosca <erosca@de.adit-jv.com> wrote:

> Commit 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in
> deactivate_slab()") suffered an update when picked up from LKML [1].
> 
> Specifically, relocating 'freelist = NULL' into 'freelist_corrupted()'
> created a no-op statement. Fix it by sticking to the behavior intended
> in the original patch [1]. Prefer the lowest-line-count solution.
> 
> [1] https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/
> 
> ...
>
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -677,7 +677,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
>  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
>  	    !check_valid_pointer(s, page, nextfree)) {
>  		object_err(s, page, freelist, "Freechain corrupt");
> -		freelist = NULL;
>  		slab_fix(s, "Isolate corrupted freechain");
>  		return true;
>  	}
> @@ -2184,8 +2183,10 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>  		 * 'freelist' is already corrupted.  So isolate all objects
>  		 * starting at 'freelist'.
>  		 */
> -		if (freelist_corrupted(s, page, freelist, nextfree))
> +		if (freelist_corrupted(s, page, freelist, nextfree)) {
> +			freelist = NULL;
>  			break;
> +		}
>  
>  		do {
>  			prior = page->freelist;

Looks right.

What are the runtime effects of this change?  In other words, what are
the end user visible effects of the present defect?

Re: [PATCH] mm: slub: fix conversion of freelist_corrupted()

[Eugeniu Rosca]

Dear Andrew,

On Tue, Aug 11, 2020 at 01:49:09PM -0700, Andrew Morton wrote:
> On Tue, 11 Aug 2020 14:46:56 +0200 Eugeniu Rosca <erosca@de.adit-jv.com> wrote:
> 
> > Commit 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in
> > deactivate_slab()") suffered an update when picked up from LKML [1].
> > 
> > Specifically, relocating 'freelist = NULL' into 'freelist_corrupted()'
> > created a no-op statement. Fix it by sticking to the behavior intended
> > in the original patch [1]. Prefer the lowest-line-count solution.
> > 
> > [1] https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/
> > 
> > ...
> >
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -677,7 +677,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
> >  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
> >  	    !check_valid_pointer(s, page, nextfree)) {
> >  		object_err(s, page, freelist, "Freechain corrupt");
> > -		freelist = NULL;
> >  		slab_fix(s, "Isolate corrupted freechain");
> >  		return true;
> >  	}
> > @@ -2184,8 +2183,10 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
> >  		 * 'freelist' is already corrupted.  So isolate all objects
> >  		 * starting at 'freelist'.
> >  		 */
> > -		if (freelist_corrupted(s, page, freelist, nextfree))
> > +		if (freelist_corrupted(s, page, freelist, nextfree)) {
> > +			freelist = NULL;
> >  			break;
> > +		}
> >  
> >  		do {
> >  			prior = page->freelist;
> 
> Looks right.
> 
> What are the runtime effects of this change?  In other words, what are
> the end user visible effects of the present defect?

Thank you for the prompt feedback.

The issue popped up as a result of static analysis and code review.
Therefore, I lack any specific runtime behavior example being fixed.
Nevertheless, I think this does not diminish the concern expressed in
the description of the patch.

-- 
Best regards,
Eugeniu Rosca

Re: [PATCH] mm: slub: fix conversion of freelist_corrupted()

[Dongli Zhang]

On 8/11/20 5:46 AM, Eugeniu Rosca wrote:
> Commit 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in
> deactivate_slab()") suffered an update when picked up from LKML [1].
> 
> Specifically, relocating 'freelist = NULL' into 'freelist_corrupted()'
> created a no-op statement. Fix it by sticking to the behavior intended
> in the original patch [1]. Prefer the lowest-line-count solution.
> 
> [1] https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/
> 
> Fixes: 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in deactivate_slab()")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Dongli Zhang <dongli.zhang@oracle.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
> ---
>  mm/slub.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 68c02b2eecd9..9a3e963b02a3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -677,7 +677,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
>  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
>  	    !check_valid_pointer(s, page, nextfree)) {
>  		object_err(s, page, freelist, "Freechain corrupt");
> -		freelist = NULL;
>  		slab_fix(s, "Isolate corrupted freechain");
>  		return true;
>  	}
> @@ -2184,8 +2183,10 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>  		 * 'freelist' is already corrupted.  So isolate all objects
>  		 * starting at 'freelist'.
>  		 */
> -		if (freelist_corrupted(s, page, freelist, nextfree))
> +		if (freelist_corrupted(s, page, freelist, nextfree)) {
> +			freelist = NULL;

This is good to me.

However, this would confuse people when CONFIG_SLUB_DEBUG is not defined.

While reading the source code, people may be curious why to reset freelist when
CONFIG_SLUB_DEBUG is even not defined.

Dongli Zhang

Re: [PATCH] mm: slub: fix conversion of freelist_corrupted()

[Eugeniu Rosca]

Hello Dongli,

On Thu, Aug 13, 2020 at 11:57:51PM -0700, Dongli Zhang wrote:
> On 8/11/20 5:46 AM, Eugeniu Rosca wrote:
> > Commit 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in
> > deactivate_slab()") suffered an update when picked up from LKML [1].
> > 
> > Specifically, relocating 'freelist = NULL' into 'freelist_corrupted()'
> > created a no-op statement. Fix it by sticking to the behavior intended
> > in the original patch [1]. Prefer the lowest-line-count solution.
> > 
> > [1] https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/
> > 
> > Fixes: 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in deactivate_slab()")
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Dongli Zhang <dongli.zhang@oracle.com>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
> > ---
> >  mm/slub.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 68c02b2eecd9..9a3e963b02a3 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -677,7 +677,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
> >  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
> >  	    !check_valid_pointer(s, page, nextfree)) {
> >  		object_err(s, page, freelist, "Freechain corrupt");
> > -		freelist = NULL;
> >  		slab_fix(s, "Isolate corrupted freechain");
> >  		return true;
> >  	}
> > @@ -2184,8 +2183,10 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
> >  		 * 'freelist' is already corrupted.  So isolate all objects
> >  		 * starting at 'freelist'.
> >  		 */
> > -		if (freelist_corrupted(s, page, freelist, nextfree))
> > +		if (freelist_corrupted(s, page, freelist, nextfree)) {
> > +			freelist = NULL;
> 
> This is good to me.
> 
> However, this would confuse people when CONFIG_SLUB_DEBUG is not defined.
> 
> While reading the source code, people may be curious why to reset freelist when
> CONFIG_SLUB_DEBUG is even not defined.

This is a fair point. To address it, the `freelist = NULL` assignment
should be then moved into the body of freelist_corrupted(). If no
concerns on that, I will soon push a v2 implementing this proposal.

-- 
Best regards,
Eugeniu Rosca

Re: [PATCH] mm: slub: fix conversion of freelist_corrupted()

[Dongli Zhang]

On 8/14/20 12:46 AM, Eugeniu Rosca wrote:
> Hello Dongli,
> 
> On Thu, Aug 13, 2020 at 11:57:51PM -0700, Dongli Zhang wrote:
>> On 8/11/20 5:46 AM, Eugeniu Rosca wrote:
>>> Commit 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in
>>> deactivate_slab()") suffered an update when picked up from LKML [1].
>>>
>>> Specifically, relocating 'freelist = NULL' into 'freelist_corrupted()'
>>> created a no-op statement. Fix it by sticking to the behavior intended
>>> in the original patch [1]. Prefer the lowest-line-count solution.
>>>
>>> [1] https://urldefense.com/v3/__https://lore.kernel.org/linux-mm/20200331031450.12182-1-dongli.zhang@oracle.com/__;!!GqivPVa7Brio!LkxH4qJ3WzKnO_nmONLWV-HAougEaefnp8UnI6qC_8j0SS9_9fkO6bOe68flixlQzx8$ 
>>>
>>> Fixes: 52f23478081ae0 ("mm/slub.c: fix corrupted freechain in deactivate_slab()")
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Dongli Zhang <dongli.zhang@oracle.com>
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
>>> ---
>>>  mm/slub.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 68c02b2eecd9..9a3e963b02a3 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -677,7 +677,6 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
>>>  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
>>>  	    !check_valid_pointer(s, page, nextfree)) {
>>>  		object_err(s, page, freelist, "Freechain corrupt");
>>> -		freelist = NULL;
>>>  		slab_fix(s, "Isolate corrupted freechain");
>>>  		return true;
>>>  	}
>>> @@ -2184,8 +2183,10 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>>>  		 * 'freelist' is already corrupted.  So isolate all objects
>>>  		 * starting at 'freelist'.
>>>  		 */
>>> -		if (freelist_corrupted(s, page, freelist, nextfree))
>>> +		if (freelist_corrupted(s, page, freelist, nextfree)) {
>>> +			freelist = NULL;
>>
>> This is good to me.
>>
>> However, this would confuse people when CONFIG_SLUB_DEBUG is not defined.
>>
>> While reading the source code, people may be curious why to reset freelist when
>> CONFIG_SLUB_DEBUG is even not defined.
> 
> This is a fair point. To address it, the `freelist = NULL` assignment
> should be then moved into the body of freelist_corrupted(). If no
> concerns on that, I will soon push a v2 implementing this proposal.
> 

I do have have concern with that if that can make both of static analysis tool
and the people reading code happy :)

Thank you very much!

Dongli Zhang