Re: [PATCH v2] mm: slub: Print the broken data before restoring slub.

[Hyesoo Yu <hyesoo.yu@samsung.com>]

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On Thu, Feb 06, 2025 at 03:08:48PM +0100, Vlastimil Babka wrote:
> On 2/5/25 01:44, Hyesoo Yu wrote:
> > Previously, the restore occured after printing the object in slub.
> > After commit 47d911b02cbe ("slab: make check_object() more consistent"),
> > the bytes are printed after the restore. This information about the bytes
> > before the restore is highly valuable for debugging purpose.
> > For instance, in a event of cache issue, it displays byte patterns
> > by breaking them down into 64-bytes units. Without this information,
> > we can only speculate on how it was broken. Hence the corrupted regions
> > should be printed prior to the restoration process. However if an object breaks
> > in multiple places, the same log may be output multiple times.
> > Therefore the slub log is reported only once to prevent redundant printing,
> > by sending a parameter indicating whether an error has occurred previously.
> > 
> > Changes in v2:
> > - Instead of using print_section every time on check_bytes_and_report,
> > just print it once for the entire slub object before the restore.
> > 
> > Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>
> > ---
> >  mm/slub.c | 25 ++++++++++++-------------
> >  1 file changed, 12 insertions(+), 13 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index ea956cb4b8be..7a9f7a2c17d7 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -1182,7 +1182,7 @@ static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
> >  static pad_check_attributes int
> >  check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
> >  		       u8 *object, char *what,
> > -		       u8 *start, unsigned int value, unsigned int bytes)
> > +		       u8 *start, unsigned int value, unsigned int bytes, int slab_obj_print)
> 
> It would be better to redistribute the arguments among lines to fit each <80
> chars. The previous line is underutilized. Also could the new argument be bool?
>

I used interger to make the types match, but it does seem like using a boolean would be more
readable. I will change it to a boolean and modify it to pass !!ret from check_objects().

> >  {
> >  	u8 *fault;
> >  	u8 *end;
> > @@ -1205,6 +1205,10 @@ check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
> >  	pr_err("0x%p-0x%p @offset=%tu. First byte 0x%x instead of 0x%x\n",
> >  					fault, end - 1, fault - addr,
> >  					fault[0], value);
> 
> Hm we have slab_bug() above this, not slab_err(). So this is another place
> that would need to take care a WARN is called with your other patch.
> 
> > +	if (slab_obj_print) {
> > +		print_trailer(s, slab, object);
> > +		add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> 
> I guess we could do the WARN here. If panic_on_warn is enabled it will not
> report all problems that check_object() could find and panic on the first
> one. But that would have happened too with your slab_fix() approach
> (slab_fix() called from restore_bytes() below). I think we can live with
> that instead of needing two separate reporting and fixing rounds from
> check_object().
> 
> Could you send the two patches as a series in v3, as they are
> inter-dependent? Thanks.
> 

I guess we can call object_err() here. That would include the WARN() from other
patch. I will send the two pathces as a series in v3.

Thanks,
Regards.

> > +	}
> >  
> >  skip_bug_print:
> >  	restore_bytes(s, what, value, fault, end);
> > @@ -1268,7 +1272,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
> >  		return 1;
> >  
> >  	return check_bytes_and_report(s, slab, p, "Object padding",
> > -			p + off, POISON_INUSE, size_from_object(s) - off);
> > +			p + off, POISON_INUSE, size_from_object(s) - off, 1);
> >  }
> >  
> >  /* Check the pad bytes at the end of a slab page */
> > @@ -1318,11 +1322,11 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> >  
> >  	if (s->flags & SLAB_RED_ZONE) {
> >  		if (!check_bytes_and_report(s, slab, object, "Left Redzone",
> > -			object - s->red_left_pad, val, s->red_left_pad))
> > +			object - s->red_left_pad, val, s->red_left_pad, ret))
> >  			ret = 0;
> >  
> >  		if (!check_bytes_and_report(s, slab, object, "Right Redzone",
> > -			endobject, val, s->inuse - s->object_size))
> > +			endobject, val, s->inuse - s->object_size, ret))
> >  			ret = 0;
> >  
> >  		if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
> > @@ -1331,7 +1335,7 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> >  			if (s->object_size > orig_size  &&
> >  				!check_bytes_and_report(s, slab, object,
> >  					"kmalloc Redzone", p + orig_size,
> > -					val, s->object_size - orig_size)) {
> > +					val, s->object_size - orig_size, ret)) {
> >  				ret = 0;
> >  			}
> >  		}
> > @@ -1339,7 +1343,7 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> >  		if ((s->flags & SLAB_POISON) && s->object_size < s->inuse) {
> >  			if (!check_bytes_and_report(s, slab, p, "Alignment padding",
> >  				endobject, POISON_INUSE,
> > -				s->inuse - s->object_size))
> > +				s->inuse - s->object_size, ret))
> >  				ret = 0;
> >  		}
> >  	}
> > @@ -1355,11 +1359,11 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> >  			if (kasan_meta_size < s->object_size - 1 &&
> >  			    !check_bytes_and_report(s, slab, p, "Poison",
> >  					p + kasan_meta_size, POISON_FREE,
> > -					s->object_size - kasan_meta_size - 1))
> > +					s->object_size - kasan_meta_size - 1, ret))
> >  				ret = 0;
> >  			if (kasan_meta_size < s->object_size &&
> >  			    !check_bytes_and_report(s, slab, p, "End Poison",
> > -					p + s->object_size - 1, POISON_END, 1))
> > +					p + s->object_size - 1, POISON_END, 1, ret))
> >  				ret = 0;
> >  		}
> >  		/*
> > @@ -1385,11 +1389,6 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> >  		ret = 0;
> >  	}
> >  
> > -	if (!ret && !slab_in_kunit_test()) {
> > -		print_trailer(s, slab, object);
> > -		add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> > -	}
> > -
> >  	return ret;
> >  }
> >  
> 
> 

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