Re: [PATCH v3 1/2] mm: slub: Print the broken data before restoring slub.

[Harry Yoo]

On Thu, Feb 20, 2025 at 12:39:43PM +0900, 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 v3:
> - Change the parameter type of check_bytes_and_report.
> 
> 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@xxxxxxxxxxx>
> Change-Id: I73cf76c110eed62506643913517c957c05a29520

As previously mentioned by others, Change-Id is not used in Linux
kernel development.

> ---
>  mm/slub.c | 29 ++++++++++++++---------------
>  1 file changed, 14 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index b3969d63cc04..de62fed12236 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1192,12 +1192,13 @@ 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 *object, char *what, u8 *start, unsigned int value,
> +		       unsigned int bytes, bool slab_obj_print)
>  {
>  	u8 *fault;
>  	u8 *end;
>  	u8 *addr = slab_address(slab);
> +	char buf[100];
>  
>  	metadata_access_enable();
>  	fault = memchr_inv(kasan_reset_tag(start), value, bytes);
> @@ -1212,11 +1213,14 @@ check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
>  	if (slab_add_kunit_errors())
>  		goto skip_bug_print;
>  
> -	slab_bug(s, "%s overwritten", what);
>  	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);
>  
> +	scnprintf(buf, 100, "%s overwritten", what);

How about moving this into the if block and changing 100 to sizeof(buf)?

> +	if (slab_obj_print)
> +		object_err(s, slab, object, buf);
> +
>  skip_bug_print:
>  	restore_bytes(s, what, value, fault, end);
>  	return 0;
> @@ -1279,7 +1283,7 @@ 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, true);
>  }
>  
>  /* Check the pad bytes at the end of a slab page */
> @@ -1329,11 +1333,11 @@ 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))

I think you don't have to add !! to ret.
Converting from int to _Bool is legal in C99 and it will work as intended. 

>  			ret = 0;

-- 
Cheers,
Harry