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

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Mon, Jan 20, 2025 at 5:31 PM Hyesoo Yu <hyesoo.yu@samsung.com> wrote:

Let's add Chengming, the author of the commit, to Cc,
as he might have some opinions about it.

> Previously, the restore occured after printing the object in slub.
> After commit 47d911b ("slab: make check_object() more consistent"),

at least 12 characters of the commit hash should be used to refer to a commit.
Documentation/process/submitting-patches.rst states that:
    You should also be sure to use at least the first twelve
characters of the SHA-1 ID.
    The kernel repository holds a lot of objects, making collisions
with shorter IDs a real
    possibility. Bear in mind that, even if there is no collision with
your six-character ID
    now, that condition may change five years from now.

> 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
> are printed prior to the restoration process.

Probably this should be considered for -stable releases. What do you think?
[1] https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html

> diff --git a/mm/slub.c b/mm/slub.c
> index c2151c9fee22..48cefc969480 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1207,6 +1207,7 @@ check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
>                                         fault[0], value);
>
>  skip_bug_print:
> +       print_section(KERN_ERR, "Corrupt  ", fault, end - fault);

I don't think it's supposed to report an error here, per the name of
the label "skip_bug_print".

Maybe move print_trailer() and add_taint() back to
check_bytes_and_report(), and report an error
only once and skip reporting if it's already reported?

Best,
Hyeonggon

>         restore_bytes(s, what, value, fault, end);
>         return 0;
>  }
> --
> 2.48.0
>