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mm, slub: make some slub_debug related attributes read-only 

SLUB_DEBUG creates several files under /sys/kernel/slab/<cache>/ that can
be read to check if the respective debugging options are enabled for given
cache.  The options can be also toggled at runtime by writing into the
files.  Some of those, namely red_zone, poison, and store_user can be
toggled only when no objects yet exist in the cache.

Vijayanand reports [1] that there is a problem with freelist randomization
if changing the debugging option's state results in different number of
objects per page, and the random sequence cache needs thus needs to be
recomputed.

However, another problem is that the check for "no objects yet exist in
the cache" is racy, as noted by Jann [2] and fixing that would add
overhead or otherwise complicate the allocation/freeing paths.  Thus it
would be much simpler just to remove the runtime toggling support.  The
documentation describes it's "In case you forgot to enable debugging on
the kernel command line", but the neccessity of having no objects limits
its usefulness anyway for many caches.

Vijayanand describes an use case [3] where debugging is enabled for all
but zram caches for memory overhead reasons, and using the runtime toggles
was the only way to achieve such configuration.  After the previous patch
it's now possible to do that directly from the kernel boot option, so we
can remove the dangerous runtime toggles by making the /sys attribute
files read-only.

While updating it, also improve the documentation of the debugging /sys files.

[1] https://lkml.kernel.org/r/1580379523-32272-1-git-send-email-vjitta@codeaurora.org
[2] https://lore.kernel.org/r/CAG48ez31PP--h6_FzVyfJ4H86QYczAFPdxtJHUEEan+7VJETAQ@mail.gmail.com
[3] https://lore.kernel.org/r/1383cd32-1ddc-4dac-b5f8-9c42282fa81c@codeaurora.org

Reported-by: Vijayanand Jitta <vjitta@codeaurora.org>
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/20200610163135.17364-3-vbabka@suse.cz
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Vlastimil Babka 2020-08-06 23:18:38 -07:00 committed by Linus Torvalds
parent e17f1dfba3
commit ad38b5b113
2 changed files with 19 additions and 53 deletions
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26
Documentation/vm/slub.rst
View File

@ -101,20 +101,26 @@ with "-" as options::
slub_debug=FZ;-,zs_handle,zspage
In case you forgot to enable debugging on the kernel command line: It is
possible to enable debugging manually when the kernel is up. Look at the
contents of::
The state of each debug option for a slab can be found in the respective files
under::
/sys/kernel/slab/<slab name>/
Look at the writable files. Writing 1 to them will enable the
corresponding debug option. All options can be set on a slab that does
not contain objects. If the slab already contains objects then sanity checks
and tracing may only be enabled. The other options may cause the realignment
of objects.
If the file contains 1, the option is enabled, 0 means disabled. The debug
options from the ``slub_debug`` parameter translate to the following files::
Careful with tracing: It may spew out lots of information and never stop if
used on the wrong slab.
F sanity_checks
Z red_zone
P poison
U store_user
T trace
A failslab
The sanity_checks, trace and failslab files are writable, so writing 1 or 0
will enable or disable the option at runtime. The writes to trace and failslab
may return -EINVAL if the cache is subject to slab merging. Careful with
tracing: It may spew out lots of information and never stop if used on the
wrong slab.
Slab merging
============

46
mm/slub.c
View File

@ -5335,61 +5335,21 @@ static ssize_t red_zone_show(struct kmem_cache *s, char *buf)
return sprintf(buf, "%d\n", !!(s->flags & SLAB_RED_ZONE));
}
static ssize_t red_zone_store(struct kmem_cache *s,
const char *buf, size_t length)
{
if (any_slab_objects(s))
return -EBUSY;
s->flags &= ~SLAB_RED_ZONE;
if (buf[0] == '1') {
s->flags |= SLAB_RED_ZONE;
}
calculate_sizes(s, -1);
return length;
}
SLAB_ATTR(red_zone);
SLAB_ATTR_RO(red_zone);
static ssize_t poison_show(struct kmem_cache *s, char *buf)
{
return sprintf(buf, "%d\n", !!(s->flags & SLAB_POISON));
}
static ssize_t poison_store(struct kmem_cache *s,
const char *buf, size_t length)
{
if (any_slab_objects(s))
return -EBUSY;
s->flags &= ~SLAB_POISON;
if (buf[0] == '1') {
s->flags |= SLAB_POISON;
}
calculate_sizes(s, -1);
return length;
}
SLAB_ATTR(poison);
SLAB_ATTR_RO(poison);
static ssize_t store_user_show(struct kmem_cache *s, char *buf)
{
return sprintf(buf, "%d\n", !!(s->flags & SLAB_STORE_USER));
}
static ssize_t store_user_store(struct kmem_cache *s,
const char *buf, size_t length)
{
if (any_slab_objects(s))
return -EBUSY;
s->flags &= ~SLAB_STORE_USER;
if (buf[0] == '1') {
s->flags &= ~__CMPXCHG_DOUBLE;
s->flags |= SLAB_STORE_USER;
}
calculate_sizes(s, -1);
return length;
}
SLAB_ATTR(store_user);
SLAB_ATTR_RO(store_user);
static ssize_t validate_show(struct kmem_cache *s, char *buf)
{