Re: [PATCH] mm/damon/core: reset nr_dests on allocation failure in damos_commit_dests()

[SeongJae Park <sj@kernel.org>]

On Thu, 19 Mar 2026 07:07:18 +0000 Josh Law <objecting@objecting.org> wrote:

> 
> 
> On 19 March 2026 04:33:09 GMT, SeongJae Park <sj@kernel.org> wrote:
> >Hello Josh,
> >
> >On Wed, 18 Mar 2026 21:49:39 +0000 Josh Law <objecting@objecting.org> wrote:
> >
> >> damos_commit_dests() frees the old node_id_arr and weight_arr before
> >> reallocating.  If kmalloc_array() fails, the function returns -ENOMEM but
> >> leaves dst->nr_dests at its previous value.  A subsequent call with the
> >> same nr_dests will skip the reallocation (the sizes match), and the loop
> >> at the end will dereference the now-NULL array pointers.
> >
> >Nice catch.  But, this is a sort of intended behavior.
> >
> >The idea behind the code is that, if the function fails, the caller will
> >not resue 'dst' but discard it.  Hence the function is only ensuring the 'dst'
> >after the failure can be deallocated using the deallocation helper function
> >like 'damon_destroy_scheme()'.  For this, the function is setting weight_arr as
> >NULL in the allocation failure.
> >
> >> 
> >> Fix this by resetting dst->nr_dests to 0 immediately after freeing the
> >> old arrays, so any later call always enters the reallocation path.
> >> 
> >> Fixes: cbc4eea4ffb5 ("mm/damon/core: commit damos->migrate_dests")
> >> Signed-off-by: Josh Law <objecting@objecting.org>
> >> ---
> >>  mm/damon/core.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >> 
> >> diff --git a/mm/damon/core.c b/mm/damon/core.c
> >> index 7f74982535ac..e233eb84a2d5 100644
> >> --- a/mm/damon/core.c
> >> +++ b/mm/damon/core.c
> >> @@ -1060,6 +1060,7 @@ static int damos_commit_dests(struct damos_migrate_dests *dst,
> >>  	if (dst->nr_dests != src->nr_dests) {
> >>  		kfree(dst->node_id_arr);
> >>  		kfree(dst->weight_arr);
> >> +		dst->nr_dests = 0;
> >>  
> >>  		dst->node_id_arr = kmalloc_array(src->nr_dests,
> >>  			sizeof(*dst->node_id_arr), GFP_KERNEL);
> >
> >Someone (including a part of myself) could argue anyway initializing the field
> >is better to do, for code readability and completeness of the data structure.
> >But I'd argue that might only encourage calllers to reuse 'dst' after the
> >failure.  Also, the 0 nr_dests could still meaning something incorrect, if the
> >first kmalloc_array() for node_id_arr success but the following kmalloc_array()
> >for weight_arr failed.  In the case, nr_dests is zero, but the size of
> >node_id_arr is not zero.
> >
> >I think the intention behind the code is not well documented and that might
> >confused you.  Sorry if that was the case.  I think this could better be
> >documented by adding comments for the function.  The single line comment in the
> >function body was for the purpose, but having more detailed comments at the top
> >of the function may be better.  If you'd like to send such documentation,
> >please do so.  If not, I will do that.  Whatever is your preference, thank you
> >for finding and sharing this room to improve!
> >
> >... And, this patch helped me finding something actually broken.  As I
> >mentioned above, callers of damos_commit_dests() are assumed to discard the
> >'dst' when the function failed.  And the only caller, sysfs.c, does so, except
> >for the final commit to the running context (kdmond->damon_ctx).  It can result
> >in DAMON running with the incorrect data structure, doing NULL dereference.
> >Similar issue might exist for DAMON_RECLAIM and DAMON_LRU_SORT.  Because those
> >modules use only limited parameters, there might be not.  I will double check
> >and make a fix soon.  Again, thank you for helping me finding this issue, Josh!
> >
> >
> >Thanks,
> >SJ
> 
> 
> Well, I guess hardening this patch is useful for then..

Agreed.  Maybe adding another sanity check (e.g., WARN_ON(dst->nr_dests &&
(!dst->weight_arr || !dst->node_id_arr), "foo")) under DAMON_DEBUG_SANITY might
make sense.


Thanks,
SJ

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