Re: [PATCH v2] mm/slub: clean up create_unique_id()

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On 9/25/22 09:12, Chao Yu wrote:
From: Chao Yu <chao.yu@xxxxxxxx>

As Christophe JAILLET suggested:

In create_unique_id(),

"looks that ID_STR_LENGTH could even be reduced to 32 or 16.

The 2nd BUG_ON at the end of the function could certainly be just
removed as well or remplaced by a:
         if (p > name + ID_STR_LENGTH - 1) {
                 kfree(name);
                 return -E<something>;
         }
"

According to above suggestion, let's do below cleanups:
1. reduce ID_STR_LENGTH to 32, as the buffer size should be enough;
2. use WARN_ON instead of BUG_ON() and return error if check condition
is true;
3. use snprintf instead of sprintf to avoid overflow.

Link: https://lore.kernel.org/linux-mm/2025305d-16db-abdf-6cd3-1fb93371c2b4@xxxxxxxxxx/
Fixes: 81819f0fc828 ("SLUB core")
Suggested-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
Signed-off-by: Chao Yu <chao.yu@xxxxxxxx>
---
v2:
- add WARN_ON() instead of return error silently;
- use snprintf instead of sprintf to avoid overflow.
  mm/slub.c | 10 +++++++---
  1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 4b98dff9be8e..3d37a8a7b965 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5890,7 +5890,7 @@ static inline struct kset *cache_kset(struct kmem_cache *s)
  	return slab_kset;
  }
-#define ID_STR_LENGTH 64
+#define ID_STR_LENGTH 32
/* Create a unique string id for a slab cache:
   *
@@ -5924,9 +5924,13 @@ static char *create_unique_id(struct kmem_cache *s)
  		*p++ = 'A';
  	if (p != name + 1)
  		*p++ = '-';
-	p += sprintf(p, "%07u", s->size);
+	p += snprintf(p, ID_STR_LENGTH - 1 - (p - name), "%07u", s->size);

I think we don't need "- 1" here as snprintf() says:
@size: The size of the buffer, including the trailing null space

- BUG_ON(p > name + ID_STR_LENGTH - 1);
+	if (p > name + ID_STR_LENGTH - 1) {
+		WARN_ON(1);

This would be shorter: if (WARN_ON(p > name...))

+		kfree(name);
+		return ERR_PTR(-EINVAL);
+	}
  	return name;
  }





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