Thread (5 messages) 5 messages, 2 authors, 2020-06-16

Re: [PATCH v5 2/2] mm, treewide: Rename kzfree() to kfree_sensitive()

From: Andrew Morton <akpm@linux-foundation.org>
Date: 2020-06-16 18:09:51
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On Tue, 16 Jun 2020 11:43:11 -0400 Waiman Long [off-list ref] wrote:
quoted hunk ↗ jump to hunk
As said by Linus:

  A symmetric naming is only helpful if it implies symmetries in use.
  Otherwise it's actively misleading.

  In "kzalloc()", the z is meaningful and an important part of what the
  caller wants.

  In "kzfree()", the z is actively detrimental, because maybe in the
  future we really _might_ want to use that "memfill(0xdeadbeef)" or
  something. The "zero" part of the interface isn't even _relevant_.

The main reason that kzfree() exists is to clear sensitive information
that should not be leaked to other future users of the same memory
objects.

Rename kzfree() to kfree_sensitive() to follow the example of the
recently added kvfree_sensitive() and make the intention of the API
more explicit. In addition, memzero_explicit() is used to clear the
memory to make sure that it won't get optimized away by the compiler.

The renaming is done by using the command sequence:

  git grep -w --name-only kzfree |\
  xargs sed -i 's/\bkzfree\b/kfree_sensitive/'

followed by some editing of the kfree_sensitive() kerneldoc and adding
a kzfree backward compatibility macro in slab.h.

...
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -186,10 +186,12 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *, struct mem_cgroup *);
  */
 void * __must_check krealloc(const void *, size_t, gfp_t);
 void kfree(const void *);
-void kzfree(const void *);
+void kfree_sensitive(const void *);
 size_t __ksize(const void *);
 size_t ksize(const void *);
 
+#define kzfree(x)	kfree_sensitive(x)	/* For backward compatibility */
+
What was the thinking here?  Is this really necessary?

I suppose we could keep this around for a while to ease migration.  But
not for too long, please.
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