[PATCH v4 10/13] fs: dcache: manually unpoison dname after allocation to shut up kasan's reports

[Andrey Ryabinin]

We need to manually unpoison rounded up allocation size for dname
to avoid kasan's reports in dentry_string_cmp().
When CONFIG_DCACHE_WORD_ACCESS=y dentry_string_cmp may access
few bytes beyound requested in kmalloc() size.

dentry_string_cmp() relates on that fact that dentry allocated
using kmalloc and kmalloc internally round up allocation size.
So this is not a bug, but this makes kasan to complain about
such accesses.
To avoid such reports we mark rounded up allocation size in
shadow as accessible.

Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
---
 fs/dcache.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 876ac08..584b283 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -38,6 +38,7 @@
 #include <linux/prefetch.h>
 #include <linux/ratelimit.h>
 #include <linux/list_lru.h>
+#include <linux/kasan.h>
 #include "internal.h"
 #include "mount.h"
 
@@ -1395,6 +1396,10 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 			kmem_cache_free(dentry_cache, dentry); 
 			return NULL;
 		}
+#ifdef CONFIG_DCACHE_WORD_ACCESS
+		kasan_unpoison_shadow(dname,
+				round_up(name->len + 1,	sizeof(unsigned long)));
+#endif
 	} else  {
 		dname = dentry->d_iname;
 	}	
-- 
2.1.2

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