[PATCH 5.10.y 1/2] Fix unefficient call to memset before memcpu in nla_strlcpy.

[Sasha Levin <sashal@kernel.org>]

From: Francis Laniel <laniel_francis@privacyrequired.com>

[ Upstream commit 8eeb99bc81bc1cb3d5e5323d9a82d8392e3a27b4 ]

Before this commit, nla_strlcpy first memseted dst to 0 then wrote src into it.
This is inefficient because bytes whom number is less than src length are written
twice.

This patch solves this issue by first writing src into dst then fill dst with
0's.
Note that, in the case where src length is higher than dst, only 0 is written.
Otherwise there are as many 0's written to fill dst.

For example, if src is "foo\0" and dst is 5 bytes long, the result will be:
1. "fooGG" after memcpy (G means garbage).
2. "foo\0\0" after memset.

Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 1f58a5335cdd ("taskstats: fix cpumask parsing cutting off the last character")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 lib/nlattr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/nlattr.c b/lib/nlattr.c
index 247426c0af0dc..e08c4b12510b4 100644
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -756,8 +756,9 @@ size_t nla_strlcpy(char *dst, const struct nlattr *nla, size_t dstsize)
 	if (dstsize > 0) {
 		size_t len = (srclen >= dstsize) ? dstsize - 1 : srclen;
 
-		memset(dst, 0, dstsize);
 		memcpy(dst, src, len);
+		/* Zero pad end of dst. */
+		memset(dst + len, 0, dstsize - len);
 	}
 
 	return srclen;
-- 
2.53.0