From e825196d48d2b89a6ec3a8eff280098d2a78207e Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Sun, 23 Mar 2014 00:28:40 -0400
Subject: make prepend_name() work correctly when called with negative *buflen

In all callchains leading to prepend_name(), the value left in *buflen
is eventually discarded unused if prepend_name() has returned a negative.
So we are free to do what prepend() does, and subtract from *buflen
*before* checking for underflow (which turns into checking the sign
of subtraction result, of course).

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/dcache.c b/fs/dcache.c
index 265e0ce..ca02c13 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2833,9 +2833,9 @@ static int prepend_name(char **buffer, int *buflen, struct qstr *name)
 	u32 dlen = ACCESS_ONCE(name->len);
 	char *p;
 
-	if (*buflen < dlen + 1)
-		return -ENAMETOOLONG;
 	*buflen -= dlen + 1;
+	if (*buflen < 0)
+		return -ENAMETOOLONG;
 	p = *buffer -= dlen + 1;
 	*p++ = '/';
 	while (dlen--) {
-- 
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