linux-kernel - [PATCH 03/18] fs_context: fix detecting full log buffer

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Date:   Sun,  8 Jul 2018 14:01:39 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     David Howells <dhowells@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>
Subject: [PATCH 03/18] fs_context: fix detecting full log buffer

From: Eric Biggers <ebiggers@...gle.com>

When 'head' and 'tail' wrap around, 'log->head - log->tail' will be
something like '4 - 252 = -248', and comparing that directly to the
array size is wrong.  Fix by casting to 'u8'.

Fixes: 09aeca629fb3 ("vfs: Implement logging through fs_context")
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 fs/fs_context.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 97e8c1dc4e3b1..a0e22f4c6b64a 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -418,7 +418,9 @@ void logfc(struct fs_context *fc, const char *fmt, ...)
 	freeable = 0;
 store_string:
 	index = log->head & (logsize - 1);
-	if ((int)log->head - (int)log->tail == 8) {
+	BUILD_BUG_ON(sizeof(log->head) != sizeof(u8) ||
+		     sizeof(log->tail) != sizeof(u8));
+	if ((u8)(log->head - log->tail) == logsize) {
 		/* The buffer is full, discard the oldest message */
 		if (log->need_free & (1 << index))
 			kfree(log->buffer[index]);
-- 
2.18.0