[PATCH 3/3] index-helper: take extra care with readlink

[Ramsay Jones <ramsay@ramsayjones.plus.com>]

It took me a few minutes to convince myself that, if index-helper is
the only writer for the symlink, that the call to readlink would
result in a properly NULL terminated string. This relies on the
initialization of the address variable setting each byte of the
'struct sockaddr_un' to zero and the contents of the symlink being
no more than 'sizeof(address.sun_path) - 1' bytes long.

In order to make the call easier to reason about, introduce a wrapper
function, read_link(), which copes with overlong symlinks and ensures
correct NULL termination.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
---
 read-cache.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/read-cache.c b/read-cache.c
index c7053d8..39330a1 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1722,12 +1722,23 @@ static void post_read_index_from(struct index_state *istate)
 	tweak_untracked_cache(istate);
 }
 
+static int read_link(const char *path, char *buf, size_t bufsize)
+{
+	int len;
+
+	len = readlink(path, buf, bufsize);
+	if (len < 0 || len >= bufsize)
+		return -1;
+	buf[len] = '\0';
+	return 0;
+}
+
 int connect_to_index_helper(void)
 {
 	struct sockaddr_un address = {0};
 	int fd;
 
-	if (readlink(git_path("index-helper.path"), address.sun_path,
+	if (read_link(git_path("index-helper.path"), address.sun_path,
 		     sizeof(address.sun_path)) < 0)
 		return -1;
 
-- 
2.8.0