linux-ext4 - [PATCH 10/8] fuse2fs: fix lockfile creation, again

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Message-ID: <20250708173333.GD2672022@frogsfrogsfrogs>
Date: Tue, 8 Jul 2025 10:33:33 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: linux-ext4@...r.kernel.org
Subject: [PATCH 10/8] fuse2fs: fix lockfile creation, again

From: Darrick J. Wong <djwong@...nel.org>

On closer examination of the lockfile code, there is still a fatal flaw
in the locking logic.  This is born out by the fact that you can run:

# truncate -s 300m /tmp/a
# mkfs.ext2 /tmp/a
# fuse2fs -o kernel /tmp/a /mnt -o lockfile=/tmp/fuselock
# fuse2fs -o kernel /tmp/a /mnt -o lockfile=/tmp/fuselock

and the second mount attempt succeeds where it really shouldn't.  This
is due to the use of fopen(..., "w"), because "w" means "truncate or
create".  It does /not/ imply O_CREAT | O_EXCL, which fails if the file
already exists.  Theoretically that could have been done with mode
string "wx", but that's a glibc extension.

Fix this by calling open() directly with the O_ modes that we want.

Cc: <linux-ext4@...r.kernel.org> # v1.47.3-rc3
Fixes: e50fbaa4d156a6 ("fuse2fs: clean up the lockfile handling")
Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse2fs.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index b7201f7c8ed185..ff8d4668cee217 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -4473,11 +4473,15 @@ int main(int argc, char *argv[])
 	}

 	if (fctx.lockfile) {
-		FILE *lockfile = fopen(fctx.lockfile, "w");
 		char *resolved;
+		int lockfd;

-		if (!lockfile) {
-			err = errno;
+		lockfd = open(fctx.lockfile, O_RDWR | O_CREAT | O_EXCL, 0400);
+		if (lockfd < 0) {
+			if (errno == EEXIST)
+				err = EWOULDBLOCK;
+			else
+				err = errno;
 			err_printf(&fctx, "%s: %s: %s\n", fctx.lockfile,
 				   _("opening lockfile failed"),
 				   strerror(err));
@@ -4485,7 +4489,7 @@ int main(int argc, char *argv[])
 			ret |= 32;
 			goto out;
 		}
-		fclose(lockfile);
+		close(lockfd);

 		resolved = realpath(fctx.lockfile, NULL);
 		if (!resolved) {