linux-ext4 - [PATCH 2/2] libext2fs: don't look for O_EXCL in the F

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Message-ID: <20250828144600.GB8084@frogsfrogsfrogs>
Date: Thu, 28 Aug 2025 07:46:00 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Theodore Ts'o <tytso@....edu>
Cc: linux-ext4 <linux-ext4@...r.kernel.org>
Subject: [PATCH 2/2] libext2fs: don't look for O_EXCL in the F_GETFL output

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

For decades, Linux has never propagated O_EXCL into the user-visible
file flags in do_dentry_open:

	f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);

Therefore, one cannot use F_GETFL to determine if the file was opened
with O_EXCL.  The unixfd IO manager will have to trust that the caller
opened the file in O_EXCL mode.  Without this patch, the upcoming flock
patch will not work correctly in determining the lock mode to keep other
copies of fuse4fs and/or systemd from touching a fuse4fs mounted
filesystem.

Cc: <linux-ext4@...r.kernel.org> # v1.43.2
Fixes: 4ccf9e4fe165cf ("libext2fs: add unixfd_io_manager")
Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 lib/ext2fs/unix_io.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/lib/ext2fs/unix_io.c b/lib/ext2fs/unix_io.c
index e839628aa1b74d..55007ad7d2ae15 100644
--- a/lib/ext2fs/unix_io.c
+++ b/lib/ext2fs/unix_io.c
@@ -1172,11 +1172,10 @@ static errcode_t unixfd_open(const char *str_fd, int flags,
 	if (fd_flags == -1)
 		return EBADF;

-	flags = 0;
+	/* O_EXCL is cleared by Linux at open and not returned by F_GETFL */
+	flags &= IO_FLAG_EXCLUSIVE;
 	if (fd_flags & O_RDWR)
 		flags |= IO_FLAG_RW;
-	if (fd_flags & O_EXCL)
-		flags |= IO_FLAG_EXCLUSIVE;
 #if defined(O_DIRECT)
 	if (fd_flags & O_DIRECT)
 		flags |= IO_FLAG_DIRECT_IO;