linux-ext4 - [PATCH 04/12] fuse2fs: allow O_APPEND and O

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Message-ID: <175573318676.4130038.2598964686073896057.stgit@frogsfrogsfrogs>
Date: Wed, 20 Aug 2025 16:41:55 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: tytso@....edu
Cc: linux-ext4@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: [PATCH 04/12] fuse2fs: allow O_APPEND and O_TRUNC opens

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

Commit 9f69dfc4e275cc didn't quite get the permissions checking correct:

generic/362       - output mismatch (see /var/tmp/fstests/generic/362.out.bad)
    --- tests/generic/362.out   2025-04-30 16:20:44.563833050 -0700
    +++ /var/tmp/fstests/generic/362.out.bad    2025-06-11 17:04:24.061193618 -0700
    @@ -1,2 +1,3 @@
     QA output created by 362
    +Failed to open/create file: Operation not permitted
     Silence is golden
    ...
    (Run 'diff -u /run/fstests/bin/tests/generic/362.out /var/tmp/fstests/generic/362.out.bad'  to see the entire diff)

The kernel allows opening a file for append and truncation.  What it
doesn't allow is opening an append-only file for truncation.  Note that
this causes generic/079 to regress, but the root cause of that problem
is actually that fuse oddly supports FS_IOC_[GS]ETFLAGS but doesn't
actually set the VFS inode flags.

Cc: <linux-ext4@...r.kernel.org> # v1.47.3
Fixes: 9f69dfc4e275cc ("fuse2fs: implement O_APPEND correctly")
Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 misc/fuse2fs.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/misc/fuse2fs.c b/misc/fuse2fs.c
index cce1c97c81c075..1debbd3355ec8f 100644
--- a/misc/fuse2fs.c
+++ b/misc/fuse2fs.c
@@ -2639,15 +2639,13 @@ static int __op_open(struct fuse2fs *ff, const char *path,
 		file->open_flags |= EXT2_FILE_WRITE;
 		break;
 	}
-	if (fp->flags & O_APPEND) {
-		/* the kernel doesn't allow truncation of an append-only file */
-		if (fp->flags & O_TRUNC) {
-			ret = -EPERM;
-			goto out;
-		}

+	/*
+	 * If the caller wants to truncate the file, we need to ask for full
+	 * write access even if the caller claims to be appending.
+	 */
+	if ((fp->flags & O_APPEND) && !(fp->flags & O_TRUNC))
 		check |= A_OK;
-	}

 	detect_linux_executable_open(fp->flags, &check, &file->open_flags);