From: Tao Cui <cui.tao@linux.dev>
To: clm@fb.com, dsterba@suse.com, sforshee@kernel.org, wqu@suse.com
Cc: josef@toxicpanda.com, brauner@kernel.org,
linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, cui.tao@linux.dev,
Tao Cui <cuitao@kylinos.cn>
Subject: [PATCH v2] btrfs: allow idmapped DEFRAG ioctls
Date: Sat, 15 Aug 2026 21:22:59 +0800 [thread overview]
Message-ID: <20260815132259.3935817-1-cui.tao@linux.dev> (raw)
From: Tao Cui <cuitao@kylinos.cn>
btrfs_ioctl_defrag() checks MAY_WRITE against nop_mnt_idmap, so on an
idmapped mount the owner comparison uses the caller's fsuid against the
raw on-disk uid and the ioctl fails with -EPERM even for the file's
owner. Pass the mount idmap down so defrag works on idmapped mounts.
The check, added in 616d374efa23 ("btrfs: allow defrag on a file opened
read-only that has rw permissions"), is not a privilege gate. It only
tests whether the file could have been opened for writing, and a user
who owns the file on the idmapped mount can already open it O_RDWR,
write() to it or fallocate() it. Defragmenting a regular file only
rearranges the caller's own extents; there is nothing it can rewrite
that the caller could not rewrite anyway. Whole-subvolume defrag on a
directory keeps its capable(CAP_SYS_ADMIN) requirement, and the
!capable() guard around this check is left as is.
FIDEDUPERANGE already works this way for unprivileged callers:
may_dedupe_file() in fs/remap_range.c compares the inode owner through
file_mnt_idmap(file) and falls back to inode_permission() with the same
idmap, and dedupe can rewrite one file's extents from another file's
contents, which is a stronger operation than defrag.
On regular mounts file_mnt_idmap() is nop_mnt_idmap and nothing
changes.
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
v2: rewrite the commit message to explain why defrag is safe for
idmapped users (Seth Forshee); the code is unchanged.
---
fs/btrfs/ioctl.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 72bc9d4f7708..607329dedd50 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2458,7 +2458,7 @@ static int btrfs_ioctl_defrag(struct file *file, void __user *argp)
* running and allows defrag on files open in read-only mode.
*/
if (!capable(CAP_SYS_ADMIN) &&
- inode_permission(&nop_mnt_idmap, inode, MAY_WRITE)) {
+ inode_permission(file_mnt_idmap(file), inode, MAY_WRITE)) {
ret = -EPERM;
goto out;
}
--
2.43.0
reply other threads:[~2026-08-15 13:23 UTC|newest]
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