linux-ext4 - [PATCH 6/9] fuse: let the kernel handle KILL_SUID/KILL

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Message-ID: <176169811721.1426244.10570675160710429432.stgit@frogsfrogsfrogs>
Date: Tue, 28 Oct 2025 17:55:22 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: djwong@...nel.org, miklos@...redi.hu
Cc: joannelkoong@...il.com, bernd@...ernd.com, neal@...pa.dev,
 linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [PATCH 6/9] fuse: let the kernel handle KILL_SUID/KILL_SGID for iomap
 filesystems

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

Let the kernel handle killing the suid/sgid bits because the
write/falloc/truncate/chown code already does this, and we don't have to
worry about external modifications that are only visible to the fuse
server (i.e. we're not a cluster fs).

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
---
 fs/fuse/dir.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 135c601230e547..9435d6b8d14ea4 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -2261,6 +2261,7 @@ static int fuse_setattr(struct mnt_idmap *idmap, struct dentry *entry,
 	struct inode *inode = d_inode(entry);
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	struct file *file = (attr->ia_valid & ATTR_FILE) ? attr->ia_file : NULL;
+	const bool is_iomap = fuse_inode_has_iomap(inode);
 	int ret;

 	if (fuse_is_bad(inode))
@@ -2269,15 +2270,19 @@ static int fuse_setattr(struct mnt_idmap *idmap, struct dentry *entry,
 	if (!fuse_allow_current_process(get_fuse_conn(inode)))
 		return -EACCES;

-	if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
+	if (!is_iomap &&
+	    (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID))) {
 		attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
 				    ATTR_MODE);

 		/*
 		 * The only sane way to reliably kill suid/sgid is to do it in
-		 * the userspace filesystem
+		 * the userspace filesystem if this isn't an iomap file.  For
+		 * iomap filesystems we let the kernel kill the setuid/setgid
+		 * bits.
 		 *
-		 * This should be done on write(), truncate() and chown().
+		 * This should be done on write(), truncate(), chown(), and
+		 * fallocate().
 		 */
 		if (!fc->handle_killpriv && !fc->handle_killpriv_v2) {
 			/*