It makes little sense for fsmount() to output the warning message when
mount_too_revealing() is violated to kmsg. Instead, the warning should
be output (with a "VFS" prefix) to the fscontext log. In addition,
include the same log message for mount_too_revealing() when doing a
regular mount for consistency.

With the newest fsopen()-based mount(8) from util-linux, the error
messages now look like

  # mount -t proc proc /tmp
  mount: /tmp: fsmount() failed: VFS: Mount too revealing.
         dmesg(1) may have more information after failed mount system call.

which could finally result in mount_too_revealing() errors being easier
for users to detect and understand.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 fs/namespace.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 55f28cebbe7d..b2146857cbbd 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3820,8 +3820,10 @@ static int do_new_mount_fc(struct fs_context *fc, struct path *mountpoint,
 	int error;
 
 	error = security_sb_kern_mount(sb);
-	if (!error && mount_too_revealing(sb, &mnt_flags))
+	if (!error && mount_too_revealing(sb, &mnt_flags)) {
 		error = -EPERM;
+		errorfcp(fc, "VFS", "Mount too revealing");
+	}
 
 	if (unlikely(error)) {
 		fc_drop_locked(fc);
@@ -4547,7 +4549,7 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 
 	ret = -EPERM;
 	if (mount_too_revealing(fc->root->d_sb, &mnt_flags)) {
-		pr_warn("VFS: Mount too revealing\n");
+		errorfcp(fc, "VFS", "Mount too revealing");
 		goto err_unlock;
 	}
 

-- 
2.50.1

On Wed, Aug 06, 2025 at 02:48:30PM +1000, Aleksa Sarai wrote:

>  	error = security_sb_kern_mount(sb);
> -	if (!error && mount_too_revealing(sb, &mnt_flags))
> +	if (!error && mount_too_revealing(sb, &mnt_flags)) {
>  		error = -EPERM;
> +		errorfcp(fc, "VFS", "Mount too revealing");
> +	}

Hmm...  For aesthetics sake, I'd probably do logging first; otherwise
fine by me.

On 2025-08-06, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Wed, Aug 06, 2025 at 02:48:30PM +1000, Aleksa Sarai wrote:
> 
> >  	error = security_sb_kern_mount(sb);
> > -	if (!error && mount_too_revealing(sb, &mnt_flags))
> > +	if (!error && mount_too_revealing(sb, &mnt_flags)) {
> >  		error = -EPERM;
> > +		errorfcp(fc, "VFS", "Mount too revealing");
> > +	}
> 
> Hmm...  For aesthetics sake, I'd probably do logging first; otherwise
> fine by me.

Good point, I'll send a v2.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/