fs/filesystems.c: downgrade user-reachable WARN_ONCE() to pr_warn_once()

After request_module(), nothing is stopping the module from being
unloaded until someone takes a reference to it via try_get_module().

The WARN_ONCE() in get_fs_type() is thus user-reachable, via userspace
running 'rmmod' concurrently.

Since WARN_ONCE() is for kernel bugs only, not for user-reachable
situations, downgrade this warning to pr_warn_once().

Keep it printed once only, since the intent of this warning is to detect
a bug in modprobe at boot time.  Printing the warning more than once
wouldn't really provide any useful extra information.

Fixes: 41124db869b7 ("fs: warn in case userspace lied about modprobe return")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jessica Yu <jeyu@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jeff Vander Stoep <jeffv@google.com>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: NeilBrown <neilb@suse.com>
Cc: <stable@vger.kernel.org>		[4.13+]
Link: http://lkml.kernel.org/r/20200312202552.241885-3-ebiggers@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/filesystems.c b/fs/filesystems.c
index 77bf5f95362da5ec767c4919e4271c11120a18ff..90b8d879fbaf3d4c8aa6846c33304f400889b6b9 100644 (file)
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -272,7 +272,9 @@ struct file_system_type *get_fs_type(const char *name)
        fs = __get_fs_type(name, len);
        if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
                fs = __get_fs_type(name, len);
-               WARN_ONCE(!fs, "request_module fs-%.*s succeeded, but still no fs?\n", len, name);
+               if (!fs)
+                       pr_warn_once("request_module fs-%.*s succeeded, but still no fs?\n",
+                                    len, name);
        }
 
        if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {