Re: [PATCH v3 2/5] fs/filesystems.c: downgrade user-reachable WARN_ONCE() to pr_warn_once()

[Jessica Yu <jeyu@kernel.org>]

+++ Eric Biggers [14/03/20 14:34 -0700]:
>From: Eric Biggers <ebiggers@google.com>
>
>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().
>
>Acked-by: Luis Chamberlain <mcgrof@kernel.org>
>Cc: stable@vger.kernel.org
>Cc: Alexei Starovoitov <ast@kernel.org>
>Cc: Andrew Morton <akpm@linux-foundation.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: NeilBrown <neilb@suse.com>
>Signed-off-by: Eric Biggers <ebiggers@google.com>
>---
> fs/filesystems.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/fs/filesystems.c b/fs/filesystems.c
>index 77bf5f95362da..90b8d879fbaf3 100644
>--- 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);

Hm, what was the rationale for warning only once again? It might be useful
for debugging issues to see each instance of request_module() failure
(and with which fs). However, I don't have a concrete use case to
support this argument, so:

Reviewed-by: Jessica Yu <jeyu@kernel.org>