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

[Jessica Yu]

+++ Eric Biggers [14/03/20 14:34 -0700]:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Jeff Vander Stoep <jeffv@xxxxxxxxxx>
> Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: NeilBrown <neilb@xxxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>