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

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Date:   Wed, 18 Mar 2020 16:43:15 +0100
From:   Jessica Yu <jeyu@...nel.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeff Vander Stoep <jeffv@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        NeilBrown <neilb@...e.com>, stable@...r.kernel.org
Subject: Re: [PATCH v3 2/5] fs/filesystems.c: downgrade user-reachable
 WARN_ONCE() to pr_warn_once()

+++ Eric Biggers [14/03/20 14:34 -0700]:
>From: Eric Biggers <ebiggers@...gle.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@...nel.org>
>Cc: stable@...r.kernel.org
>Cc: Alexei Starovoitov <ast@...nel.org>
>Cc: Andrew Morton <akpm@...ux-foundation.org>
>Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>Cc: Jeff Vander Stoep <jeffv@...gle.com>
>Cc: Jessica Yu <jeyu@...nel.org>
>Cc: Kees Cook <keescook@...omium.org>
>Cc: NeilBrown <neilb@...e.com>
>Signed-off-by: Eric Biggers <ebiggers@...gle.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@...nel.org>