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

From: Eric Biggers <ebiggers@kernel.org>
To: Jessica Yu <jeyu@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jeff Vander Stoep <jeffv@google.com>,
	Kees Cook <keescook@chromium.org>,
	Luis Chamberlain <mcgrof@kernel.org>, NeilBrown <neilb@suse.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH v3 2/5] fs/filesystems.c: downgrade user-reachable WARN_ONCE() to pr_warn_once()
Date: Wed, 18 Mar 2020 15:54:15 -0700	[thread overview]
Message-ID: <20200318225415.GD2334@sol.localdomain> (raw)
In-Reply-To: <20200318154315.GB4144@linux-8ccs>

On Wed, Mar 18, 2020 at 04:43:15PM +0100, Jessica Yu wrote:
> +++ 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>

This was discussed on v2, see
https://lkml.kernel.org/lkml/20200313010053.GS11244@42.do-not-panic.com/.
If the warning triggers, then it indicates a broken modprobe program.
Printing the warning multiple times wouldn't really add any new information.

And in any case, it's printed once both before and after this patch.

- Eric