linux-kernel - Re: [PATCH] fcntl: Distribute switch variables for initialization

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Date:   Tue, 3 Mar 2020 13:11:12 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alexander Potapenko <glider@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fcntl: Distribute switch variables for initialization

On Tue, Mar 03, 2020 at 10:55:22AM -0500, Jeff Layton wrote:
> On Mon, 2020-03-02 at 20:41 -0800, Kees Cook wrote:
> > On Wed, Feb 19, 2020 at 10:22:43PM -0800, Kees Cook wrote:
> > > Variables declared in a switch statement before any case statements
> > > cannot be automatically initialized with compiler instrumentation (as
> > > they are not part of any execution flow). With GCC's proposed automatic
> > > stack variable initialization feature, this triggers a warning (and they
> > > don't get initialized). Clang's automatic stack variable initialization
> > > (via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
> > > doesn't initialize such variables[1]. Note that these warnings (or silent
> > > skipping) happen before the dead-store elimination optimization phase,
> > > so even when the automatic initializations are later elided in favor of
> > > direct initializations, the warnings remain.
> > > 
> > > To avoid these problems, move such variables into the "case" where
> > > they're used or lift them up into the main function body.
> > > 
> > > fs/fcntl.c: In function ‘send_sigio_to_task’:
> > > fs/fcntl.c:738:20: warning: statement will never be executed [-Wswitch-unreachable]
> > >   738 |   kernel_siginfo_t si;
> > >       |                    ^~
> > > 
> > > [1] https://bugs.llvm.org/show_bug.cgi?id=44916
> > > 
> > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > 
> > Ping. Can someone pick this up, please?
> > 
> > Thanks!
> > 
> > -Kees
> > 
> > > ---
> > >  fs/fcntl.c |    6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/fcntl.c b/fs/fcntl.c
> > > index 9bc167562ee8..2e4c0fa2074b 100644
> > > --- a/fs/fcntl.c
> > > +++ b/fs/fcntl.c
> > > @@ -735,8 +735,9 @@ static void send_sigio_to_task(struct task_struct *p,
> > >  		return;
> > >  
> > >  	switch (signum) {
> > > -		kernel_siginfo_t si;
> > > -		default:
> > > +		default: {
> > > +			kernel_siginfo_t si;
> > > +
> > >  			/* Queue a rt signal with the appropriate fd as its
> > >  			   value.  We use SI_SIGIO as the source, not 
> > >  			   SI_KERNEL, since kernel signals always get 
> > > @@ -769,6 +770,7 @@ static void send_sigio_to_task(struct task_struct *p,
> > >  			si.si_fd    = fd;
> > >  			if (!do_send_sig_info(signum, &si, p, type))
> > >  				break;
> > > +		}
> > >  		/* fall-through - fall back on the old plain SIGIO signal */
> > >  		case 0:
> > >  			do_send_sig_info(SIGIO, SEND_SIG_PRIV, p, type);
> > > 
> 
> Sure, looks straightforward enough. I'll pick it up for v5.7.

Awesome; thank you!

-Kees

> 
> Thanks,
> -- 
> Jeff Layton <jlayton@...nel.org>
> 

-- 
Kees Cook