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

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Date:   Mon, 2 Mar 2020 20:41:34 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>
Cc:     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 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);
> 

-- 
Kees Cook