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

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Message-Id: <20200220.100032.1712605490922863986.davem@davemloft.net>
Date:   Thu, 20 Feb 2020 10:00:32 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     keescook@...omium.org
Cc:     kuba@...nel.org, glider@...gle.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: core: Distribute switch variables for
 initialization

From: Kees Cook <keescook@...omium.org>
Date: Wed, 19 Feb 2020 22:23:04 -0800

> 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.
> 
> net/core/skbuff.c: In function ‘skb_checksum_setup_ip’:
> net/core/skbuff.c:4809:7: warning: statement will never be executed [-Wswitch-unreachable]
>  4809 |   int err;
>       |       ^~~
> 
> [1] https://bugs.llvm.org/show_bug.cgi?id=44916
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>

Applied.