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Date:   Wed, 2 Nov 2022 12:08:00 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Thomas Sailer <t.sailer@alumni.ethz.ch>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-hams@vger.kernel.org,
        netdev@vger.kernel.org, Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>,
        Sami Tolvanen <samitolvanen@google.com>, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev
Subject: Re: [PATCH] hamradio: baycom_epp: Fix return type of
 baycom_send_packet()
Message-ID: <202211021207.E41AE398@keescook>
References: <20221102160610.1186145-1-nathan@kernel.org>
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On Wed, Nov 02, 2022 at 09:06:10AM -0700, Nathan Chancellor wrote:
> With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
> indirect call targets are validated against the expected function
> pointer prototype to make sure the call target is valid to help mitigate
> ROP attacks. If they are not identical, there is a failure at run time,
> which manifests as either a kernel panic or thread getting killed. A
> proposed warning in clang aims to catch these at compile time, which
> reveals:
> 
>   drivers/net/hamradio/baycom_epp.c:1119:25: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
>           .ndo_start_xmit      = baycom_send_packet,
>                                 ^~~~~~~~~~~~~~~~~~
>   1 error generated.
> 
> ->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
> 'netdev_tx_t', not 'int'. Adjust the return type of baycom_send_packet()
> to match the prototype's to resolve the warning and CFI failure.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1750
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Heh, this one was already using the enum for the argument to "return",
too. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook