linux-kernel - Re: [PATCH] drm/meson: Fix return type of meson_encoder_cvbs_mode

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Date:   Wed, 2 Nov 2022 12:06:34 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Neil Armstrong <neil.armstrong@...aro.org>,
        dri-devel@...ts.freedesktop.org, linux-amlogic@...ts.infradead.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Sami Tolvanen <samitolvanen@...gle.com>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] drm/meson: Fix return type of
 meson_encoder_cvbs_mode_valid()

On Wed, Nov 02, 2022 at 08:52:42AM -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/gpu/drm/meson/meson_encoder_cvbs.c:211:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_bridge *, const struct drm_display_info *, const struct drm_display_mode *)' with an expression of type 'int (struct drm_bridge *, const struct drm_display_info *, const struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
>           .mode_valid = meson_encoder_cvbs_mode_valid,
>                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   1 error generated.
> 
> ->mode_valid() in 'struct drm_bridge_funcs' expects a return type of
> 'enum drm_mode_status', not 'int'. Adjust the return type of
> meson_encoder_cvbs_mode_valid() to match the prototype's to resolve the
> warning and CFI failure.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1750
> Reported-by: Sami Tolvanen <samitolvanen@...gle.com>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

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

-- 
Kees Cook