linux-kernel - Re: [PATCH] drm/i915: fix build issue when using clang

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Date:   Sun, 13 Feb 2022 11:39:23 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Tong Zhang <ztong0001@...il.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] drm/i915: fix build issue when using clang

On Sat, Feb 12, 2022 at 10:51:06PM -0800, Tong Zhang wrote:
> drm/i915 target adds some extra cflags, especially it does re-apply -Wall.
> In clang this will override -Wno-format-security and cause the build to
> fail when CONFIG_DRM_I915_WERROR=y. While with GCC this does not happen.
> We reapply -Wno-format-security here to get around this issue.

For what it's worth, GCC would warn in the exact same way if
-Wformat-security was included within -Wall like it is for clang:

drivers/gpu/drm/i915/gt/intel_gt.c: In function ‘intel_gt_invalidate_tlbs’:
drivers/gpu/drm/i915/gt/intel_gt.c:988:9: error: format not a string literal and no format arguments [-Werror=format-security]
  988 |         GEM_TRACE("\n");
      |         ^~~~~~~~~
cc1: all warnings being treated as errors

drivers/gpu/drm/i915/gt/selftest_execlists.c: In function ‘live_sanitycheck’:
drivers/gpu/drm/i915/gt/selftest_execlists.c:142:25: error: format not a string literal and no format arguments [-Werror=format-security]
  142 |                         GEM_TRACE("spinner failed to start\n");
      |                         ^~~~~~~~~
drivers/gpu/drm/i915/gt/selftest_execlists.c: In function ‘live_preempt’:
drivers/gpu/drm/i915/gt/selftest_execlists.c:1775:25: error: format not a string literal and no format arguments [-Werror=format-security]
 1775 |                         GEM_TRACE("lo spinner failed to start\n");
      |                         ^~~~~~~~~
drivers/gpu/drm/i915/gt/selftest_execlists.c:1792:25: error: format not a string literal and no format arguments [-Werror=format-security]
 1792 |                         GEM_TRACE("hi spinner failed to start\n");
      |                         ^~~~~~~~~
cc1: all warnings being treated as errors

If fixing these warnings instead of just disabling the warning is
undesirable (since I feel like the entire point of the i195 cflags
situation is to enable more warnings than the main Makefile), I think
the commit message should be rewritten to something along the lines of:

"drm/i915 adds some extra cflags, namely -Wall, which causes
instances of -Wformat-security to appear when building with clang, even
though this warning is turned off kernel-wide in the main Makefile:"

> drivers/gpu/drm/i915/gt/intel_gt.c:983:2: error: format string is not a string literal (potentially insecure) [-Werror,-Wformat-security]
>         GEM_TRACE("ERROR\n");
>         ^~~~~~~~~~~~~~~~~~~~
> ./drivers/gpu/drm/i915/i915_gem.h:76:24: note: expanded from macro 'GEM_TRACE'
>  #define GEM_TRACE(...) trace_printk(__VA_ARGS__)
>                        ^~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/kernel.h:369:3: note: expanded from macro 'trace_printk'
>                 do_trace_printk(fmt, ##__VA_ARGS__);    \
>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/kernel.h:383:30: note: expanded from macro 'do_trace_printk'
>                 __trace_bprintk(_THIS_IP_, trace_printk_fmt, ##args);   \
>                                            ^~~~~~~~~~~~~~~~
> drivers/gpu/drm/i915/gt/intel_gt.c:983:2: note: treat the string as an argument to avoid this

"This does not happen with GCC because it does not enable
-Wformat-security with -Wall. Disable -Wformat-security within the i915
Makefile so that these warnings do not show up with clang."

The actual diff itself looks fine to me.

Cheers,
Nathan

> Signed-off-by: Tong Zhang <ztong0001@...il.com>
> ---
>  drivers/gpu/drm/i915/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> index 1b62b9f65196..c04e05a3d39f 100644
> --- a/drivers/gpu/drm/i915/Makefile
> +++ b/drivers/gpu/drm/i915/Makefile
> @@ -13,6 +13,7 @@
>  # will most likely get a sudden build breakage... Hopefully we will fix
>  # new warnings before CI updates!
>  subdir-ccflags-y := -Wall -Wextra
> +subdir-ccflags-y += -Wno-format-security
>  subdir-ccflags-y += -Wno-unused-parameter
>  subdir-ccflags-y += -Wno-type-limits
>  subdir-ccflags-y += -Wno-missing-field-initializers
> -- 
> 2.25.1
>