linux-kernel - Re: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate

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Message-ID: <87v9o965gg.fsf@intel.com>
Date:   Fri, 14 Feb 2020 08:36:15 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate_vma

On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@...il.com> wrote:
> A recent commit in clang added -Wtautological-compare to -Wall, which is
> enabled for i915 after -Wtautological-compare is disabled for the rest
> of the kernel so we see the following warning on x86_64:
>
>  ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: warning:
>  result of comparison of constant 576460752303423487 with expression of
>  type 'unsigned int' is always false
>  [-Wtautological-constant-out-of-range-compare]
>          if (unlikely(remain > N_RELOC(ULONG_MAX)))
>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
>  ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely'
>  # define unlikely(x)    __builtin_expect(!!(x), 0)
>                                             ^
>  1 warning generated.
>
> It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not
> account for the case where this file is built for 32-bit x86, where
> ULONG_MAX == UINT_MAX and this check is still relevant.
>
> Cast remain to unsigned long, which keeps the generated code the same
> (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and
> the warning is silenced so we can catch more potential issues in the
> future.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/778
> Suggested-by: Michel Dänzer <michel@...nzer.net>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Works for me as a workaround,

Reviewed-by: Jani Nikula <jani.nikula@...el.com>


> ---
>
> Round 3 :)
>
> Previous threads/patches:
>
> https://lore.kernel.org/lkml/20191123195321.41305-1-natechancellor@gmail.com/
> https://lore.kernel.org/lkml/20200211050808.29463-1-natechancellor@gmail.com/
>
>  drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index 60c984e10c4a..47f4d8ab281e 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -1430,7 +1430,7 @@ static int eb_relocate_vma(struct i915_execbuffer *eb, struct i915_vma *vma)
>  
>  	urelocs = u64_to_user_ptr(entry->relocs_ptr);
>  	remain = entry->relocation_count;
> -	if (unlikely(remain > N_RELOC(ULONG_MAX)))
> +	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
>  		return -EINVAL;
>  
>  	/*

-- 
Jani Nikula, Intel Open Source Graphics Center