linux-kernel - [PATCH 5.4 04/18] drm/i915: Cast remain to unsigned long in eb_relocate

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Date:   Fri, 13 May 2022 16:23:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Michel Dänzer <michel@...nzer.net>,
        Jani Nikula <jani.nikula@...el.com>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH 5.4 04/18] drm/i915: Cast remain to unsigned long in eb_relocate_vma

From: Nathan Chancellor <natechancellor@...il.com>

commit 7bf03e7504e433da274963c447648876902b86df upstream.

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.

Closes: https://github.com/ClangBuiltLinux/linux/issues/778
Suggested-by: Michel Dänzer <michel@...nzer.net>
Reviewed-by: Jani Nikula <jani.nikula@...el.com>
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
Signed-off-by: Jani Nikula <jani.nikula@...el.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200214054706.33870-1-natechancellor@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -1452,7 +1452,7 @@ static int eb_relocate_vma(struct i915_e
 
 	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;
 
 	/*