linux-kernel - [PATCH] bitops: Always inline sign extension helpers

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Message-Id: <740179324b2b18b750b16295c48357f00b5fa9ed.1582982020.git.jpoimboe@redhat.com>
Date:   Sat, 29 Feb 2020 07:15:26 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Randy Dunlap <rdunlap@...radead.org>,
        intel-gfx@...ts.freedesktop.org
Subject: [PATCH] bitops: Always inline sign extension helpers

With CONFIG_CC_OPTIMIZE_FOR_SIZE, objtool reports:

  drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: i915_gem_execbuffer2_ioctl()+0x5b7: call to gen8_canonical_addr() with UACCESS enabled

This means i915_gem_execbuffer2_ioctl() is calling gen8_canonical_addr()
from the user_access_begin/end critical region (i.e, with SMAP
disabled).

While it's probably harmless in this case, in general we like to avoid
extra function calls in SMAP-disabled regions because it can open up
inadvertent security holes.

Fix the warning by changing the sign extension helpers to
__always_inline.  This convinces GCC to inline gen8_canonical_addr().

The sign extension functions are trivial anyway, so it makes sense to
always inline them.  With my test optimize-for-size-based config, this
actually shrinks the text size of i915_gem_execbuffer.o by 45 bytes --
and no change for vmlinux.

Reported-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 include/linux/bitops.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 47f54b459c26..9acf654f0b19 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -162,7 +162,7 @@ static inline __u8 ror8(__u8 word, unsigned int shift)
  *
  * This is safe to use for 16- and 8-bit types as well.
  */
-static inline __s32 sign_extend32(__u32 value, int index)
+static __always_inline __s32 sign_extend32(__u32 value, int index)
 {
 	__u8 shift = 31 - index;
 	return (__s32)(value << shift) >> shift;
@@ -173,7 +173,7 @@ static inline __s32 sign_extend32(__u32 value, int index)
  * @value: value to sign extend
  * @index: 0 based bit index (0<=index<64) to sign bit
  */
-static inline __s64 sign_extend64(__u64 value, int index)
+static __always_inline __s64 sign_extend64(__u64 value, int index)
 {
 	__u8 shift = 63 - index;
 	return (__s64)(value << shift) >> shift;
-- 
2.21.1