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.

Josh Poimboeuf committed 6 years ago

Commit f80ac981 parent 66801251 file · +2 −2Browse files

Showing the changes to include/linux (1 of 1 file). Show all

Unified · Split

include/linux/bitops.h+2−2

@@ -162,7 +162,7 @@ static inline __u8 ror8(__u8 word, unsigned int shift) · 161 unmodified lines
162	162	*
163	163	* This is safe to use for 16- and 8-bit types as well.
164	164	*/
165		−static inline __s32 sign_extend32(__u32 value, int index)
	165	+static __always_inline __s32 sign_extend32(__u32 value, int index)
166	166	{
167	167	__u8 shift = 31 - index;
168	168	return (__s32)(value << shift) >> shift;
@@ -173,7 +173,7 @@ static inline __s32 sign_extend32(__u32 value, int index) · 4 unmodified lines
173	173	* @value: value to sign extend
174	174	* @index: 0 based bit index (0<=index<64) to sign bit
175	175	*/
176		−static inline __s64 sign_extend64(__u64 value, int index)
	176	+static __always_inline __s64 sign_extend64(__u64 value, int index)
177	177	{
178	178	__u8 shift = 63 - index;
179	179	return (__s64)(value << shift) >> shift;