[PATCH v2] x86: bitops: fix build regression

[Nick Desaulniers]

From: Ilie Halip <ilie.halip@xxxxxxxxx>

It turns out that if your config tickles __builtin_constant_p via
differences in choices to inline or not, this now produces invalid
assembly:

$ cat foo.c
long a(long b, long c) {
  asm("orb\t%1, %0" : "+q"(c): "r"(b));
  return c;
}
$ gcc foo.c
foo.c: Assembler messages:
foo.c:2: Error: `%rax' not allowed with `orb'

The "q" constraint only has meanting on -m32 otherwise is treated as
"r".

This is easily reproducible via Clang+CONFIG_STAGING=y+CONFIG_VT6656=m.

Without the cast to u8, gcc and clang will not select low-8-bit
registers required for the `b` suffix on `orb` and `andb`, which results
in an assembler error.  Without the mask, sparse will warn about the
upper bytes of the value being truncated.

 [Nick: reworded]

Cc: Jesse Brandeburg <jesse.brandeburg@xxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/961
Link: https://lore.kernel.org/lkml/20200504193524.GA221287@xxxxxxxxxx/
Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved cast")
Reported-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Reported-by: kernelci.org bot <bot@xxxxxxxxxxxx>
Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
Suggested-by: Ilie Halip <ilie.halip@xxxxxxxxx>
Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Signed-off-by: Ilie Halip <ilie.halip@xxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Changes V1 -> V2:
* change authorship/signed-off-by to Ilie
* add Nathan's Tested by/reviewed by
* update commit message slightly with info sent to HPA.

 arch/x86/include/asm/bitops.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index b392571c1f1d..139122e5b25b 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
 	if (__builtin_constant_p(nr)) {
 		asm volatile(LOCK_PREFIX "orb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" (CONST_MASK(nr) & 0xff)
+			: "iq" ((u8)(CONST_MASK(nr) & 0xff))
 			: "memory");
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
@@ -74,7 +74,7 @@ arch_clear_bit(long nr, volatile unsigned long *addr)
 	if (__builtin_constant_p(nr)) {
 		asm volatile(LOCK_PREFIX "andb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" (CONST_MASK(nr) ^ 0xff));
+			: "iq" ((u8)(CONST_MASK(nr) ^ 0xff)));
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
 			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");
-- 
2.26.2.526.g744177e7f7-goog