linux-kernel - [PATCH v4] x86: bitops: fix build regression

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Date:   Fri,  8 May 2020 11:28:35 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        "kernelci . org bot" <bot@...nelci.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Brian Gerst <brgerst@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Ilie Halip <ilie.halip@...il.com>, x86@...nel.org,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH v4] x86: bitops: fix build regression

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

It turns out that if your config tickles __builtin_constant_p via
differences in choices to inline or not, these statements produce
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'

Use the `%b` "x86 Operand Modifier" to instead force register allocation
to select a lower-8-bit GPR operand.

The "q" constraint only has meaning on -m32 otherwise is treated as
"r". Not all GPRs have low-8-bit aliases for -m32.

Cc: Jesse Brandeburg <jesse.brandeburg@...el.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/961
Link: https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/
Link: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers
Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved cast")
Reported-by: Sedat Dilek <sedat.dilek@...il.com>
Reported-by: kernelci.org bot <bot@...nelci.org>
Suggested-by: Andy Shevchenko <andriy.shevchenko@...el.com>
Suggested-by: Brian Gerst <brgerst@...il.com>
Suggested-by: H. Peter Anvin <hpa@...or.com>
Suggested-by: Ilie Halip <ilie.halip@...il.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Changes V3 -> V4:
* drop (u8) cast from arch_change_bit() as well.

Changes V2 -> V3:
* use `%b` "x86 Operand Modifier" instead of bitwise op then cast.
* reword commit message.
* add Brian and HPA suggested by tags
* drop Nathan & Sedat Tested by/reviewed by tags (new patch is different
  enough).
* Take over authorship.

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 | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index b392571c1f1d..8a8b7bb9677b 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -52,9 +52,9 @@ static __always_inline void
 arch_set_bit(long nr, volatile unsigned long *addr)
 {
 	if (__builtin_constant_p(nr)) {
-		asm volatile(LOCK_PREFIX "orb %1,%0"
+		asm volatile(LOCK_PREFIX "orb %b1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" (CONST_MASK(nr) & 0xff)
+			: "iq" (CONST_MASK(nr))
 			: "memory");
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
@@ -72,9 +72,9 @@ static __always_inline void
 arch_clear_bit(long nr, volatile unsigned long *addr)
 {
 	if (__builtin_constant_p(nr)) {
-		asm volatile(LOCK_PREFIX "andb %1,%0"
+		asm volatile(LOCK_PREFIX "andb %b1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" (CONST_MASK(nr) ^ 0xff));
+			: "iq" (~CONST_MASK(nr)));
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
 			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");
@@ -125,7 +125,7 @@ arch_change_bit(long nr, volatile unsigned long *addr)
 	if (__builtin_constant_p(nr)) {
 		asm volatile(LOCK_PREFIX "xorb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" ((u8)CONST_MASK(nr)));
+			: "iq" (CONST_MASK(nr)));
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(btc) " %1,%0"
 			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");
-- 
2.26.2.645.ge9eca65c58-goog