linux-kernel - [PATCH v2] x86: bitops: fix build regression

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Message-Id: <20200506170554.54635-1-ndesaulniers@google.com>
Date:   Wed,  6 May 2020 10:05:53 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     Ilie Halip <ilie.halip@...il.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>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Daniel Axtens <dja@...ens.net>,
        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 v2] x86: bitops: fix build regression

From: Ilie Halip <ilie.halip@...il.com>

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@...el.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/961
Link: https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/
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: Ilie Halip <ilie.halip@...il.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>
Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Signed-off-by: Ilie Halip <ilie.halip@...il.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
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