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From: Nick Desaulniers <ndesaulniers@google.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>
Cc: Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
	Nick Desaulniers <ndesaulniers@google.com>,
	Arnd Bergmann <arnd@arndb.de>,
	David Woodhouse <dwmw2@infradead.org>,
	Dmitry Golovin <dima@golovin.in>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH v3 11/11] x86: support i386 with Clang
Date: Mon, 20 Jul 2020 13:49:25 -0700	[thread overview]
Message-ID: <20200720204925.3654302-12-ndesaulniers@google.com> (raw)
In-Reply-To: <20200720204925.3654302-1-ndesaulniers@google.com>

GCC and Clang are architecturally different, which leads to subtle
issues for code that's invalid but clearly dead. This can happen with
code that emulates polymorphism with the preprocessor and sizeof.

GCC will perform semantic analysis after early inlining and dead code
elimination, so it will not warn on invalid code that's dead. Clang
strictly performs optimizations after semantic analysis, so it will warn
for dead code.

Neither Clang nor GCC like this very much with -m32:

long long ret;
asm ("movb $5, %0" : "=q" (ret));

However, GCC can tolerate this variant:

long long ret;
switch (sizeof(ret)) {
case 1:
        asm ("movb $5, %0" : "=q" (ret));
        break;
case 8:;
}

Clang, on the other hand, won't accept that because it validates the
inline asm for the '1' case *before* the optimisation phase where it
realises that it wouldn't have to emit it anyway.

If LLVM (Clang's "back end") fails such as during instruction selection
or register allocation, it cannot provide accurate diagnostics
(warnings/errors) that contain line information, as the AST has been
discarded from memory at that point.

While there have been early discussions about having C/C++ specific
language optimizations in Clang via the use of MLIR, which would enable
such earlier optimizations, such work is not scoped and likely a
multi-year endeavor.

We also don't want to swap the use of "=q" with "=r". For 64b, it
doesn't matter. For 32b, it's possible that a 32b register without a 8b
lower alias (i.e. ESI, EDI, EBP) is selected which the assembler will
then reject.

With this, Clang can finally build an i386 defconfig.

Link: https://bugs.llvm.org/show_bug.cgi?id=33587
Link: https://github.com/ClangBuiltLinux/linux/issues/3
Link: https://github.com/ClangBuiltLinux/linux/issues/194
Link: https://github.com/ClangBuiltLinux/linux/issues/781
Link: https://lore.kernel.org/lkml/20180209161833.4605-1-dwmw2@infradead.org/
Link: https://lore.kernel.org/lkml/CAK8P3a1EBaWdbAEzirFDSgHVJMtWjuNt2HGG8z+vpXeNHwETFQ@mail.gmail.com/
Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: David Woodhouse <dwmw2@infradead.org>
Reported-by: Dmitry Golovin <dima@golovin.in>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 arch/x86/include/asm/uaccess.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index dd3261f9f4ea..9d57556ad42f 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -314,11 +314,13 @@ do {									\
 
 #define __get_user_size(x, ptr, size, retval)				\
 do {									\
+	unsigned char x_u8__;						\
 	retval = 0;							\
 	__chk_user_ptr(ptr);						\
 	switch (size) {							\
 	case 1:								\
-		__get_user_asm(x, ptr, retval, "b", "=q");		\
+		__get_user_asm(x_u8__, ptr, retval, "b", "=q");		\
+		(x) = x_u8__;						\
 		break;							\
 	case 2:								\
 		__get_user_asm(x, ptr, retval, "w", "=r");		\
-- 
2.28.0.rc0.105.gf9edc3c819-goog


  parent reply	other threads:[~2020-07-20 20:50 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-20 20:49 [PATCH v3 00/11] i386 Clang support Nick Desaulniers
2020-07-20 20:49 ` [PATCH v3 01/11] x86/percpu: Introduce size abstraction macros Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 02/11] x86/percpu: Clean up percpu_to_op() Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 03/11] x86/percpu: Clean up percpu_from_op() Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 04/11] x86/percpu: Clean up percpu_add_op() Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 05/11] x86/percpu: Remove "e" constraint from XADD Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 06/11] x86/percpu: Clean up percpu_add_return_op() Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 07/11] x86/percpu: Clean up percpu_xchg_op() Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 08/11] x86/percpu: Clean up percpu_cmpxchg_op() Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 09/11] x86/percpu: Clean up percpu_stable_op() Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` [PATCH v3 10/11] x86/percpu: Remove unused PER_CPU() macro Nick Desaulniers
2020-07-23 10:43   ` [tip: x86/asm] " tip-bot2 for Brian Gerst
2020-07-20 20:49 ` Nick Desaulniers [this message]
2020-07-23  9:14   ` [PATCH v3 11/11] x86: support i386 with Clang Thomas Gleixner
2020-07-23  9:17     ` Thomas Gleixner
2020-07-23 11:06       ` Sedat Dilek
2020-07-23 11:42         ` Arnd Bergmann
2020-07-23 13:14           ` Sedat Dilek
2020-07-23 13:55             ` Arnd Bergmann
2020-07-24 13:22               ` Sedat Dilek
2020-07-23 10:43   ` [tip: x86/asm] x86/uaccess: Make __get_user_size() Clang compliant on 32-bit tip-bot2 for Nick Desaulniers
2020-07-21  8:09 ` [PATCH v3 00/11] i386 Clang support Sedat Dilek
2020-07-21 22:27 ` Dennis Zhou
2020-07-22 23:08   ` Thomas Gleixner
2020-07-22 23:25     ` Dennis Zhou

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