linux-kernel - [PATCH] x86/extable: prefer local labels in .set directives

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Date:   Tue, 29 Mar 2022 13:21:45 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Bernardo Meurer Costa <beme@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] x86/extable: prefer local labels in .set directives

Bernardo reported an error that Nathan bisected down to
(x86_64) defconfig+LTO_CLANG_FULL+X86_PMEM_LEGACY.

    LTO     vmlinux.o
  ld.lld: error: <instantiation>:1:13: redefinition of 'found'
  .set found, 0
              ^

  <inline asm>:29:1: while in macro instantiation
  extable_type_reg reg=%eax, type=(17 | ((0) << 16))
  ^

This appears to be another LTO specific issue similar to what was folded
into commit 4b5305decc84 ("x86/extable: Extend extable functionality"),
where the `.set found, 0` in DEFINE_EXTABLE_TYPE_REG in
arch/x86/include/asm/asm.h conflicts with the symbol for the static
function `found` in arch/x86/kernel/pmem.c.

Assembler .set directive declare symbols with global visibility, so the
assembler may not rename such symbols in the event of a conflict. LTO
could rename static functions if there was a conflict in C sources, but
it cannot see into symbols defined in inline asm.

The symbols are also retained in the symbol table, regardless of LTO.

Give the symbols .L prefixes making them locally visible, so that they
may be renamed for LTO to avoid conflicts, and to drop them from the
symbol table regardless of LTO.

Fixes: 4b5305decc84 ("x86/extable: Extend extable functionality")
Link: https://github.com/ClangBuiltLinux/linux/issues/1612
Link: https://sourceware.org/binutils/docs/as/Symbol-Names.html#Local-Symbol-Names
Reported-by: Bernardo Meurer Costa <beme@...gle.com>
Debugged-by: Nathan Chancellor <nathan@...nel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 arch/x86/include/asm/asm.h | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
index c878fed3056f..fbcfec4dc4cc 100644
--- a/arch/x86/include/asm/asm.h
+++ b/arch/x86/include/asm/asm.h
@@ -154,24 +154,24 @@
 
 # define DEFINE_EXTABLE_TYPE_REG \
 	".macro extable_type_reg type:req reg:req\n"						\
-	".set found, 0\n"									\
-	".set regnr, 0\n"									\
+	".set .Lfound, 0\n"									\
+	".set .Lregnr, 0\n"									\
 	".irp rs,rax,rcx,rdx,rbx,rsp,rbp,rsi,rdi,r8,r9,r10,r11,r12,r13,r14,r15\n"		\
 	".ifc \\reg, %%\\rs\n"									\
-	".set found, found+1\n"									\
-	".long \\type + (regnr << 8)\n"								\
+	".set .Lfound, .Lfound+1\n"								\
+	".long \\type + (.Lregnr << 8)\n"							\
 	".endif\n"										\
-	".set regnr, regnr+1\n"									\
+	".set .Lregnr, .Lregnr+1\n"								\
 	".endr\n"										\
-	".set regnr, 0\n"									\
+	".set .Lregnr, 0\n"									\
 	".irp rs,eax,ecx,edx,ebx,esp,ebp,esi,edi,r8d,r9d,r10d,r11d,r12d,r13d,r14d,r15d\n"	\
 	".ifc \\reg, %%\\rs\n"									\
-	".set found, found+1\n"									\
-	".long \\type + (regnr << 8)\n"								\
+	".set .Lfound, .Lfound+1\n"								\
+	".long \\type + (.Lregnr << 8)\n"							\
 	".endif\n"										\
-	".set regnr, regnr+1\n"									\
+	".set .Lregnr, .Lregnr+1\n"								\
 	".endr\n"										\
-	".if (found != 1)\n"									\
+	".if (.Lfound != 1)\n"									\
 	".error \"extable_type_reg: bad register argument\"\n"					\
 	".endif\n"										\
 	".endm\n"
-- 
2.35.1.1021.g381101b075-goog