linux-kernel - Re: [PATCH 4/9] x86/alternative: Implement .retpoline

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Date:   Thu, 14 Oct 2021 12:05:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     x86@...nel.org, jpoimboe@...hat.com, linux-kernel@...r.kernel.org,
        alexei.starovoitov@...il.com, ndesaulniers@...gle.com
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

On Wed, Oct 13, 2021 at 05:12:13PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 13, 2021 at 03:38:27PM +0100, Andrew Cooper wrote:
> > On 13/10/2021 13:22, Peter Zijlstra wrote:

> > > +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> > > +{
> > > +	void (*target)(void);
> > > +	int reg, i = 0;
> > > +
> > > +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> > > +		return -1;
> > > +
> > > +	target = addr + insn->length + insn->immediate.value;
> > > +	reg = (target - &__x86_indirect_thunk_rax) /
> > > +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> > 
> > This is equal measures beautiful and terrifying.
> 
> Thanks! :-)

Would something like this appease people? If the toolchain can mess this
up everything is broken.

That makes the symtab looks like:

(and arguably, that array symbol could be local)

...
35: 0000000000000000   512 NOTYPE  GLOBAL DEFAULT    4 __x86_indirect_thunk_array
36: 0000000000000000    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rax
37: 0000000000000020    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rcx
38: 0000000000000040    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rdx
39: 0000000000000060    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rbx
40: 0000000000000080    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rsp
41: 00000000000000a0    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rbp
42: 00000000000000c0    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rsi
43: 00000000000000e0    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_rdi
44: 0000000000000100    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r8
45: 0000000000000120    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r9
46: 0000000000000140    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r10
47: 0000000000000160    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r11
48: 0000000000000180    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r12
49: 00000000000001a0    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r13
50: 00000000000001c0    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r14
51: 00000000000001e0    17 FUNC    GLOBAL DEFAULT    4 __x86_indirect_thunk_r15


---
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -30,7 +30,7 @@
 
 	.align 32
 
-SYM_FUNC_START(__x86_indirect_thunk_\reg)
+SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL)
 
 	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
 		      __stringify(RETPOLINE \reg), X86_FEATURE_RETPOLINE, \
@@ -55,10 +55,16 @@ SYM_FUNC_END(__x86_indirect_thunk_\reg)
 #define __EXPORT_THUNK(sym)	_ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)
 #define EXPORT_THUNK(reg)	__EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
 
+	.align 32
+SYM_CODE_START(__x86_indirect_thunk_array)
+
 #define GEN(reg) THUNK reg
 #include <asm/GEN-for-each-reg.h>
 #undef GEN
 
+	.align 32
+SYM_CODE_END(__x86_indirect_thunk_array)
+
 #define GEN(reg) EXPORT_THUNK(reg)
 #include <asm/GEN-for-each-reg.h>
 #undef GEN