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SubjectRe: [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
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