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SubjectRe: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions
On Fri, Jun 07, 2019 at 05:41:42AM +0000, Nadav Amit wrote:

> > int poke_int3_handler(struct pt_regs *regs)
> > {
> > + long ip = regs->ip - INT3_INSN_SIZE + CALL_INSN_SIZE;
> > + struct opcode {
> > + u8 insn;
> > + s32 rel;
> > + } __packed opcode;
> > +
> > /*
> > * Having observed our INT3 instruction, we now must observe
> > * bp_patching_in_progress.
> > *
> > - * in_progress = TRUE INT3
> > - * WMB RMB
> > - * write INT3 if (in_progress)
> > + * in_progress = TRUE INT3
> > + * WMB RMB
> > + * write INT3 if (in_progress)
>
> I don’t see what has changed in this chunk… Whitespaces?

Yep, my editor kept marking that stuff red (space before tab), which
annoyed me enough to fix it.

> > *
> > - * Idem for bp_int3_handler.
> > + * Idem for bp_int3_opcode.
> > */
> > smp_rmb();
> >
> > @@ -943,8 +949,21 @@ int poke_int3_handler(struct pt_regs *re
> > if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
> > return 0;
> >
> > - /* set up the specified breakpoint handler */
> > - regs->ip = (unsigned long) bp_int3_handler;
> > + opcode = *(struct opcode *)bp_int3_opcode;
> > +
> > + switch (opcode.insn) {
> > + case 0xE8: /* CALL */
> > + int3_emulate_call(regs, ip + opcode.rel);
> > + break;
> > +
> > + case 0xE9: /* JMP */
> > + int3_emulate_jmp(regs, ip + opcode.rel);
> > + break;
>
> Consider using RELATIVECALL_OPCODE and RELATIVEJUMP_OPCODE instead of the
> constants (0xE8, 0xE9), just as you do later in the patch.

Those are private to kprobes..

but I can do something like so:

--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -48,8 +48,14 @@ static inline void int3_emulate_jmp(stru
regs->ip = ip;
}

-#define INT3_INSN_SIZE 1
-#define CALL_INSN_SIZE 5
+#define INT3_INSN_SIZE 1
+#define INT3_INSN_OPCODE 0xCC
+
+#define CALL_INSN_SIZE 5
+#define CALL_INSN_OPCODE 0xE8
+
+#define JMP_INSN_SIZE 5
+#define JMP_INSN_OPCODE 0xE9

static inline void int3_emulate_push(struct pt_regs *regs, unsigned long val)
{
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -952,11 +952,11 @@ int poke_int3_handler(struct pt_regs *re
opcode = *(struct opcode *)bp_int3_opcode;

switch (opcode.insn) {
- case 0xE8: /* CALL */
+ case CALL_INSN_OPCODE:
int3_emulate_call(regs, ip + opcode.rel);
break;

- case 0xE9: /* JMP */
+ case JMP_INSN_OPCODE:
int3_emulate_jmp(regs, ip + opcode.rel);
break;

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