Messages in this thread | | | Date | Thu, 3 Oct 2019 10:27:51 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/3] x86/alternatives: Teach text_poke_bp() to emulate instructions |
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On Thu, Oct 03, 2019 at 02:00:50PM +0900, Masami Hiramatsu wrote:
> > This fits almost all text_poke_bp() users, except > > arch_unoptimize_kprobe() which restores random text, and for that site > > we have to build an explicit emulate instruction. > > OK, and in this case, I would like to change RELATIVEJUMP_OPCODE > to JMP32_INSN_OPCODE for readability. (or at least > making RELATIVEJUMP_OPCODE as an alias of JMP32_INSN_OPCODE)
> > @@ -448,12 +447,18 @@ void arch_optimize_kprobes(struct list_h > > void arch_unoptimize_kprobe(struct optimized_kprobe *op) > > { > > u8 insn_buff[RELATIVEJUMP_SIZE]; > > + u8 emulate_buff[RELATIVEJUMP_SIZE]; > > > > /* Set int3 to first byte for kprobes */ > > insn_buff[0] = BREAKPOINT_INSTRUCTION; > > memcpy(insn_buff + 1, op->optinsn.copied_insn, RELATIVE_ADDR_SIZE); > > + > > + emulate_buff[0] = RELATIVEJUMP_OPCODE; > > + *(s32 *)(&emulate_buff[1]) = (s32)((long)op->optinsn.insn - > > + ((long)op->kp.addr + RELATIVEJUMP_SIZE));
I'm halfway through a patch introducing:
union text_poke_insn { u8 code[POKE_MAX_OPCODE_SUZE]; struct { u8 opcode; s32 disp; } __attribute__((packed)); };
to text-patching.h to unify all such custom unions we have all over the place. I'll mob up the above in that.
> > + > > text_poke_bp(op->kp.addr, insn_buff, RELATIVEJUMP_SIZE, > > - op->optinsn.insn); > > + emulate_buff); > > }
As argued in a previous thread, text_poke_bp() is broken when it changes more than a single instruction at a time.
Now, ISTR optimized kprobes does something like:
poke INT3 synchronize_rcu_tasks() /* waits for all tasks to schedule guarantees instructions after INT3 are unused */ install optimized probe /* overwrites multiple instrctions with JMP.d32 */
And the above then undoes that by:
poke INT3 on top of the optimzed probe
poke tail instructions back /* guaranteed safe because the above INT3 poke ensures the JMP.d32 instruction is unused */
poke head byte back
Is this correct? If so, we should probably put a comment in there explaining how all this is unusual but safe.
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