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Re: kprobes vs __ex_table[]

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On Thu, 23 Feb 2017 19:30:02 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> Hi Masami,
> 
> I just wondered what would happen if I put a probe on an instruction
> that was listed in __ex_table[] or __bug_table[].

Ah, thanks for reporting, I know __ex_table issue and fixed, but
I didn't care about __bug_table.

> And it looks like it will happily do that. It will then run the
> instruction out-of-line, and when said instruction traps, the
> instruction address will not match the one listed in either __ex_table[]
> or __bug_table[] and badness will happen.

For the __ex_table[], at least on x86, kprobes already handles it in
kprobe_fault_handler, which restore regs->ip to original place when
a pagefault happens on singlestepping.

> If kprobes does indeed not check this, we should probably fix it, if it
> does do check this, could you point me to it?

Yeah, for BUG() case, as far as I can see, there is no check about that.
So, there are 2 ways to fix it up, one is to just reject to put kprobes on
UD2, another is fixup trap address as we did for exceptions_table.
I think latter is better because if there is a divide error happening
on single-step, anyway we should fixup the address...

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>

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Thread

kprobes vs __ex_table[] Peter Zijlstra <peterz@infradead.org> - 2017-02-23 19:40 +0100
  Re: kprobes vs __ex_table[] Masami Hiramatsu <mhiramat@kernel.org> - 2017-02-24 02:10 +0100
    Re: kprobes vs __ex_table[] Peter Zijlstra <peterz@infradead.org> - 2017-02-24 10:30 +0100
      Re: kprobes vs __ex_table[] Masami Hiramatsu <mhiramat@kernel.org> - 2017-02-24 17:40 +0100
        Re: kprobes vs __ex_table[] Peter Zijlstra <peterz@infradead.org> - 2017-02-24 18:50 +0100
          [RFC PATCH 1/2] kprobes/x86: Use probe_kernel_read instead of memcpy Masami Hiramatsu <mhiramat@kernel.org> - 2017-02-27 17:20 +0100
          [RFC PATCH 2/2] kprobes/x86: Exit single-stepping before trying fixup_exception Masami Hiramatsu <mhiramat@kernel.org> - 2017-02-27 17:20 +0100
          [RFC PATCH 0/2] kprobes/x86: Handle probing on ex_table cases Masami Hiramatsu <mhiramat@kernel.org> - 2017-02-27 17:20 +0100
      Re: kprobes vs __ex_table[] Masami Hiramatsu <mhiramat@kernel.org> - 2017-02-28 17:30 +0100
        [PATCH] [BUGFIX] kprobes/x86: Fix to check __ex_table entry by probed address Masami Hiramatsu <mhiramat@kernel.org> - 2017-02-28 18:50 +0100

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