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    SubjectRe: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support
    (2013/11/11 19:58), Will Deacon wrote:
    > On Mon, Nov 11, 2013 at 10:51:52AM +0000, Masami Hiramatsu wrote:
    >> (2013/11/11 16:54), Masami Hiramatsu wrote:
    >>>>>> In fact, how do you avoid a race with hardware breakpoints? E.g., somebody
    >>>>>> places a hardware breakpoint on an instruction in the kernel for which
    >>>>>> kprobes has patched in a brk. We take the hardware breakpoint, disable the
    >>>>>> breakpoint and set up a single step before returning to the brk. The brk
    >>>>>> then traps, but we must take care not to disable single-step and/or unmask
    >>>>>> debug exceptions, because that will cause the hardware breakpoint code to
    >>>>>> re-arm its breakpoint before we've stepped off the brk instruction.
    >>>>>
    >>>>> Hmm, frankly to say, this kind of race issue is not seriously discussed
    >>>>> on x86 too, since kgdb is still a special tool (not used on the production
    >>>>> system).
    >>>>> I think under such situation kgdb operator must have full control of the
    >>>>> system, and he can (and has to) avoid such kind of race.
    >>>> Masami,
    >>>>
    >>>> Hmm I think in same lines, but not sure if we expect kprobes to be
    >>>> able to work fool-proof along with kgdb or hw breakpoints ?
    >>>
    >>> For hw breakpoint, yes, we finally get check each other to safely
    >>> use it even if one rejects the other one at some points(address).
    >>> Since the hw breakpoint is already open for normal user via perf,
    >>> we should do it. But the policy still needs to be discussed.
    >>
    >> OK, I've ensured that the hw_breakpoint (from perf) can work
    >> with kprobes (from ftrace) at the same address on x86.
    >> So if arm64 already support hw_breakpoint on perf, kprobes should
    >> work with it.
    >
    > Single-stepping on x86 is different to the step behaviour on arm64 afaik. On
    > ARM, we have to manually remove the breakpoint, perform a single-step, then
    > add the breakpoint again. If we re-enable debug exceptions in the kprobe
    > handler, the step will complete early and we'll never step off the
    > breakpoint.

    I'm unsure about arm64's debug feature behavior, what does happen when
    it performs a single-step on sw-breakpoint?

    > Sandeepa: I think you need to retry Masami's test on the arm64 model, since
    > I'm fairly sure it won't work as expected without some additional code.

    OK, anyway, for testing same one, we need to port ftrace first. So the next
    plan is to make a kprobe module to put a probe (which just printk something)
    on a specific function (e.g. vfs_symlink), and run perf record with
    hw-breakpoint as below

    $ perf record -e "mem:0xXXXXXX:k" ln -s /dev/null /tmp/foo

    Note that 0xXXXXXX is the address of vfs_symlink.

    After that, you can see the message in dmesg and also check the perf result
    with "sudo perf script --dump" (you can find a PERF_RECORD_SAMPLE entry if
    it works)

    Thank you,

    --
    Masami HIRAMATSU
    IT Management Research Dept. Linux Technology Center
    Hitachi, Ltd., Yokohama Research Laboratory
    E-mail: masami.hiramatsu.pt@hitachi.com




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