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    SubjectRe: [PATCH] ptrace: add ability to get clear_tid_address
    On 02/08/2012 07:02 PM, Oleg Nesterov wrote:
    > On 02/08, Pedro Alves wrote:
    >>
    >> On 02/08/2012 05:31 PM, Oleg Nesterov wrote:
    >>> On 02/08, Pedro Alves wrote:
    >>>>
    >>>> I just tried it. This is &pthread->tid in glibc/libpthread, so with debug
    >>>> info it's easy to figure out where to set the watchpoint manually with gdb
    >>>> without asking the kernel. Doesn't work. ptrace doesn't show any trap
    >>>> for the kernel writes.
    >>>
    >>> The tracee simply can't report this trap. it is already dead ;) and
    >>> hw breakpoint (used by ptrace) is "pinned" to the thread.
    >>
    >> Right, as I said. :-) I saw that a watchpoint trap isn't reported either
    >> for the CLONE_CHILD_SETTID case (that is, within clone, when the kernel
    >> writes the tid to the memory address passed in to the clone syscall).
    >
    > Yes. But in this case the new thread has no bps even if it is auto-
    > attached.

    Ah, right. It used to be the kernel copied the debug registers from
    parent->child, but they're always cleared in the child nowadays
    (since 72f674d203cd230426437cdcf7dd6f681dad8b0d).

    >> I wouldn't have been surprised to see the trap in userspace in either
    >> the parent
    >
    > It would be just wrong. Please note that it is child, not parent, who
    > does the write.

    Okay, I didn't know which it was that touched the memory,
    hence the "either". Thanks. Paired with the
    we-now-clear-debug-regs-on-clone thing, it makes sense.

    --
    Pedro Alves


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