Messages in this thread | | | Date | Wed, 08 Feb 2012 19:42:58 +0000 | From | Pedro Alves <> | Subject | Re: [PATCH] ptrace: add ability to get clear_tid_address |
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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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