Messages in this thread | | | Date | Wed, 8 Feb 2012 20:02:50 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] ptrace: add ability to get clear_tid_address |
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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.
IOW, I think that hw bp can detect the write from the kernel space, but I didn't check.
> 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.
If only I understood why do we need CLONE_CHILD_SETTID... at least I certainly do not understand why glibc translates fork() into clone(CLONE_CHILD_SETTID) on my system. The child write into its memory, the parent can't see this change. IIRC, initially CLONE_CHILD_SETTID wrote child->pid into the parent's memory, and even before the child was actually created.
Oleg.
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