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    Subject[PATCH] fix for zeroed user-space tids in multi-threaded core dumps
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    Hi, Andrew.  Please consider the patch below for the next open 2.6 release.

    The NPTL library uses the CLONE_CHILD_CLEARTID flag on clone() syscalls
    on behalf of pthread_create() library calls. This feature is used to
    request that the kernel clear the thread-id in user space (at an address
    provided in the syscall) when the thread disassociates itself from the
    address space, which is done in mm_release().

    Unfortunately, when a multi-threaded process incurs a core dump (such as
    from a SIGSEGV), the core-dumping thread sends SIGKILL signals to all of
    the other threads, which then proceed to clear their user-space tids
    before synchronizing in exit_mm() with the start of core dumping. This
    misrepresents the state of process's address space at the time of the
    SIGSEGV and makes it more difficult for someone to debug NPTL and glibc
    problems (misleading him/her to conclude that the threads had gone away
    before the fault).

    The fix below is to simply avoid the CLONE_CHILD_CLEARTID action if a
    core dump has been initiated.

    Cheers. -ernie



    --- linux-2.6.18/kernel/fork.c.orig
    +++ linux-2.6.18/kernel/fork.c
    @@ -439,7 +439,9 @@ void mm_release(struct task_struct *tsk,
    tsk->vfork_done = NULL;
    complete(vfork_done);
    }
    - if (tsk->clear_child_tid && atomic_read(&mm->mm_users) > 1) {
    + if (tsk->clear_child_tid &&
    + mm->core_waiters == 0 &&
    + atomic_read(&mm->mm_users) > 1) {
    u32 __user * tidptr = tsk->clear_child_tid;
    tsk->clear_child_tid = NULL;

    -
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