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    Subject[PATCH 3/3] ptrace: do not use task_lock() for attach
    Remove the "Nasty, nasty" lock dance in ptrace_attach()/ptrace_traceme().
    From now task_lock() has nothing to do with ptrace at all.

    With the recent changes nobody uses task_lock() to serialize with ptrace,
    but in fact it was never needed and it was never used consistently.

    However ptrace_attach() calls __ptrace_may_access() and needs task_lock()
    to pin task->mm for get_dumpable(). But we can call __ptrace_may_access()
    before we take tasklist_lock, ->cred_exec_mutex protects us against
    do_execve() path which can change creds and MMF_DUMP* flags.

    (ugly, but we can't use ptrace_may_access() because it hides the error
    code, so we have to take task_lock() and use __ptrace_may_access()).

    NOTE: this change assumes that LSM hooks, security_ptrace_may_access() and
    security_ptrace_traceme(), can be called without task_lock() held.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---

    kernel/ptrace.c | 59 ++++++++++++--------------------------------------------
    1 file changed, 13 insertions(+), 46 deletions(-)

    --- PTRACE/kernel/ptrace.c~3_TASK_LOCK 2009-05-05 23:49:15.000000000 +0200
    +++ PTRACE/kernel/ptrace.c 2009-05-06 00:16:17.000000000 +0200
    @@ -177,7 +177,6 @@ bool ptrace_may_access(struct task_struc
    int ptrace_attach(struct task_struct *task)
    {
    int retval;
    - unsigned long flags;

    audit_ptrace(task);

    @@ -193,34 +192,19 @@ int ptrace_attach(struct task_struct *ta
    retval = mutex_lock_interruptible(&task->cred_exec_mutex);
    if (retval < 0)
    goto out;
    -repeat:
    - /*
    - * Nasty, nasty.
    - *
    - * We want to hold both the task-lock and the
    - * tasklist_lock for writing at the same time.
    - * But that's against the rules (tasklist_lock
    - * is taken for reading by interrupts on other
    - * cpu's that may have task_lock).
    - */
    - task_lock(task);
    - if (!write_trylock_irqsave(&tasklist_lock, flags)) {
    - task_unlock(task);
    - do {
    - cpu_relax();
    - } while (!write_can_lock(&tasklist_lock));
    - goto repeat;
    - }

    + task_lock(task);
    retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH);
    + task_unlock(task);
    if (retval)
    - goto bad;
    + goto unlock_creds;

    + write_lock_irq(&tasklist_lock);
    retval = -EPERM;
    if (unlikely(task->exit_state))
    - goto bad;
    + goto unlock_tasklist;
    if (task->ptrace)
    - goto bad;
    + goto unlock_tasklist;

    task->ptrace = PT_PTRACED;
    if (capable(CAP_SYS_PTRACE))
    @@ -230,9 +214,9 @@ repeat:
    send_sig_info(SIGSTOP, SEND_SIG_FORCED, task);

    retval = 0;
    -bad:
    - write_unlock_irqrestore(&tasklist_lock, flags);
    - task_unlock(task);
    +unlock_tasklist:
    + write_unlock_irq(&tasklist_lock);
    +unlock_creds:
    mutex_unlock(&task->cred_exec_mutex);
    out:
    return retval;
    @@ -248,26 +232,10 @@ int ptrace_traceme(void)
    {
    int ret = -EPERM;

    - /*
    - * Are we already being traced?
    - */
    -repeat:
    - task_lock(current);
    + write_lock_irq(&tasklist_lock);
    + /* Are we already being traced? */
    if (!current->ptrace) {
    - /*
    - * See ptrace_attach() comments about the locking here.
    - */
    - unsigned long flags;
    - if (!write_trylock_irqsave(&tasklist_lock, flags)) {
    - task_unlock(current);
    - do {
    - cpu_relax();
    - } while (!write_can_lock(&tasklist_lock));
    - goto repeat;
    - }
    -
    ret = security_ptrace_traceme(current->parent);
    -
    /*
    * Check PF_EXITING to ensure ->real_parent has not passed
    * exit_ptrace(). Otherwise we don't report the error but
    @@ -277,10 +245,9 @@ repeat:
    current->ptrace = PT_PTRACED;
    __ptrace_link(current, current->real_parent);
    }
    -
    - write_unlock_irqrestore(&tasklist_lock, flags);
    }
    - task_unlock(current);
    + write_unlock_irq(&tasklist_lock);
    +
    return ret;
    }



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