Messages in this thread Patch in this message | | | Date | Wed, 6 May 2009 00:47:29 +0200 | From | Oleg Nesterov <> | 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; }
| |