On 11/22, KOSAKI Motohiro wrote: > > (11/22/2013 12:54 PM), Oleg Nesterov wrote: > > fs_struct->in_exec == T means that this ->fs is used by a single > > process (thread group), and one of the treads does do_execve(). > > > > To avoid the mt-exec races this code has the following complications: > > > > 1. check_unsafe_exec() returns -EBUSY if ->in_exec was > > already set by another thread. > > > > 2. do_execve_common() records "clear_in_exec" to ensure > > that the error path can only clear ->in_exec if it was > > set by current. > > > > However, after 9b1bf12d5d51 "signals: move cred_guard_mutex from > > task_struct to signal_struct" we do not need these complications: > > > > 1. We can't race with our sub-thread, this is called under > > per-process ->cred_guard_mutex. And we can't race with > > another CLONE_FS task, we already checked that this fs > > is not shared. > > > > We can remove the dead -EAGAIN logic. > > > > 2. "out_unmark:" in do_execve_common() is either called > > under ->cred_guard_mutex, or after de_thread() which > > kills other threads, so we can't race with sub-thread > > which could set ->in_exec. And if ->fs is shared with > > another process ->in_exec should be false anyway. > > > > We can clear in_exec unconditionally. > > > > This also means that check_unsafe_exec() can be void. > > > > Signed-off-by: Oleg Nesterov <o...@redhat.com> > > I have found no problem in this patch. However, I have a very basic question. > Why do we need to keep fs->in_exec?
To ensure that a sub-thread can't create a new process with the same ->fs while we are doing exec without LSM_UNSAFE_SHARE, I guess. This is only for security/ code. > If it is correct, > can't we move it it to signal->in_exec? Yes, perhaps, I am thinking about more cleanups too. But not that this will add the subtle change. CLONE_THREAD doesn't require CLONE_FS, so copy_fs() can fail even it the caller doesn't share ->fs with the execing thread. And we still need fs->lock to set signal->in_exec, this looks a bit strange. > I am not expert in this area and I may overlook something. Neither me ;) So this patch tries to not change the current logic. I feel that perhaps we can do more cleanups, but I am not really sure and this needs a separate change. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/