| Message ID | 20200817220425.9389-12-ebiederm@xmission.com |
|---|---|
| State | Not Applicable |
| Delegated to: | David Miller |
| Headers | show |
| Series | [01/17] exec: Move unshare_files to fix posix file locking during exec | expand |
On Mon, Aug 17, 2020 at 3:11 PM Eric W. Biederman <ebiederm@xmission.com> wrote: > > Instead hold task_lock for the duration that task->files needs to be > stable in seq_show. The task_lock was already taken in > get_files_struct, and so skipping get_files_struct performs less work > overall, and avoids the problems with the files_struct reference > count. Hmm. Do we even need that task_lock() at all? Couldn't we do this all with just the RCU lock held for reading? As far as I can tell, we don't really need the task lock. We don't even need the get/pid_task_struct(). Can't we just do rcu_read_lock(); task = pid_task(proc_pid(inode), PIDTYPE_PID); if (task) { unsigned int fd = proc_fd(m->private); struct file *file = fcheck_task(task, fd); if (file) .. do the seq_printf .. and do it all with no refcount games or task locking at all? Anyway, I don't dislike your patch per se, I just get the feeling that you could go a bit further in that cleanup.. And it's quite possible that I'm wrong, and you can't just use the RCU lock, but as far as I can tell, both the task lookup and the file lookup *already* really both depend on the RCU lock anyway, so the other locking and reference counting games really do seem superfluous. Please just send me a belittling email telling me what a nincompoop I am if I have missed something. Linus
On Mon, Aug 17, 2020 at 6:13 PM Eric W. Biederman <ebiederm@xmission.com> wrote: > > task_lock is needed to protect task->files. Hah. Right you are. I found a few cases where we didn't do that, but I hadn't noticed that they were all of the pattern struct task_struct *tsk = current; so "tsk->files" was safe for that reason.. So never mind. Linus
On Mon, Aug 17, 2020 at 05:04:20PM -0500, Eric W. Biederman wrote: > When discussing[1] exec and posix file locks it was realized that none > of the callers of get_files_struct fundamentally needed to call > get_files_struct, and that by switching them to helper functions > instead it will both simplify their code and remove unnecessary > increments of files_struct.count. Those unnecessary increments can > result in exec unnecessarily unsharing files_struct which breaking > posix locks, and it can result in fget_light having to fallback to > fget reducing system performance. > > Instead hold task_lock for the duration that task->files needs to be > stable in seq_show. The task_lock was already taken in > get_files_struct, and so skipping get_files_struct performs less work > overall, and avoids the problems with the files_struct reference > count. > > [1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com > Suggested-by: Oleg Nesterov <oleg@redhat.com> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> > --- Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
diff --git a/fs/proc/fd.c b/fs/proc/fd.c index d9fee5390fd7..0b46eea154b7 100644 --- a/fs/proc/fd.c +++ b/fs/proc/fd.c @@ -28,9 +28,8 @@ static int seq_show(struct seq_file *m, void *v) if (!task) return -ENOENT; - files = get_files_struct(task); - put_task_struct(task); - + task_lock(task); + files = task->files; if (files) { unsigned int fd = proc_fd(m->private); @@ -47,8 +46,9 @@ static int seq_show(struct seq_file *m, void *v) ret = 0; } spin_unlock(&files->file_lock); - put_files_struct(files); } + task_unlock(task); + put_task_struct(task); if (ret) return ret; @@ -57,6 +57,7 @@ static int seq_show(struct seq_file *m, void *v) (long long)file->f_pos, f_flags, real_mount(file->f_path.mnt)->mnt_id); + /* show_fd_locks() never deferences files so a stale value is safe */ show_fd_locks(m, file, files); if (seq_has_overflowed(m)) goto out;
When discussing[1] exec and posix file locks it was realized that none of the callers of get_files_struct fundamentally needed to call get_files_struct, and that by switching them to helper functions instead it will both simplify their code and remove unnecessary increments of files_struct.count. Those unnecessary increments can result in exec unnecessarily unsharing files_struct which breaking posix locks, and it can result in fget_light having to fallback to fget reducing system performance. Instead hold task_lock for the duration that task->files needs to be stable in seq_show. The task_lock was already taken in get_files_struct, and so skipping get_files_struct performs less work overall, and avoids the problems with the files_struct reference count. [1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com Suggested-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> --- fs/proc/fd.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)