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SubjectRe: [PATCH] Allow threads to rename siblings via /proc/pid/tasks/tid/comm
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(cc to linux-security-module and James)

> On Mon, 16 Nov 2009 13:11:07 -0800
> john stultz <johnstul@us.ibm.com> wrote:
>
> > Setting a thread's comm to be something unique is a very useful ability
> > and is helpful for debugging complicated threaded applications. However
> > currently the only way to set a thread name is for the thread to name
> > itself via the PR_SET_NAME prctl.
> >
> > However, there may be situations where it would be advantageous for a
> > thread dispatcher to be naming the threads its managing, rather then
> > having the threads self-describe themselves. This sort of behavior is
> > available on other systems via the pthread_setname_np() interface.
> >
> > This patch exports a task's comm via proc/pid/comm and
> > proc/pid/task/tid/comm interfaces, and allows thread siblings to write
> > to these values.
> >
>
> Would be nice to document the new userspace interface.
> Documentation/filesystems/proc.txt, perhaps.
>
> >
> > diff --git a/fs/exec.c b/fs/exec.c
> > index d49be6b..90003f8 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -926,6 +926,15 @@ char *get_task_comm(char *buf, struct task_struct *tsk)
> > void set_task_comm(struct task_struct *tsk, char *buf)
> > {
> > task_lock(tsk);
> > +
> > + /*
> > + * Threads may access current->comm without holding
> > + * the task lock, so write the string carefully.
> > + * Readers without a lock may see incomplete new
> > + * names but are safe from non-terminating string reads.
> > + */
> > + memset(tsk->comm, 0, TASK_COMM_LEN);
> > + wmb();
>
> OK.

Hmm, I don't like mix TASK_COMM_LEN and sizeof(tsk->comm).
John, Is there any reason?

>
> > strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> > task_unlock(tsk);
> > perf_event_comm(tsk);
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 837469a..7f59af1 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1265,6 +1265,78 @@ static const struct file_operations proc_pid_sched_operations = {
> >
> > #endif
> >
> > +
> > +
> > +static ssize_t
> > +comm_write(struct file *file, const char __user *buf,
> > + size_t count, loff_t *offset)
> > +{
> > + struct inode *inode = file->f_path.dentry->d_inode;
> > + struct task_struct *p;
> > + char buffer[TASK_COMM_LEN];
> > +
> > + memset(buffer, 0, sizeof(buffer));
> > + if (count > sizeof(buffer) - 1)
> > + count = sizeof(buffer) - 1;
>
> Is this the best policy? If userspace tries to apply a too-long name
> to a thread, the kernel will silently truncate (ie: corrupt) it? I'd
> have thought that returning an error would be more robust?
>
> > + if (copy_from_user(buffer, buf, count))
> > + return -EFAULT;
> > +
> > + p = get_proc_task(inode);
> > + if (!p)
> > + return -ESRCH;
> > +
> > + if (same_thread_group(current, p))
> > + set_task_comm(p, buffer);
> > + else
> > + count = -EINVAL;
> > +
> > + put_task_struct(p);
> > +
> > + return count;
> > +}
>
> Is same_thread_group() sufficient? Are any security/permission-related
> checks appropriate here, for example?
>
> The restriction to a separate thread group seems a bit arbitrary,
> really. There's no reason I can see why we cannot permit unrelated
> (but suitably authorised) processes to do this.

At least, currently /proc/pid/cmdline read the process stack. A stack
can be rewritten without any security check by the same group thread.
I guess we can't make consist check of task name change.

Plus, now we don't have any LSM hook of task name change nor security
capability. I guess all security module don't need task name.

I hope security folks correct me if I misunderstood.


> This patch makes task->comm inconsistent with /prod/pid/cmdline. What
> are the implications of that for userspace? None, I guess, given that
> this can already be done.

ditto.
Process stack isn't guarded now. we can't make reasonable protection.

- kosaki

>
> > +
> > +static int comm_show(struct seq_file *m, void *v)
> > +{
> > + struct inode *inode = m->private;
> > + struct task_struct *p;
> > +
> > + p = get_proc_task(inode);
> > + if (!p)
> > + return -ESRCH;
> > +
> > + task_lock(p);
> > + seq_printf(m, "%s\n", p->comm);
> > + task_unlock(p);
> > +
> > + put_task_struct(p);
> > +
> > + return 0;
> > +}
> > +
> > +static int comm_open(struct inode *inode, struct file *filp)
> > +{
> > + int ret;
> > +
> > + ret = single_open(filp, comm_show, NULL);
> > + if (!ret) {
> > + struct seq_file *m = filp->private_data;
> > +
> > + m->private = inode;
> > + }
> > + return ret;
> > +}
> > +
> > +
>
> The patch has a seemingly-random inexplicable mixture of \n and \n\n.
>
> > +static const struct file_operations proc_pid_set_comm_operations = {
> > + .open = comm_open,
> > + .read = seq_read,
> > + .write = comm_write,
> > + .llseek = seq_lseek,
> > + .release = single_release,
> > +};
> > +
> > +
> > /*
> > * We added or removed a vma mapping the executable. The vmas are only mapped
> > * during exec and are not mapped with the mmap system call.
> > @@ -2504,6 +2576,7 @@ static const struct pid_entry tgid_base_stuff[] = {
> > #ifdef CONFIG_SCHED_DEBUG
> > REG("sched", S_IRUGO|S_IWUSR, proc_pid_sched_operations),
> > #endif
> > + REG("comm", S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
> > #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
> > INF("syscall", S_IRUSR, proc_pid_syscall),
> > #endif
> > @@ -2839,6 +2912,7 @@ static const struct pid_entry tid_base_stuff[] = {
> > #ifdef CONFIG_SCHED_DEBUG
> > REG("sched", S_IRUGO|S_IWUSR, proc_pid_sched_operations),
> > #endif
> > + REG("comm", S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
> > #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
> > INF("syscall", S_IRUSR, proc_pid_syscall),
> > #endif
>





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