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Date: Wed, 18 Nov 2009 13:54:57 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: john stultz <johnstul@...ibm.com>
Cc: Andi Kleen <andi@...stfloor.org>,
Arjan van de Ven <arjan@...radead.org>,
lkml <linux-kernel@...r.kernel.org>,
Mike Fulton <fultonm@...ibm.com>,
Sean Foley <Sean_Foley@...ibm.com>,
Darren Hart <dvhltc@...ibm.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] Allow threads to rename siblings via
/proc/pid/tasks/tid/comm
On Mon, 16 Nov 2009 13:11:07 -0800
john stultz <johnstul@...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.
> 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.
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.
> +
> +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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