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    Subject[PATCH] Allow threads to rename siblings via /proc/pid/tasks/tid/comm
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    Andrew: Would you consider this for testing in -mm?

    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.

    Signed-off-by: John Stultz <johnstul@us.ibm.com>

    diff --git a/fs/exec.c b/fs/exec.c
    index d49be6b..ce70e55 100644
    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -926,7 +926,18 @@ char *get_task_comm(char *buf, struct task_struct *tsk)
    void set_task_comm(struct task_struct *tsk, char *buf)
    {
    task_lock(tsk);
    - strlcpy(tsk->comm, buf, sizeof(tsk->comm));
    +
    + /*
    + * Threads may access current->comm without holding
    + * the task lock, so write the string carefully
    + * to avoid non-terminating reads. Readers without a lock
    + * will get the oldname, the newname or an empty string.
    + */
    + tsk->comm[0] = 0;
    + wmb();
    + strlcpy(tsk->comm+1, buf+1, sizeof(tsk->comm)-1);
    + wmb();
    + tsk->comm[0] = buf[0];
    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;
    + 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;
    +}
    +
    +
    +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;
    +}
    +
    +
    +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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