On Thu, May 28, 2015 at 01:14:35AM +0300, Cyrill Gorcunov wrote:
> On Thu, May 28, 2015 at 12:49:53AM +0300, Alexey Dobriyan wrote:
> ...
> > -static int proc_pid_cmdline(struct seq_file *m, struct pid_namespace *ns,
> > - struct pid *pid, struct task_struct *task)
> > +static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
> > + size_t _count, loff_t *pos)
> > {
> > + struct task_struct *tsk;
> > + struct mm_struct *mm;
> > + char *page;
> > + unsigned long count = _count;
> > + unsigned long arg_start, arg_end, env_start, env_end;
> > + unsigned long len1, len2, len;
> > + unsigned long p;
> > + char c;
> > + ssize_t rv;
> > +
> > + BUG_ON(*pos < 0);
> > +
> > + tsk = get_proc_task(file_inode(file));
> > + if (!tsk)
> > + return -ESRCH;
> > + mm = get_task_mm(tsk);
> > + put_task_struct(tsk);
> > + if (!mm)
> > + return 0;
> > + /* Check if process spawned far enough to have cmdline. */
> > + if (!mm->env_end) {
> > + rv = 0;
> > + goto out_mmput;
> > + }
> > +
> > + page = (char *)__get_free_page(GFP_TEMPORARY);
> > + if (!page) {
> > + rv = -ENOMEM;
> > + goto out_mmput;
> > + }
> > +
> > + down_read(&mm->mmap_sem);
> > + arg_start = mm->arg_start;
> > + arg_end = mm->arg_end;
> > + env_start = mm->env_start;
> > + env_end = mm->env_end;
> > + up_read(&mm->mmap_sem);
>
> Could you please explain why this down/up is needed?
Code is written this way to get constistent snapshot of data.
If you look at PR_SET_MM_* code, you'll notice down_read(&mm->mmap_sem)
as well which is a separate bug because you're _writing_ those fields
eventually in prctl_set_mm(), yuck!
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