Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

[Oleg Nesterov <oleg@redhat.com>]

> +/* The caller must have pinned the task */
> +static struct file *
> +get_file_raw_ptr(struct task_struct *task, unsigned int idx)
> +{
> +	struct fdtable *fdt;
> +	struct file *file;
> +
> +	spin_lock(&task->files->file_lock);

task->files can be NULL, we can race with exit_files().

> +	fdt = files_fdtable(task->files);
> +	if (idx < fdt->max_fds)
> +		file = fdt->fd[idx];

You can probably rely on rcu instead of ->file_lock, but this is minor.

> +SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
> +		unsigned long, idx1, unsigned long, idx2)
> +{
> +	struct task_struct *task1, *task2;
> +	int ret;
> +
> +	rcu_read_lock();
> +
> +	/*
> +	 * Tasks are looked up in caller's PID namespace only.
> +	 */
> +	task1 = find_task_by_vpid(pid1);
> +	task2 = find_task_by_vpid(pid2);
> +	if (!task1 || !task2)
> +		goto err_no_task;
> +
> +	get_task_struct(task1);
> +	get_task_struct(task2);
> +
> +	rcu_read_unlock();
> +
> +	/*
> +	 * One should have enough rights to inspect task details.
> +	 */
> +	if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
> +	    !ptrace_may_access(task2, PTRACE_MODE_READ)) {
> +		ret = -EACCES;

Well, probably this is fine... but may be you can add a comment.
The task can change its credentials right after ptrace_may_access()
succeeds. This _looks_ wrong, perhaps it makes sense to  add the
"we do not care" note.

Oleg.