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    SubjectRe: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree
    > +/* 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.



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