Hello!
> > Earlier on in this thread, Aleksa mentioned sched_setattr as
> > establishing precedent for the kernel modifying non-const objects. It
> > looks like glibc actually does provide a sched_setattr wrapper since
> > 2.41. The relevant argument hasn't been marked as const and the kernel
> > does modify the contents, and glibc's syscall wrapper simply passes it
> > through. So we already do this.
>
> given that
>
> SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
> unsigned int, flags)
>
> calls sched_setattr(), which is defined thus:
>
> int sched_setattr(struct task_struct *p, const struct sched_attr *attr)
> {
> return __sched_setscheduler(p, attr, true, true);
> }
>
> i think that's just a copy & paste mistake in the kernel -- carefully
> preserved in glibc and bionic -- no?
>
> (i only see the kernel updating its own _copy_ of the passed-in struct.)
Based on my understanding, it all happens before the call to the const
marked sched_setattr. Starting from line 986 (as of today) on the same
syscalls.c file [1]:
retval = sched_copy_attr(uattr, &attr);
if (retval)
return retval;
Which inside sched_copy_attr does:
ret = copy_struct_from_user(attr, sizeof(*attr), uattr, size);
if (ret) {
if (ret == -E2BIG)
goto err_size;
And that leads to:
err_size:
put_user(sizeof(*attr), &uattr->size);
return -E2BIG;
Which writes to userspace.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/sched/syscalls.c?id=e6b9dce0aeeb91dfc0974ab87f02454e24566182#n986
--
Arjun Shankar
he/him/his