Re: [PATCH 0/2] mm/userfaultfd: Fix uninitialized output field for -EAGAIN race

[James Houghton]

On Thu, Apr 24, 2025 at 5:57 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> When discussing some userfaultfd issues with Andrea, Andrea pointed out an
> ABI issue with userfaultfd that existed for years.  Luckily the issue
> should only be a very corner case one, and the fix (even if changing the
> kernel ABI) should only be in the good way, IOW there should have no risk
> breaking any userapp but only fixing.

FWIW, my userspace basically looks like this:

struct uffdio_continue uffdio_continue;
int64_t target_len = /* whatever */;
int64_t bytes_mapped = 0;
int ioctl_ret;
do {
  uffdio_continue.range = /* whatever */;
  uffdio_continue.mapped = 0;
  ioctl_ret = ioctl(uffd, UFFDIO_CONTINUE, &uffdio_continue);
  if (uffdio_continue.mapped < 0) { break; }
  bytes_mapped += uffdio_continue.mapped;
} while (bytes_mapped < target_len && errno == EAGAIN);

I think your patch would indeed break this. (Perhaps I shouldn't be
reading from `mapped` without first checking that errno == EAGAIN.)

Well, that's what I would say, except in practice I never actually hit
the mmap_changing case while invoking UFFDIO_CONTINUE. :)