[LTP] userfaultfd: Add helper for checking UFFD feature support

[linuxtestproject.agent@gmail.com]

Hi Ricardo,

--- [PATCH 1/2] ---

On Thu, 7 May 2026, Ricardo Branco wrote:
> userfaultfd: Add helper for checking UFFD feature support
>
> Add CHECK_UFFD_FEATURE() to perform the initial UFFDIO_API handshake
> without requesting optional features and verify that the kernel reports
> the requested feature bit.

The body only explains the mechanism, not the motivation. Add a sentence
explaining why this helper is needed — e.g. "Without this two-step probe,
requesting an unsupported feature in UFFDIO_API fails with EINVAL,
causing tests to TBROK instead of TCONF."

--- [PATCH 2/2] ---

On Thu, 7 May 2026, Ricardo Branco wrote:
> Fixes: 1840ee23d172b5ab04cca7c2acfa48755b041911

Please verify this SHA resolves in the full LTP history; it is not
reachable in this clone.

Pre-existing issues noticed in the surrounding code (not introduced
by this patch):

- testcases/kernel/syscalls/userfaultfd/userfaultfd05.c — `wp_fault_seen`
  is never reset to 0 at the top of run(); on multi-iteration runs (-i N)
  the second iteration may falsely pass.

---
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