[LTP] lib: New library function tst_get_free_uid

[linuxtestproject.agent@gmail.com]

Hi Wei,

On Wed, 15 Apr 2026, Wei Gao wrote:
> open16: allow restricted O_CREAT of FIFOs and regular files

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

On Wed, 15 Apr 2026, Wei Gao wrote:
> open16: allow restricted O_CREAT of FIFOs and regular files

> +	SAFE_FILE_PRINTF(PROTECTED_REGULAR, "2");
> +	SAFE_FILE_PRINTF(PROTECTED_FIFOS, "2");
> +	SAFE_CHMOD(DIR, 0020 | S_ISVTX);
> +
> +	if (!SAFE_FORK()) {
> +		SAFE_SETUID(uid2);
> +		TST_EXP_FAIL(openat(dir_fd, TEST_FILE, O_RDWR | O_CREAT, 0777), EACCES);
> +		TST_EXP_FAIL(open(TEST_FIFO_PATH, O_RDWR | O_CREAT, 0777), EACCES);

uid2 is a free (unused) UID with no group memberships. With mode 0020|S_ISVTX
the "other" bits are 0, so the kernel's DAC check rejects openat/open before
reaching the protected_regular/protected_fifos logic in may_open(). EACCES
fires for the wrong reason.

For a valid level-2 test the directory needs group-execute and group-write
(e.g. 0770|S_ISVTX), and uid2 must be placed in the directory's owning group
so it can access the directory but is still blocked by the protection check.

---
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