[LTP] lib: New library function tst_get_free_uid

[linuxtestproject.agent@gmail.com]

Hi Wei,

On Tue, 07 Jul 2026 02:29:41 +0000, Wei Gao wrote:
> lib: New library function tst_get_free_uid

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

> Add tst_get_free_uid() to dynamically find unused UIDs for tests.

The body restates what the subject already says.  What scenario or
test requires a dynamically allocated free UID that tst_get_uids()
cannot cover?

> +uid_t tst_get_free_uid_(const char *file, const int lineno, uid_t skip);
> +#define tst_get_free_uid(skip) tst_get_free_uid_(__FILE__, __LINE__, (skip))

tst_get_free_uid_() is a new public function in lib/ and include/.
LTP documentation rules require a kernel-doc /** ... */ comment for
new public symbols there.  Something like:

  /**
   * tst_get_free_uid() - Find a UID not assigned to any user.
   * @skip: UID value to skip (pass 0 to skip none).
   *
   * Scans the password database for the first unused UID starting
   * from 1, skipping @skip.  Calls tst_brk(TBROK) if no free UID
   * is found or a lookup error occurs.
   *
   * Return: An unused uid_t value.
   */

> - * Find unassigned gid. The skip argument can be used to ignore e.g. the main
> + * Find unassigned uid/gid. The skip argument can be used to ignore e.g. the main
>   * group of a specific user in case it's not listed in the group file. If you
> - * do not need to skip any specific gid, simply set it to 0.
> + * do not need to skip any specific id, simply set it to 0.

"not listed in the group file" applies to GIDs only.  UIDs live in
the password database (/etc/passwd or NSS passwd).  The comment
should say "password or group file", or be split per-function.

> +uid_t tst_get_free_uid_(const char *file, const int lineno, uid_t skip)
> +{
> +	uid_t ret;
> +
> +	errno = 0;
> +
> +	for (ret = 1; ret < MAX_UID; ret++) {
> +		if (ret == skip || getpwuid(ret))
> +			continue;
> +
> +		if (errno == 0 || errno == ENOENT || errno == ESRCH) {

POSIX requires callers to set errno = 0 before each call to
getpwuid() so that a NULL return with non-zero errno means error
rather than "entry not found".  The reset here is done once before
the loop.  If a successful getpwuid() call for an earlier UID leaves
errno non-zero (POSIX does not guarantee it is cleared on success),
the next NULL-returning call could be misclassified as an error and
trigger the tst_brk() path below.

A per-iteration reset:

  errno = 0;
  if (ret == skip || getpwuid(ret))
      continue;

would match the POSIX-recommended pattern and make the intent clear.

Verdict - Needs revision

Pre-existing issues:

tst_get_free_gid_() in lib/tst_uid.c has the same errno-reset issue
described above (errno set once before the loop, not before each
getgrgid() call).

---
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