[PATCH v10 1/2] lib: New library function tst_get_free_uid

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

Some tests need a completely unassigned, unused UID. This is used by
open16 to verify restricted O_CREAT in sticky directories by running
as a sandboxed user with no file ownership or privileges.

Signed-off-by: Wei Gao <wegao-IBi9RG/[email protected]>
---
 include/tst_uid.h | 27 +++++++++++++++++++++++----
 lib/tst_uid.c     | 28 ++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/include/tst_uid.h b/include/tst_uid.h
index 2237ddcbf..2ffe84613 100644
--- a/include/tst_uid.h
+++ b/include/tst_uid.h
@@ -7,10 +7,29 @@
 
 #include <sys/types.h>
 
-/*
- * Find unassigned 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.
+/**
+ * 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.
+ */
+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_gid() - Find a GID not assigned to any group.
+ * @skip: GID value to skip (pass 0 to skip none).
+ *
+ * Scans the group database for the first unused GID starting from 1,
+ * skipping @skip. 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.
+ * Calls tst_brk(TBROK) if no free GID is found or a lookup error occurs.
+ *
+ * Return: An unused gid_t value.
  */
 gid_t tst_get_free_gid_(const char *file, const int lineno, gid_t skip);
 #define tst_get_free_gid(skip) tst_get_free_gid_(__FILE__, __LINE__, (skip))
diff --git a/lib/tst_uid.c b/lib/tst_uid.c
index af4ef8cf7..93c9b83d2 100644
--- a/lib/tst_uid.c
+++ b/lib/tst_uid.c
@@ -5,6 +5,7 @@
 
 #include <sys/types.h>
 #include <grp.h>
+#include <pwd.h>
 #include <errno.h>
 
 #define TST_NO_DEFAULT_MAIN
@@ -12,6 +13,33 @@
 #include "tst_uid.h"
 
 #define MAX_GID 32767
+#define MAX_UID 32767
+
+uid_t tst_get_free_uid_(const char *file, const int lineno, uid_t skip)
+{
+	uid_t ret;
+
+	for (ret = 1; ret < MAX_UID; ret++) {
+		if (ret == skip)
+			continue;
+
+		errno = 0;
+		if (getpwuid(ret))
+			continue;
+
+		if (errno == 0 || errno == ENOENT || errno == ESRCH) {
+			tst_res_(file, lineno, TINFO | TERRNO,
+				"Found unused UID %d", (int)ret);
+			return ret;
+		}
+
+		tst_brk_(file, lineno, TBROK | TERRNO, "User ID lookup failed");
+		return (uid_t)-1;
+	}
+
+	tst_brk_(file, lineno, TBROK, "No free user ID found");
+	return (uid_t)-1;
+}
 
 gid_t tst_get_free_gid_(const char *file, const int lineno, gid_t skip)
 {
-- 
2.54.0


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