test/zdtm: Add ZDTM test for dynamic bfd expansion on massive group

Add a new ZDTM test case `massive_groups` to verify the dynamic buffer
expansion logic in `bfd.c`

Signed-off-by: dong sunchao <dongsunchao@gmail.com>

Author: DongSunchao

test/zdtm/static/Makefile

@@ -287,6 +287,7 @@ TST_NOFILE	:=				\
 		sigtrap01			\
 		change_mnt_context		\
 		fd_offset			\
+		massive_groups			\
 #		jobctl00			\
 
 PKG_CONFIG ?= pkg-config
@@ -736,6 +737,8 @@ cgroupv2_01:		LDLIBS += -pthread
 uprobes:		CFLAGS += $(call pkg-cflags, libtracefs libtraceevent)
 uprobes:		LDLIBS += $(call pkg-libs, libtracefs libelf)
 
+massive_groups:	LDLIBS += -lrt -pthread
+
 $(LIB):	force
 	$(Q) $(MAKE) -C $(LIBDIR)
 

test/zdtm/static/massive_groups.c

@@ -0,0 +1,75 @@
+#include <grp.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+#include "zdtmtst.h"
+
+const char *test_doc = "Test dumping a process with many supplementary groups (Trigger bfd dynamic expansion)";
+const char *test_author = "DongSunchao";
+
+/*
+ * _SC_NGROUPS_MAX beyond parasite's maximum supported groups, so we use a hardcoded value.
+ * Use 900 groups: enough to test the "Groups:" line in /proc/pid/status
+ * and triggering dynamic expansion,
+ * but still within the PARASITE_MAX_GROUPS limit (~952).
+ */
+#define TEST_NGROUPS 900
+
+int main(int argc, char **argv)
+{
+	gid_t *group;
+	gid_t *restored_groups;
+	int i, ngroups;
+	int restored_ngroups;
+
+	test_init(argc, argv);
+
+	ngroups = TEST_NGROUPS;
+
+	group = malloc(ngroups * sizeof(gid_t));
+	if (!group) {
+		fail("Failed to allocate memory for group IDs: %s", strerror(errno));
+		return 1;
+	}
+
+	/*
+	 * Fill the group array with unique group IDs to reach the maximum limit.
+	 * This will trigger the dynamic expansion of the BFD buffer when dumping
+	 * the process.
+	 */
+	for (i = 0; i < ngroups; i++) {
+		group[i] = i + 1000;
+	}
+
+	if (setgroups(ngroups, group) < 0) {
+		fail("Failed to set supplementary groups: %s", strerror(errno));
+		free(group);
+		return 1;
+	}
+
+	test_daemon();
+
+	test_waitsig();
+
+	restored_ngroups = getgroups(0, NULL);
+	if (restored_ngroups != ngroups) {
+		fail("Restored number of supplementary groups (%d) does not match expected (%d)", restored_ngroups, ngroups);
+		free(group);
+		return 1;
+	}
+
+	restored_groups = malloc(restored_ngroups * sizeof(gid_t));
+	getgroups(restored_ngroups, restored_groups);
+
+	for (i = 0; i < ngroups; i++) {
+		if (restored_groups[i] != group[i]) {
+			fail("Restored group ID at index %d (%d) does not match expected (%d)", i, restored_groups[i], group[i]);
+		}
+	}
+
+	free(restored_groups);
+	free(group);
+	pass();
+	return 0;
+}

test/zdtm/static/massive_groups.desc

@@ -0,0 +1 @@
+{'flavor': 'h', 'flags': 'suid'}