selftests/seccomp: Check CAP_SYS_ADMIN capability in the test mode_filter_without_nnp

In the "mode_filter_without_nnp" test in seccomp_bpf, there is currently
a TODO which asks to check the capability CAP_SYS_ADMIN instead of euid.
This patch adds support to check if the calling process has the flag
CAP_SYS_ADMIN, and also if this flag has CAP_EFFECTIVE set.

Signed-off-by: Gautam Menghani <gautammenghani201@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220731092529.28760-1-gautammenghani201@gmail.com

Author: Gautam-Menghani

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -392,6 +392,8 @@ TEST(mode_filter_without_nnp)
 		.filter = filter,
 	};
 	long ret;
+	cap_t cap = cap_get_proc();
+	cap_flag_value_t is_cap_sys_admin = 0;
 
 	ret = prctl(PR_GET_NO_NEW_PRIVS, 0, NULL, 0, 0);
 	ASSERT_LE(0, ret) {
@@ -400,8 +402,8 @@ TEST(mode_filter_without_nnp)
 	errno = 0;
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog, 0, 0);
 	/* Succeeds with CAP_SYS_ADMIN, fails without */
-	/* TODO(wad) check caps not euid */
-	if (geteuid()) {
+	cap_get_flag(cap, CAP_SYS_ADMIN, CAP_EFFECTIVE, &is_cap_sys_admin);
+	if (!is_cap_sys_admin) {
 		EXPECT_EQ(-1, ret);
 		EXPECT_EQ(EACCES, errno);
 	} else {