selftests/bpf: Introduce __caps_unpriv annotation for tests

Add a __caps_unpriv annotation so that tests requiring specific
capabilities while dropping the rest can conveniently specify them
during selftest declaration instead of munging with capabilities at
runtime from the testing binary.

While at it, let us convert test_verifier_mtu to use this new support
instead.

Since we do not want to include linux/capability.h, we only defined the
four main capabilities BPF subsystem deals with in bpf_misc.h for use in
tests. If the user passes a CAP_SYS_NICE or anything else that's not
defined in the header, capability parsing code will return a warning.

Also reject strtol returning 0. CAP_CHOWN = 0 but we'll never need to
use it, and strtol doesn't errno on failed conversion. Fail the test in
such a case.

The original diff for this idea is available at link [0].

  [0]: https://lore.kernel.org/bpf/a1e48f5d9ae133e19adc6adf27e19d585e06bab4.camel@gmail.com

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
[ Kartikeya: rebase on bpf-next, add warn to parse_caps, convert test_verifier_mtu ]
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241204044757.1483141-4-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

tools/testing/selftests/bpf/prog_tests/verifier.c

@@ -225,24 +225,7 @@ void test_verifier_xdp(void)                  { RUN(verifier_xdp); }
 void test_verifier_xdp_direct_packet_access(void) { RUN(verifier_xdp_direct_packet_access); }
 void test_verifier_bits_iter(void) { RUN(verifier_bits_iter); }
 void test_verifier_lsm(void)                  { RUN(verifier_lsm); }
-
-void test_verifier_mtu(void)
-{
-	__u64 caps = 0;
-	int ret;
-
-	/* In case CAP_BPF and CAP_PERFMON is not set */
-	ret = cap_enable_effective(1ULL << CAP_BPF | 1ULL << CAP_NET_ADMIN, &caps);
-	if (!ASSERT_OK(ret, "set_cap_bpf_cap_net_admin"))
-		return;
-	ret = cap_disable_effective(1ULL << CAP_SYS_ADMIN | 1ULL << CAP_PERFMON, NULL);
-	if (!ASSERT_OK(ret, "disable_cap_sys_admin"))
-		goto restore_cap;
-	RUN(verifier_mtu);
-restore_cap:
-	if (caps)
-		cap_enable_effective(caps, NULL);
-}
+void test_verifier_mtu(void)		      { RUN(verifier_mtu); }
 
 static int init_test_val_map(struct bpf_object *obj, char *map_name)
 {

tools/testing/selftests/bpf/progs/bpf_misc.h

@@ -5,6 +5,10 @@
 #define XSTR(s) STR(s)
 #define STR(s) #s
 
+/* Expand a macro and then stringize the expansion */
+#define QUOTE(str) #str
+#define EXPAND_QUOTE(str) QUOTE(str)
+
 /* This set of attributes controls behavior of the
  * test_loader.c:test_loader__run_subtests().
  *
@@ -106,6 +110,7 @@
  * __arch_*          Specify on which architecture the test case should be tested.
  *                   Several __arch_* annotations could be specified at once.
  *                   When test case is not run on current arch it is marked as skipped.
+ * __caps_unpriv     Specify the capabilities that should be set when running the test.
  */
 #define __msg(msg)		__attribute__((btf_decl_tag("comment:test_expect_msg=" XSTR(__COUNTER__) "=" msg)))
 #define __xlated(msg)		__attribute__((btf_decl_tag("comment:test_expect_xlated=" XSTR(__COUNTER__) "=" msg)))
@@ -129,6 +134,13 @@
 #define __arch_x86_64		__arch("X86_64")
 #define __arch_arm64		__arch("ARM64")
 #define __arch_riscv64		__arch("RISCV64")
+#define __caps_unpriv(caps)	__attribute__((btf_decl_tag("comment:test_caps_unpriv=" EXPAND_QUOTE(caps))))
+
+/* Define common capabilities tested using __caps_unpriv */
+#define CAP_NET_ADMIN		12
+#define CAP_SYS_ADMIN		21
+#define CAP_PERFMON		38
+#define CAP_BPF			39
 
 /* Convenience macro for use with 'asm volatile' blocks */
 #define __naked __attribute__((naked))

tools/testing/selftests/bpf/progs/verifier_mtu.c

@@ -6,7 +6,9 @@
 
 SEC("tc/ingress")
 __description("uninit/mtu: write rejected")
-__failure __msg("invalid indirect read from stack")
+__success
+__caps_unpriv(CAP_BPF|CAP_NET_ADMIN)
+__failure_unpriv __msg_unpriv("invalid indirect read from stack")
 int tc_uninit_mtu(struct __sk_buff *ctx)
 {
 	__u32 mtu;

tools/testing/selftests/bpf/test_loader.c

@@ -36,6 +36,7 @@
 #define TEST_TAG_ARCH "comment:test_arch="
 #define TEST_TAG_JITED_PFX "comment:test_jited="
 #define TEST_TAG_JITED_PFX_UNPRIV "comment:test_jited_unpriv="
+#define TEST_TAG_CAPS_UNPRIV "comment:test_caps_unpriv="
 
 /* Warning: duplicated in bpf_misc.h */
 #define POINTER_VALUE	0xcafe4all
@@ -74,6 +75,7 @@ struct test_subspec {
 	struct expected_msgs jited;
 	int retval;
 	bool execute;
+	__u64 caps;
 };
 
 struct test_spec {
@@ -276,6 +278,37 @@ static int parse_int(const char *str, int *val, const char *name)
 	return 0;
 }
 
+static int parse_caps(const char *str, __u64 *val, const char *name)
+{
+	int cap_flag = 0;
+	char *token = NULL, *saveptr = NULL;
+
+	char *str_cpy = strdup(str);
+	if (str_cpy == NULL) {
+		PRINT_FAIL("Memory allocation failed\n");
+		return -EINVAL;
+	}
+
+	token = strtok_r(str_cpy, "|", &saveptr);
+	while (token != NULL) {
+		errno = 0;
+		if (!strncmp("CAP_", token, sizeof("CAP_") - 1)) {
+			PRINT_FAIL("define %s constant in bpf_misc.h, failed to parse caps\n", token);
+			return -EINVAL;
+		}
+		cap_flag = strtol(token, NULL, 10);
+		if (!cap_flag || errno) {
+			PRINT_FAIL("failed to parse caps %s\n", name);
+			return -EINVAL;
+		}
+		*val |= (1ULL << cap_flag);
+		token = strtok_r(NULL, "|", &saveptr);
+	}
+
+	free(str_cpy);
+	return 0;
+}
+
 static int parse_retval(const char *str, int *val, const char *name)
 {
 	struct {
@@ -541,6 +574,12 @@ static int parse_test_spec(struct test_loader *tester,
 			jit_on_next_line = true;
 		} else if (str_has_pfx(s, TEST_BTF_PATH)) {
 			spec->btf_custom_path = s + sizeof(TEST_BTF_PATH) - 1;
+		} else if (str_has_pfx(s, TEST_TAG_CAPS_UNPRIV)) {
+			val = s + sizeof(TEST_TAG_CAPS_UNPRIV) - 1;
+			err = parse_caps(val, &spec->unpriv.caps, "test caps");
+			if (err)
+				goto cleanup;
+			spec->mode_mask |= UNPRIV;
 		}
 	}
 
@@ -917,6 +956,13 @@ void run_subtest(struct test_loader *tester,
 			test__end_subtest();
 			return;
 		}
+		if (subspec->caps) {
+			err = cap_enable_effective(subspec->caps, NULL);
+			if (err) {
+				PRINT_FAIL("failed to set capabilities: %i, %s\n", err, strerror(err));
+				goto subtest_cleanup;
+			}
+		}
 	}
 
 	/* Implicitly reset to NULL if next test case doesn't specify */