selftests/exec: convert the binfmt_misc bpf test to the kselftest harness

The test reports its own pass and fail lines, returns a bare 4 for
KSFT_SKIP and runs both cases in one process, so a failure in the first
takes the second with it. It also open-codes the register, unregister,
file-copy and mount helpers that the tests for the upcoming transparent
and loader dispatch modes need again.

Convert it to the kselftest harness: a fixture for the common setup and
teardown, one TEST_F per case so each is reported and isolated
separately, and SKIP() for the root, BTF and binfmt_misc preconditions.
Move the helpers to a shared header on the way, with the register
helper preserving the write's errno so a caller can tell a rejected
flag combination (EINVAL) from a kernel that does not know the flag at
all. The synthetic ELF header gains an e_machine argument and uses the
elf.h constants instead of open-coded numbers.

The fixture no longer mounts bpffs. The handler is attached with
bpf_map__attach_struct_ops() and nothing is ever pinned, the mount was
carried along from a bpftool-based draft. The bpf objects are compiled
with -DBPF_NO_KFUNC_PROTOTYPES - the guard bpftool emits for exactly
this - instead of sed'ing the prototypes out of the generated
vmlinux.h. And the config fragment records the options the binfmt_misc
tests need so a merge-config kernel can run them.

No change in what is tested.

Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-7-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

4 files changed, 187 insertions, 143 deletions

diff --git a/tools/testing/selftests/exec/Makefile b/tools/testing/selftests/exec/Makefile
index ec66c1fecfc0..d2a5a58f9432 100644
--- a/tools/testing/selftests/exec/Makefile
+++ b/tools/testing/selftests/exec/Makefile
@@ -44,6 +44,8 @@ endif
 EXTRA_CLEAN := $(OUTPUT)/subdir.moved $(OUTPUT)/execveat.moved $(OUTPUT)/xxxxx*	\
 	       $(OUTPUT)/S_I*.test
 
+LOCAL_HDRS += binfmt_misc_common.h
+
 include ../lib.mk
 
 CHECK_EXEC_SAMPLES := $(top_srcdir)/samples/check-exec
@@ -86,12 +88,13 @@ LIBBPF_LDLIBS ?= -lbpf -lelf -lz
 
 $(OUTPUT)/vmlinux.h:
 	$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
-	sed -i '/__ksym;$$/d' $@
 
+# BPF_NO_KFUNC_PROTOTYPES: the programs declare the kfuncs they use themselves.
 $(OUTPUT)/%.bpf.o: %.bpf.c $(OUTPUT)/vmlinux.h
-	$(CLANG) -g -O2 -target bpf -mcpu=v3 $(BPF_CFLAGS) $(LIBBPF_CFLAGS) -c $< -o $@
+	$(CLANG) -g -O2 -target bpf -mcpu=v3 -DBPF_NO_KFUNC_PROTOTYPES \
+		$(BPF_CFLAGS) $(LIBBPF_CFLAGS) -c $< -o $@
 
-$(OUTPUT)/binfmt_misc_bpf: binfmt_misc_bpf.c
+$(OUTPUT)/binfmt_misc_bpf: binfmt_misc_bpf.c binfmt_misc_common.h
 	$(CC) $(CFLAGS) $(LIBBPF_CFLAGS) $(LDFLAGS) $< $(LIBBPF_LDLIBS) -o $@
 
 $(OUTPUT)/binfmt_bpf_interp: binfmt_bpf_interp.c
@@ -102,4 +105,4 @@ $(OUTPUT)/binfmt_bpf_interp: binfmt_bpf_interp.c
 $(OUTPUT)/binfmt_bpf_app: binfmt_bpf_app.c
 	$(CC) $(CFLAGS) $(LDFLAGS) -Wl,--dynamic-linker,'$$ORIGIN/binfmt_bpf_interp' $< -o $@
 
-EXTRA_CLEAN += $(OUTPUT)/vmlinux.h $(OUTPUT)/bpf_interp.bpf.o $(OUTPUT)/nix_origin.bpf.o
+EXTRA_CLEAN += $(OUTPUT)/vmlinux.h $(OUTPUT)/*.bpf.o
diff --git a/tools/testing/selftests/exec/binfmt_misc_bpf.c b/tools/testing/selftests/exec/binfmt_misc_bpf.c
index cb89d2766fe2..c41fb80f2a72 100644
--- a/tools/testing/selftests/exec/binfmt_misc_bpf.c
+++ b/tools/testing/selftests/exec/binfmt_misc_bpf.c
@@ -23,68 +23,42 @@
  * program's chosen interpreter actually ran.
  */
 #define _GNU_SOURCE
+#include <elf.h>
+#include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <fcntl.h>
-#include <libgen.h>
-#include <sys/mount.h>
-#include <sys/stat.h>
 
 #include <bpf/btf.h>
 #include <bpf/libbpf.h>
 
+#include "binfmt_misc_common.h"
+#include "kselftest_harness.h"
+
 #define INTERP_PATH	"/tmp/binfmt_bpf_interp"
 #define AARCH64_PATH	"/tmp/binfmt_bpf_aarch64"
-#define RELOC_DIR	"/tmp/binfmt_reloc"
-#define BINFMT_REG	"/proc/sys/fs/binfmt_misc/register"
+#define RELOC_TEMPLATE	"/tmp/binfmt_relocXXXXXX"
 #define EXPECT		"BPF_INTERP_RAN"
 
-static char testdir[512]; /* directory holding this test's built artifacts */
-
-static int copy_file(const char *src, const char *dst)
-{
-	char buf[4096];
-	int in, out;
-	ssize_t n;
-
-	in = open(src, O_RDONLY);
-	if (in < 0)
-		return -1;
-	out = open(dst, O_WRONLY | O_CREAT | O_TRUNC, 0755);
-	if (out < 0) {
-		close(in);
-		return -1;
-	}
-	while ((n = read(in, buf, sizeof(buf))) > 0) {
-		if (write(out, buf, n) != n) {
-			close(in);
-			close(out);
-			return -1;
-		}
-	}
-	close(in);
-	close(out);
-	return n < 0 ? -1 : 0;
-}
-
-/* A minimal 64-bit little-endian aarch64 ELF header, padded to the read size. */
-static int create_fake_aarch64(const char *path)
+/* A minimal 64-bit little-endian ELF header, padded to the read size. */
+static int create_fake_elf(const char *path, unsigned short machine)
 {
 	unsigned char hdr[256] = {0};
 	int fd;
 
 	hdr[0] = 0x7f; hdr[1] = 'E'; hdr[2] = 'L'; hdr[3] = 'F';
-	hdr[4] = 2;			/* ELFCLASS64 */
-	hdr[5] = 1;			/* ELFDATA2LSB */
-	hdr[6] = 1;			/* EV_CURRENT */
-	hdr[16] = 2;			/* e_type = ET_EXEC */
-	hdr[18] = 183 & 0xff;		/* e_machine = EM_AARCH64 */
-	hdr[19] = (183 >> 8) & 0xff;
-	hdr[20] = 1;			/* e_version */
-
-	fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0755);
+	hdr[4] = ELFCLASS64;
+	hdr[5] = ELFDATA2LSB;
+	hdr[6] = EV_CURRENT;
+	hdr[16] = ET_EXEC;
+	hdr[18] = machine & 0xff;	/* e_machine, little-endian */
+	hdr[19] = machine >> 8;
+	hdr[20] = EV_CURRENT;
+
+	unlink(path);
+	fd = open(path, O_WRONLY | O_CREAT | O_EXCL, 0755);
 	if (fd < 0)
 		return -1;
 	if (write(fd, hdr, sizeof(hdr)) != (ssize_t)sizeof(hdr)) {
@@ -97,31 +71,10 @@ static int create_fake_aarch64(const char *path)
 
 static int register_entry(const char *name, const char *handler)
 {
-	char rule[128];
-	int fd;
-	ssize_t n;
+	char rule[PATH_MAX];
 
 	snprintf(rule, sizeof(rule), ":%s:B::::%s:", name, handler);
-	fd = open(BINFMT_REG, O_WRONLY);
-	if (fd < 0)
-		return -1;
-	n = write(fd, rule, strlen(rule));
-	close(fd);
-	return n < 0 ? -1 : 0;
-}
-
-static void unregister_entry(const char *name)
-{
-	char path[128];
-	int fd;
-
-	snprintf(path, sizeof(path), "/proc/sys/fs/binfmt_misc/%s", name);
-	fd = open(path, O_WRONLY);
-	if (fd >= 0) {
-		if (write(fd, "-1", 2) < 0)
-			; /* best effort */
-		close(fd);
-	}
+	return write_reg(rule);
 }
 
 static int check_output(const char *cmd, const char *expected)
@@ -178,7 +131,7 @@ static int run_case(const char *objfile, const char *handler,
 		goto detach;
 	}
 	ret = check_output(target, expect);
-	unregister_entry(entry);
+	unregister(entry);
 detach:
 	bpf_link__destroy(link);
 close:
@@ -186,92 +139,73 @@ close:
 	return ret;
 }
 
-int main(void)
+FIXTURE(bpf_handler) {
+	char obj[PATH_MAX];	/* struct_ops object of the case under test */
+};
+
+FIXTURE_SETUP(bpf_handler)
 {
-	char src[600], obj[600], appdst[600], interpdst[600];
-	char exe[512];
-	ssize_t n;
-	int fail = 0;
-	struct stat st;
+	char src[PATH_MAX];
 	struct btf *btf;
 
-	if (getuid() != 0) {
-		fprintf(stderr, "Skipping: test must be run as root\n");
-		return 4; /* KSFT_SKIP */
-	}
+	if (getuid() != 0)
+		SKIP(return, "test must be run as root");
 
 	/* The kernel must know struct binfmt_misc_ops (CONFIG_BINFMT_MISC_BPF). */
 	btf = btf__load_vmlinux_btf();
 	if (!btf || btf__find_by_name_kind(btf, "binfmt_misc_ops",
 					   BTF_KIND_STRUCT) < 0) {
-		fprintf(stderr,
-			"Skipping: no struct binfmt_misc_ops in the kernel BTF (CONFIG_BINFMT_MISC_BPF)\n");
 		btf__free(btf);
-		return 4; /* KSFT_SKIP */
+		SKIP(return,
+		     "no struct binfmt_misc_ops in the kernel BTF (CONFIG_BINFMT_MISC_BPF)");
 	}
 	btf__free(btf);
 
-	n = readlink("/proc/self/exe", exe, sizeof(exe) - 1);
-	if (n < 0) {
-		perror("readlink");
-		return 1;
-	}
-	exe[n] = '\0';
-	snprintf(testdir, sizeof(testdir), "%s", dirname(exe));
-
-	if (stat("/sys/fs/bpf", &st) < 0)
-		mkdir("/sys/fs/bpf", 0755);
-	mount("bpf", "/sys/fs/bpf", "bpf", 0, NULL);
-	if (access(BINFMT_REG, F_OK) < 0)
-		mount("binfmt_misc", "/proc/sys/fs/binfmt_misc", "binfmt_misc", 0, NULL);
+	if (!binfmt_misc_available())
+		SKIP(return, "no binfmt_misc");
 
 	/* Shared test interpreter. */
-	snprintf(src, sizeof(src), "%s/binfmt_bpf_interp", testdir);
-	if (copy_file(src, INTERP_PATH)) {
-		fprintf(stderr, "cannot install %s\n", INTERP_PATH);
-		return 1;
-	}
-
-	/* Case 1: match a synthetic aarch64 header -> fixed interpreter. */
-	printf("[*] case 1: match aarch64 header -> program-chosen interpreter\n");
-	if (create_fake_aarch64(AARCH64_PATH)) {
-		fprintf(stderr, "cannot create %s\n", AARCH64_PATH);
-		return 1;
-	}
-	snprintf(obj, sizeof(obj), "%s/bpf_interp.bpf.o", testdir);
-	if (run_case(obj, "bpf_interp", "test_bpf_interp", AARCH64_PATH, EXPECT) == 0)
-		printf("[+] case 1 passed\n");
-	else {
-		printf("[-] case 1 FAILED\n");
-		fail = 1;
-	}
-	unlink(AARCH64_PATH);
+	ASSERT_EQ(artifact_path(src, sizeof(src), "binfmt_bpf_interp"), 0);
+	ASSERT_EQ(copy_file(src, INTERP_PATH), 0);
+}
 
-	/* Case 2: $ORIGIN-relative PT_INTERP -> co-located interpreter. */
-	printf("[*] case 2: $ORIGIN interpreter resolved relative to the binary\n");
-	mkdir(RELOC_DIR, 0755);
-	snprintf(appdst, sizeof(appdst), "%s/app", RELOC_DIR);
-	snprintf(interpdst, sizeof(interpdst), "%s/binfmt_bpf_interp", RELOC_DIR);
-	snprintf(src, sizeof(src), "%s/binfmt_bpf_app", testdir);
-	if (copy_file(src, appdst) ||
-	    copy_file(INTERP_PATH, interpdst)) {
-		fprintf(stderr, "cannot set up %s\n", RELOC_DIR);
-		fail = 1;
-	} else {
-		snprintf(obj, sizeof(obj), "%s/nix_origin.bpf.o", testdir);
-		if (run_case(obj, "nix_origin", "test_bpf_origin", appdst, EXPECT) == 0)
-			printf("[+] case 2 passed\n");
-		else {
-			printf("[-] case 2 FAILED\n");
-			fail = 1;
-		}
-	}
-	unlink(appdst);
-	unlink(interpdst);
-	rmdir(RELOC_DIR);
+FIXTURE_TEARDOWN(bpf_handler)
+{
 	unlink(INTERP_PATH);
+}
+
+/* The match program matches a synthetic header, the load program routes it. */
+TEST_F(bpf_handler, fixed_interpreter)
+{
+	ASSERT_EQ(create_fake_elf(AARCH64_PATH, EM_AARCH64), 0);
+	ASSERT_EQ(artifact_path(self->obj, sizeof(self->obj),
+				"bpf_interp.bpf.o"), 0);
+	EXPECT_EQ(run_case(self->obj, "bpf_interp", "test_bpf_interp",
+			   AARCH64_PATH, EXPECT), 0);
+	unlink(AARCH64_PATH);
+}
 
-	if (!fail)
-		printf("[*] all binfmt_misc bpf cases passed\n");
-	return fail;
+/* A "$ORIGIN/..." PT_INTERP resolved to an interpreter next to the binary. */
+TEST_F(bpf_handler, origin_relative_interpreter)
+{
+	char src[PATH_MAX], app[PATH_MAX], interp[PATH_MAX];
+	char dir[] = RELOC_TEMPLATE;
+
+	ASSERT_NE(mkdtemp(dir), NULL);
+	snprintf(app, sizeof(app), "%s/app", dir);
+	snprintf(interp, sizeof(interp), "%s/binfmt_bpf_interp", dir);
+	ASSERT_EQ(artifact_path(src, sizeof(src), "binfmt_bpf_app"), 0);
+	ASSERT_EQ(copy_file(src, app), 0);
+	ASSERT_EQ(copy_file(INTERP_PATH, interp), 0);
+
+	ASSERT_EQ(artifact_path(self->obj, sizeof(self->obj),
+				"nix_origin.bpf.o"), 0);
+	EXPECT_EQ(run_case(self->obj, "nix_origin", "test_bpf_origin",
+			   app, EXPECT), 0);
+
+	unlink(app);
+	unlink(interp);
+	rmdir(dir);
 }
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/exec/binfmt_misc_common.h b/tools/testing/selftests/exec/binfmt_misc_common.h
new file mode 100644
index 000000000000..70ae66082e40
--- /dev/null
+++ b/tools/testing/selftests/exec/binfmt_misc_common.h
@@ -0,0 +1,100 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Helpers shared by the binfmt_misc selftests. */
+#ifndef __SELFTESTS_EXEC_BINFMT_MISC_COMMON_H
+#define __SELFTESTS_EXEC_BINFMT_MISC_COMMON_H
+
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <limits.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <unistd.h>
+
+#define BINFMT_DIR	"/proc/sys/fs/binfmt_misc"
+#define BINFMT_REG	BINFMT_DIR "/register"
+
+static inline int copy_file(const char *src, const char *dst)
+{
+	char buf[4096];
+	int in, out;
+	ssize_t n;
+
+	in = open(src, O_RDONLY);
+	if (in < 0)
+		return -1;
+	/* The tests share /tmp, so never write through a name they don't own. */
+	unlink(dst);
+	out = open(dst, O_WRONLY | O_CREAT | O_EXCL, 0755);
+	if (out < 0) {
+		close(in);
+		return -1;
+	}
+	while ((n = read(in, buf, sizeof(buf))) > 0) {
+		if (write(out, buf, n) != n) {
+			close(in);
+			close(out);
+			return -1;
+		}
+	}
+	close(in);
+	close(out);
+	return n < 0 ? -1 : 0;
+}
+
+/* Write @rule to the register file, preserving the write's errno. */
+static inline int write_reg(const char *rule)
+{
+	int fd, saved;
+	ssize_t n;
+
+	fd = open(BINFMT_REG, O_WRONLY);
+	if (fd < 0)
+		return -1;
+	n = write(fd, rule, strlen(rule));
+	saved = errno;
+	close(fd);
+	errno = saved;
+	return n < 0 ? -1 : 0;
+}
+
+static inline void unregister(const char *name)
+{
+	char path[PATH_MAX];
+	int fd;
+
+	snprintf(path, sizeof(path), BINFMT_DIR "/%s", name);
+	fd = open(path, O_WRONLY);
+	if (fd >= 0) {
+		if (write(fd, "-1", 2) < 0)
+			; /* best effort */
+		close(fd);
+	}
+}
+
+/* Mount binfmt_misc unless it already is, and report whether it is usable. */
+static inline bool binfmt_misc_available(void)
+{
+	if (access(BINFMT_REG, F_OK) < 0)
+		mount("binfmt_misc", BINFMT_DIR, "binfmt_misc", 0, NULL);
+	return access(BINFMT_REG, F_OK) == 0;
+}
+
+/* Absolute path of @name in the directory this test was built into. */
+static inline int artifact_path(char *out, size_t sz, const char *name)
+{
+	char exe[PATH_MAX];
+	ssize_t n;
+
+	n = readlink("/proc/self/exe", exe, sizeof(exe) - 1);
+	if (n < 0)
+		return -1;
+	exe[n] = '\0';
+	if ((size_t)snprintf(out, sz, "%s/%s", dirname(exe), name) >= sz)
+		return -1;
+	return 0;
+}
+
+#endif /* __SELFTESTS_EXEC_BINFMT_MISC_COMMON_H */
diff --git a/tools/testing/selftests/exec/config b/tools/testing/selftests/exec/config
index c308079867b3..2b1973e14291 100644
--- a/tools/testing/selftests/exec/config
+++ b/tools/testing/selftests/exec/config
@@ -1,2 +1,9 @@
 CONFIG_BLK_DEV=y
 CONFIG_BLK_DEV_LOOP=y
+CONFIG_BINFMT_MISC=y
+CONFIG_BINFMT_MISC_BPF=y
+CONFIG_BPF_JIT=y
+CONFIG_BPF_SYSCALL=y
+CONFIG_DEBUG_INFO=y
+CONFIG_DEBUG_INFO_BTF=y
+CONFIG_DEBUG_INFO_DWARF4=y