selftests/bpf: make veristat skip non-BPF and failing-to-open BPF objects

Make veristat ignore non-BPF object files. This allows simpler
mass-verification (e.g., `sudo ./veristat *.bpf.o` in selftests/bpf
directory). Note that `sudo ./veristat *.o` would also work, but with
selftests's multiple copies of BPF object files (.bpf.o and
.bpf.linked{1,2,3}.o) it's 4x slower.

Also, given some of BPF object files could be incomplete in the sense
that they are meant to be statically linked into final BPF object file
(like linked_maps, linked_funcs, linked_vars), note such instances in
stderr, but proceed anyways. This seems like a better trade off between
completely silently ignoring BPF object file and aborting
mass-verification altogether.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220923175913.3272430-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/veristat.c

@@ -15,6 +15,8 @@
 #include <sys/sysinfo.h>
 #include <sys/stat.h>
 #include <bpf/libbpf.h>
+#include <libelf.h>
+#include <gelf.h>
 
 enum stat_id {
 	VERDICT,
@@ -78,6 +80,11 @@ static struct env {
 	struct filter *deny_filters;
 	int allow_filter_cnt;
 	int deny_filter_cnt;
+
+	int files_processed;
+	int files_skipped;
+	int progs_processed;
+	int progs_skipped;
 } env;
 
 static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
@@ -226,8 +233,41 @@ static bool should_process_file(const char *filename)
 	return false;
 }
 
-static bool should_process_prog(const char *filename, const char *prog_name)
+static bool is_bpf_obj_file(const char *path) {
+	Elf64_Ehdr *ehdr;
+	int fd, err = -EINVAL;
+	Elf *elf = NULL;
+
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return true; /* we'll fail later and propagate error */
+
+	/* ensure libelf is initialized */
+	(void)elf_version(EV_CURRENT);
+
+	elf = elf_begin(fd, ELF_C_READ, NULL);
+	if (!elf)
+		goto cleanup;
+
+	if (elf_kind(elf) != ELF_K_ELF || gelf_getclass(elf) != ELFCLASS64)
+		goto cleanup;
+
+	ehdr = elf64_getehdr(elf);
+	/* Old LLVM set e_machine to EM_NONE */
+	if (!ehdr || ehdr->e_type != ET_REL || (ehdr->e_machine && ehdr->e_machine != EM_BPF))
+		goto cleanup;
+
+	err = 0;
+cleanup:
+	if (elf)
+		elf_end(elf);
+	close(fd);
+	return err == 0;
+}
+
+static bool should_process_prog(const char *path, const char *prog_name)
 {
+	const char *filename = basename(path);
 	int i;
 
 	if (env.deny_filter_cnt > 0) {
@@ -303,7 +343,7 @@ static int append_filter_file(const char *path)
 	f = fopen(path, "r");
 	if (!f) {
 		err = -errno;
-		fprintf(stderr, "Failed to open '%s': %d\n", path, err);
+		fprintf(stderr, "Failed to open filters in '%s': %d\n", path, err);
 		return err;
 	}
 
@@ -463,8 +503,10 @@ static int process_prog(const char *filename, struct bpf_object *obj, struct bpf
 	int err = 0;
 	void *tmp;
 
-	if (!should_process_prog(basename(filename), bpf_program__name(prog)))
+	if (!should_process_prog(filename, bpf_program__name(prog))) {
+		env.progs_skipped++;
 		return 0;
+	}
 
 	tmp = realloc(env.prog_stats, (env.prog_stat_cnt + 1) * sizeof(*env.prog_stats));
 	if (!tmp)
@@ -487,6 +529,7 @@ static int process_prog(const char *filename, struct bpf_object *obj, struct bpf
 	verif_log_buf[0] = '\0';
 
 	err = bpf_object__load(obj);
+	env.progs_processed++;
 
 	stats->file_name = strdup(basename(filename));
 	stats->prog_name = strdup(bpf_program__name(prog));
@@ -513,18 +556,37 @@ static int process_obj(const char *filename)
 	LIBBPF_OPTS(bpf_object_open_opts, opts);
 	int err = 0, prog_cnt = 0;
 
-	if (!should_process_file(basename(filename)))
+	if (!should_process_file(basename(filename))) {
+		if (env.verbose)
+			printf("Skipping '%s' due to filters...\n", filename);
+		env.files_skipped++;
 		return 0;
+	}
+	if (!is_bpf_obj_file(filename)) {
+		if (env.verbose)
+			printf("Skipping '%s' as it's not a BPF object file...\n", filename);
+		env.files_skipped++;
+		return 0;
+	}
 
 	old_libbpf_print_fn = libbpf_set_print(libbpf_print_fn);
 
 	obj = bpf_object__open_file(filename, &opts);
 	if (!obj) {
-		err = -errno;
-		fprintf(stderr, "Failed to open '%s': %d\n", filename, err);
+		/* if libbpf can't open BPF object file, it could be because
+		 * that BPF object file is incomplete and has to be statically
+		 * linked into a final BPF object file; instead of bailing
+		 * out, report it into stderr, mark it as skipped, and
+		 * proceeed
+		 */
+		fprintf(stderr, "Failed to open '%s': %d\n", filename, -errno);
+		env.files_skipped++;
+		err = 0;
 		goto cleanup;
 	}
 
+	env.files_processed++;
+
 	bpf_object__for_each_program(prog, obj) {
 		prog_cnt++;
 	}
@@ -732,8 +794,8 @@ static void output_stats(const struct verif_stats *s, enum resfmt fmt, bool last
 
 	if (last && fmt == RESFMT_TABLE) {
 		output_header_underlines();
-		printf("Done. Processed %d object files, %d programs.\n",
-		       env.filename_cnt, env.prog_stat_cnt);
+		printf("Done. Processed %d files, %d programs. Skipped %d files, %d programs.\n",
+		       env.files_processed, env.files_skipped, env.progs_processed, env.progs_skipped);
 	}
 }