libbpf: attempt to load kernel BTF from sysfs first

Add support for loading kernel BTF from sysfs (/sys/kernel/btf/vmlinux)
as a target BTF. Also extend the list of on disk search paths for
vmlinux ELF image with entries that perf is searching for.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 3abf2dd..8462dab 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2807,15 +2807,61 @@ static int bpf_core_reloc_insn(struct bpf_program *prog, int insn_off,
        return 0;
 }
 
+static struct btf *btf_load_raw(const char *path)
+{
+       struct btf *btf;
+       size_t read_cnt;
+       struct stat st;
+       void *data;
+       FILE *f;
+
+       if (stat(path, &st))
+               return ERR_PTR(-errno);
+
+       data = malloc(st.st_size);
+       if (!data)
+               return ERR_PTR(-ENOMEM);
+
+       f = fopen(path, "rb");
+       if (!f) {
+               btf = ERR_PTR(-errno);
+               goto cleanup;
+       }
+
+       read_cnt = fread(data, 1, st.st_size, f);
+       fclose(f);
+       if (read_cnt < st.st_size) {
+               btf = ERR_PTR(-EBADF);
+               goto cleanup;
+       }
+
+       btf = btf__new(data, read_cnt);
+
+cleanup:
+       free(data);
+       return btf;
+}
+
 /*
  * Probe few well-known locations for vmlinux kernel image and try to load BTF
  * data out of it to use for target BTF.
  */
 static struct btf *bpf_core_find_kernel_btf(void)
 {
-       const char *locations[] = {
-               "/lib/modules/%1$s/vmlinux-%1$s",
-               "/usr/lib/modules/%1$s/kernel/vmlinux",
+       struct {
+               const char *path_fmt;
+               bool raw_btf;
+       } locations[] = {
+               /* try canonical vmlinux BTF through sysfs first */
+               { "/sys/kernel/btf/vmlinux", true /* raw BTF */ },
+               /* fall back to trying to find vmlinux ELF on disk otherwise */
+               { "/boot/vmlinux-%1$s" },
+               { "/lib/modules/%1$s/vmlinux-%1$s" },
+               { "/lib/modules/%1$s/build/vmlinux" },
+               { "/usr/lib/modules/%1$s/kernel/vmlinux" },
+               { "/usr/lib/debug/boot/vmlinux-%1$s" },
+               { "/usr/lib/debug/boot/vmlinux-%1$s.debug" },
+               { "/usr/lib/debug/lib/modules/%1$s/vmlinux" },
        };
        char path[PATH_MAX + 1];
        struct utsname buf;
@@ -2825,14 +2871,18 @@ static struct btf *bpf_core_find_kernel_btf(void)
        uname(&buf);
 
        for (i = 0; i < ARRAY_SIZE(locations); i++) {
-               snprintf(path, PATH_MAX, locations[i], buf.release);
+               snprintf(path, PATH_MAX, locations[i].path_fmt, buf.release);
 
                if (access(path, R_OK))
                        continue;
 
-               btf = btf__parse_elf(path, NULL);
-               pr_debug("kernel BTF load from '%s': %ld\n",
-                        path, PTR_ERR(btf));
+               if (locations[i].raw_btf)
+                       btf = btf_load_raw(path);
+               else
+                       btf = btf__parse_elf(path, NULL);
+
+               pr_debug("loading kernel BTF '%s': %ld\n",
+                        path, IS_ERR(btf) ? PTR_ERR(btf) : 0);
                if (IS_ERR(btf))
                        continue;