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>

1 files changed, 57 insertions, 7 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 3abf2dd1b3b5..8462dab02812 100644
--- 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;