This commit teaches pahole to parse symbols in .BTF_ids section in
vmlinux and discover exported kfuncs. Pahole then takes the list of
kfuncs and injects a BTF_KIND_DECL_TAG for each kfunc.
This enables downstream users and tools to dynamically discover which
kfuncs are available on a system by parsing vmlinux or module BTF, both
available in /sys/kernel/btf.
Example of encoding:
$ bpftool btf dump file .tmp_vmlinux.btf | rg DECL_TAG | wc -l
388
$ bpftool btf dump file .tmp_vmlinux.btf | rg 68940
[68940] FUNC 'bpf_xdp_get_xfrm_state' type_id=68939 linkage=static
[128124] DECL_TAG 'kfunc' type_id=68940 component_idx=-1
Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
---
btf_encoder.c | 202 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 202 insertions(+)
diff --git a/btf_encoder.c b/btf_encoder.c
index fd04008..2697214 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -34,6 +34,9 @@
#include <pthread.h>
#define BTF_ENCODER_MAX_PROTO 512
+#define BTF_IDS_SECTION ".BTF_ids"
+#define BTF_ID_FUNC_PFX "__BTF_ID__func__"
+#define BTF_KFUNC_TYPE_TAG "kfunc"
/* state used to do later encoding of saved functions */
struct btf_encoder_state {
@@ -1352,6 +1355,200 @@ out:
return err;
}
+/*
+ * Parse BTF_ID symbol and return the kfunc name.
+ *
+ * Returns:
+ * Callee-owned string containing kfunc name if successful.
+ * NULL if !kfunc or on error.
+ */
+static char *get_kfunc_name(const char *sym)
+{
+ char *kfunc, *end;
+
+ if (strncmp(sym, BTF_ID_FUNC_PFX, sizeof(BTF_ID_FUNC_PFX) - 1))
+ return NULL;
+
+ /* Strip prefix */
+ kfunc = strdup(sym + sizeof(BTF_ID_FUNC_PFX) - 1);
+
+ /* Strip suffix */
+ end = strrchr(kfunc, '_');
+ if (!end || *(end - 1) != '_') {
+ free(kfunc);
+ return NULL;
+ }
+ *(end - 1) = '\0';
+
+ return kfunc;
+}
+
+static int btf_encoder__tag_kfunc(struct btf_encoder *encoder, const char *kfunc)
+{
+ int nr_types, type_id, err = -1;
+ struct btf *btf = encoder->btf;
+
+ nr_types = btf__type_cnt(btf);
+ for (type_id = 1; type_id < nr_types; type_id++) {
+ const struct btf_type *type;
+ const char *name;
+
+ type = btf__type_by_id(btf, type_id);
+ if (!type) {
+ fprintf(stderr, "%s: malformed BTF, can't resolve type for ID %d\n",
+ __func__, type_id);
+ goto out;
+ }
+
+ if (!btf_is_func(type))
+ continue;
+
+ name = btf__name_by_offset(btf, type->name_off);
+ if (!name) {
+ fprintf(stderr, "%s: malformed BTF, can't resolve name for ID %d\n",
+ __func__, type_id);
+ goto out;
+ }
+
+ if (strcmp(name, kfunc))
+ continue;
+
+ err = btf__add_decl_tag(btf, BTF_KFUNC_TYPE_TAG, type_id, -1);
+ if (err < 0) {
+ fprintf(stderr, "%s: failed to insert kfunc decl tag for '%s': %d\n",
+ __func__, kfunc, err);
+ goto out;
+ }
+
+ err = 0;
+ break;
+ }
+
+out:
+ return err;
+}
+
+static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
+{
+ const char *filename = encoder->filename;
+ GElf_Shdr shdr_mem, *shdr;
+ int symbols_shndx = -1;
+ int idlist_shndx = -1;
+ Elf_Scn *scn = NULL;
+ Elf_Data *symbols;
+ int fd, err = -1;
+ size_t strtabidx;
+ Elf *elf = NULL;
+ size_t strndx;
+ char *secname;
+ int nr_syms;
+ int i = 0;
+
+ fd = open(filename, O_RDONLY);
+ if (fd < 0) {
+ fprintf(stderr, "Cannot open %s\n", filename);
+ goto out;
+ }
+
+ if (elf_version(EV_CURRENT) == EV_NONE) {
+ elf_error("Cannot set libelf version");
+ goto out;
+ }
+
+ elf = elf_begin(fd, ELF_C_READ, NULL);
+ if (elf == NULL) {
+ elf_error("Cannot update ELF file");
+ goto out;
+ }
+
+ /* Location symbol table and .BTF_ids sections */
+ elf_getshdrstrndx(elf, &strndx);
+ while ((scn = elf_nextscn(elf, scn)) != NULL) {
+ Elf_Data *data;
+
+ i++;
+ shdr = gelf_getshdr(scn, &shdr_mem);
+ if (shdr == NULL) {
+ elf_error("Failed to get ELF section(%d) hdr", i);
+ goto out;
+ }
+
+ secname = elf_strptr(elf, strndx, shdr->sh_name);
+ if (!secname) {
+ elf_error("Failed to get ELF section(%d) hdr name", i);
+ goto out;
+ }
+
+ data = elf_getdata(scn, 0);
+ if (!data) {
+ elf_error("Failed to get ELF section(%d) data", i);
+ goto out;
+ }
+
+ if (shdr->sh_type == SHT_SYMTAB) {
+ symbols_shndx = i;
+ symbols = data;
+ strtabidx = shdr->sh_link;
+ } else if (!strcmp(secname, BTF_IDS_SECTION)) {
+ idlist_shndx = i;
+ }
+ }
+
+ /* Cannot resolve symbol or .BTF_ids sections. Nothing to do. */
+ if (symbols_shndx == -1 || idlist_shndx == -1) {
+ err = 0;
+ goto out;
+ }
+
+ /*
+ * Look for __BTF_ID__func__ symbols in .BTF_ids section and
+ * inject BTF decl tags for each of them.
+ */
+ scn = elf_getscn(elf, symbols_shndx);
+ if (!scn) {
+ elf_error("Failed to get ELF symbol table");
+ goto out;
+ }
+
+ shdr = gelf_getshdr(scn, &shdr_mem);
+ if (!shdr) {
+ elf_error("Failed to get ELF symbol table header");
+ goto out;
+ }
+
+ nr_syms = shdr->sh_size / shdr->sh_entsize;
+ for (i = 0; i < nr_syms; i++) {
+ char *kfunc, *name;
+ GElf_Sym sym;
+ int err;
+
+ if (!gelf_getsym(symbols, i, &sym)) {
+ elf_error("Failed to get ELF symbol(%d)", i);
+ goto out;
+ }
+
+ if (sym.st_shndx != idlist_shndx)
+ continue;
+
+ name = elf_strptr(elf, strtabidx, sym.st_name);
+ kfunc = get_kfunc_name(name);
+ if (!kfunc)
+ continue;
+
+ err = btf_encoder__tag_kfunc(encoder, kfunc);
+ if (err) {
+ fprintf(stderr, "%s: failed to tag kfunc '%s'\n", __func__, kfunc);
+ free(kfunc);
+ goto out;
+ }
+ free(kfunc);
+ }
+
+ err = 0;
+out:
+ return err;
+}
+
int btf_encoder__encode(struct btf_encoder *encoder)
{
int err;
@@ -1366,6 +1563,11 @@ int btf_encoder__encode(struct btf_encoder *encoder)
if (btf__type_cnt(encoder->btf) == 1)
return 0;
+ if (btf_encoder__tag_kfuncs(encoder)) {
+ fprintf(stderr, "%s: failed to tag kfuncs!\n", __func__);
+ return -1;
+ }
+
if (btf__dedup(encoder->btf, NULL)) {
fprintf(stderr, "%s: btf__dedup failed!\n", __func__);
return -1;
--
2.42.1
