On Sun, Jan 28, 2024 at 06:30:19PM -0700, Daniel Xu wrote:
> 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.
>
> Example of encoding:
>
> $ bpftool btf dump file .tmp_vmlinux.btf | rg "DECL_TAG 'bpf_kfunc'" | wc -l
> 121
>
> $ bpftool btf dump file .tmp_vmlinux.btf | rg 56337
> [56337] FUNC 'bpf_ct_change_timeout' type_id=56336 linkage=static
> [127861] DECL_TAG 'bpf_kfunc' type_id=56337 component_idx=-1
>
> 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.
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
>
> ---
> Changes from v2:
> * More reliably detect kfunc membership in set8 by tracking set addr ranges
> * Rename some variables/functions to be more clear about kfunc vs func
>
> Changes from v1:
> * Fix resource leaks
> * Fix callee -> caller typo
> * Rename btf_decl_tag from kfunc -> bpf_kfunc
> * Only grab btf_id_set funcs tagged kfunc
> * Presort btf func list
>
> btf_encoder.c | 347 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 347 insertions(+)
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index fd04008..4f742b1 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -34,6 +34,11 @@
> #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_ID_SET8_PFX "__BTF_ID__set8__"
> +#define BTF_SET8_KFUNCS (1 << 0)
> +#define BTF_KFUNC_TYPE_TAG "bpf_kfunc"
>
> /* state used to do later encoding of saved functions */
> struct btf_encoder_state {
> @@ -79,6 +84,7 @@ struct btf_encoder {
> gen_floats,
> is_rel;
> uint32_t array_index_id;
> + struct gobuffer btf_funcs;
why does this need to be stored in encoder?
> struct {
> struct var_info vars[MAX_PERCPU_VAR_CNT];
> int var_cnt;
> @@ -94,6 +100,17 @@ struct btf_encoder {
> } functions;
> };
>
SNIP
> +/* Returns if `sym` points to a kfunc set */
> +static int is_sym_kfunc_set(GElf_Sym *sym, const char *name, Elf_Data *idlist, size_t idlist_addr)
> +{
> + int *ptr = idlist->d_buf;
> + int idx, flags;
> + bool is_set8;
> +
> + /* kfuncs are only found in BTF_SET8's */
> + is_set8 = !strncmp(name, BTF_ID_SET8_PFX, sizeof(BTF_ID_SET8_PFX) - 1);
> + if (!is_set8)
> + return false;
> +
> + idx = sym->st_value - idlist_addr;
> + if (idx >= idlist->d_size) {
> + fprintf(stderr, "%s: symbol '%s' out of bounds\n", __func__, name);
> + return false;
> + }
> +
> + /* Check the set8 flags to see if it was marked as kfunc */
> + idx = idx / sizeof(int);
> + flags = ptr[idx + 1];
> + return flags & BTF_SET8_KFUNCS;
I wonder it'd be easier to read/follow if we bring struct btf_id_set8
declaration in here and use it to get the flags field
> +}
> +
> +/*
> + * Parse BTF_ID symbol and return the func name.
> + *
> + * Returns:
> + * Caller-owned string containing func name if successful.
> + * NULL if !func or on error.
> + */
> +
SNIP
> + /* Cannot resolve symbol or .BTF_ids sections. Nothing to do. */
> + if (symbols_shndx == -1 || idlist_shndx == -1) {
> + err = 0;
> + goto out;
> + }
> +
> + if (!gelf_getshdr(symscn, &shdr)) {
> + elf_error("Failed to get ELF symbol table header");
> + goto out;
> + }
> + nr_syms = shdr.sh_size / shdr.sh_entsize;
> +
> + err = btf_encoder__collect_btf_funcs(encoder);
> + if (err) {
> + fprintf(stderr, "%s: failed to collect BTF funcs\n", __func__);
> + goto out;
> + }
> +
> + /* First collect all kfunc set ranges.
> + *
> + * Note we choose not to sort these ranges and accept a linear
> + * search when doing lookups. Reasoning is that the number of
> + * sets is ~O(100) and not worth the additional code to optimize.
> + */
I think we could event add gobuffer interface/support to sort and search
quickly the data (we use it in other place), but that can be done as follow
up when it will become a problem as you pointed out
> + for (i = 0; i < nr_syms; i++) {
> + struct btf_kfunc_set_range range = {};
> + const char *name;
> + GElf_Sym sym;
> +
> + 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);
> + if (!is_sym_kfunc_set(&sym, name, idlist, idlist_addr))
> + continue;
> +
> + range.start = sym.st_value;
> + range.end = sym.st_value + sym.st_size;
> + gobuffer__add(&btf_kfunc_ranges, &range, sizeof(range));
> + }
> +
> + /* Now inject BTF with kfunc decl tag for detected kfuncs */
> + for (i = 0; i < nr_syms; i++) {
> + const struct btf_kfunc_set_range *ranges;
> + unsigned int ranges_cnt;
> + char *func, *name;
> + GElf_Sym sym;
> + bool found;
> + int err;
> + int j;
> +
> + 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);
> + func = get_func_name(name);
> + if (!func)
> + continue;
> +
> + /* Check if function belongs to a kfunc set */
> + ranges = gobuffer__entries(&btf_kfunc_ranges);
> + ranges_cnt = gobuffer__nr_entries(&btf_kfunc_ranges);
> + found = false;
> + for (j = 0; j < ranges_cnt; j++) {
> + size_t addr = sym.st_value;
missing newline after declaration
> + if (ranges[j].start <= addr && addr < ranges[j].end) {
> + found = true;
> + break;
> + }
> + }
> + if (!found)
leaking func
jirka
> + continue;
> +
> + err = btf_encoder__tag_kfunc(encoder, func);
> + if (err) {
> + fprintf(stderr, "%s: failed to tag kfunc '%s'\n", __func__, func);
> + free(func);
> + goto out;
> + }
> + free(func);
> + }
> +
> + err = 0;
> +out:
> + __gobuffer__delete(&btf_kfunc_ranges);
> + if (elf)
> + elf_end(elf);
> + if (fd != -1)
> + close(fd);
> + return err;
> +}
> +
> int btf_encoder__encode(struct btf_encoder *encoder)
> {
> int err;
> @@ -1366,6 +1704,14 @@ int btf_encoder__encode(struct btf_encoder *encoder)
> if (btf__type_cnt(encoder->btf) == 1)
> return 0;
>
> + /* Note vmlinux may already contain btf_decl_tag's for kfuncs. So
> + * take care to call this before btf_dedup().
> + */
> + 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;
> @@ -1712,6 +2058,7 @@ void btf_encoder__delete(struct btf_encoder *encoder)
>
> btf_encoders__delete(encoder);
> __gobuffer__delete(&encoder->percpu_secinfo);
> + __gobuffer__delete(&encoder->btf_funcs);
> zfree(&encoder->filename);
> btf__free(encoder->btf);
> encoder->btf = NULL;
> --
> 2.42.1
>
