Re: [PATCH dwarves v3] pahole: Inject kfunc decl tags into BTF

[Alan Maguire <alan.maguire@oracle.com>]

On 29/01/2024 01:30, 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@dxuuu.xyz>
>

This should probably be a BTF feature supported by --btf_features; that
way we'd have a mechanism to switch it off if needed. Can you look at
adding a "tag_kfunc" or whatever name suits into the btf_features[]
array in pahole.c?  Something like:

	BTF_FEATURE(tag_kfunc, btf_tag_kfunc, false),

You'll also then need to add a btf_tag_kfunc boolean field to
struct conf_load, and generation of kfunc tags should then be guarded by

if (conf_load->btf_tag_kfunc)

...so that the tags are added conditionally depending on whether
the user wants them.

Then if a user specifies --btf_features=all or some subset of BTF
features including "tag_kfunc" they will get kfunc tags.

We probably should also move to using --btf_features instead of the
current combination of "--" parameters when pahole is bumped to v1.26.

> ---
> 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(+)rre
> 
> 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;
>  	struct {
>  		struct var_info vars[MAX_PERCPU_VAR_CNT];
>  		int		var_cnt;
> @@ -94,6 +100,17 @@ struct btf_encoder {
>  	} functions;
>  };
>  
> +struct btf_func {
> +	const char *name;
> +	int	    type_id;
> +};
> +
> +/* Half open interval representing range of addresses containing kfuncs */
> +struct btf_kfunc_set_range {
> +	size_t start;
> +	size_t end;
> +};
> +
>  static LIST_HEAD(encoders);
>  static pthread_mutex_t encoders__lock = PTHREAD_MUTEX_INITIALIZER;
>  
> @@ -1352,6 +1369,327 @@ out:
>  	return err;
>  }
>  
> +/* 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;
> +}
> +
> +/*
> + * Parse BTF_ID symbol and return the func name.
> + *
> + * Returns:
> + *	Caller-owned string containing func name if successful.
> + *	NULL if !func or on error.
> + */
> +static char *get_func_name(const char *sym)
> +{
> +	char *func, *end;
> +
> +	if (strncmp(sym, BTF_ID_FUNC_PFX, sizeof(BTF_ID_FUNC_PFX) - 1))
> +		return NULL;
> +
> +	/* Strip prefix */
> +	func = strdup(sym + sizeof(BTF_ID_FUNC_PFX) - 1);
> +
> +	/* Strip suffix */
> +	end = strrchr(func, '_');
> +	if (!end || *(end - 1) != '_') {
> +		free(func);
> +		return NULL;
> +	}
> +	*(end - 1) = '\0';
> +
> +	return func;
> +}
> +
> +static int btf_func_cmp(const void *_a, const void *_b)
> +{
> +	const struct btf_func *a = _a;
> +	const struct btf_func *b = _b;
> +
> +	return strcmp(a->name, b->name);
> +}
> +
> +/*
> + * Collects all functions described in BTF.
> + * Returns non-zero on error.
> + */
> +static int btf_encoder__collect_btf_funcs(struct btf_encoder *encoder)
> +{
> +	struct gobuffer *funcs = &encoder->btf_funcs;
> +	struct btf *btf = encoder->btf;
> +	int nr_types, type_id;
> +	int err = -1;
> +
> +	/* First collect all the func entries into an array */
> +	nr_types = btf__type_cnt(btf);
> +	for (type_id = 1; type_id < nr_types; type_id++) {
> +		const struct btf_type *type;
> +		struct btf_func func = {};
> +		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);
> +			err = -EINVAL;
> +			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);
> +			err = -EINVAL;
> +			goto out;
> +		}
> +
> +		func.name = name;
> +		func.type_id = type_id;
> +		err = gobuffer__add(funcs, &func, sizeof(func));
> +		if (err < 0)
> +			goto out;
> +	}
> +
> +	/* Now that we've collected funcs, sort them by name */
> +	qsort((void *)gobuffer__entries(funcs), gobuffer__nr_entries(funcs),
> +	      sizeof(struct btf_func), btf_func_cmp);
> +
> +	err = 0;
> +out:
> +	return err;
> +}
> +
> +static int btf_encoder__tag_kfunc(struct btf_encoder *encoder, const char *kfunc)
> +{
> +	struct btf_func key = { .name = kfunc };
> +	struct btf *btf = encoder->btf;
> +	struct btf_func *target;
> +	const void *base;
> +	unsigned int cnt;
> +	int err = -1;
> +
> +	base = gobuffer__entries(&encoder->btf_funcs);
> +	cnt = gobuffer__nr_entries(&encoder->btf_funcs);
> +	target = bsearch(&key, base, cnt, sizeof(key), btf_func_cmp);
> +	if (!target) {
> +		fprintf(stderr, "%s: failed to find kfunc '%s' in BTF\n", __func__, kfunc);
> +		goto out;
> +	}
> +
> +	/* Note we are unconditionally adding the btf_decl_tag even
> +	 * though vmlinux may already contain btf_decl_tags for kfuncs.
> +	 * We are ok to do this b/c we will later btf__dedup() to remove
> +	 * any duplicates.
> +	 */
> +	err = btf__add_decl_tag(btf, BTF_KFUNC_TYPE_TAG, target->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;
> +out:
> +	return err;
> +}
> +
> +static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> +{
> +	const char *filename = encoder->filename;
> +	struct gobuffer btf_kfunc_ranges = {};
> +	Elf_Scn *symscn = NULL;
> +	int symbols_shndx = -1;
> +	int fd = -1, err = -1;
> +	int idlist_shndx = -1;
> +	Elf_Scn *scn = NULL;
> +	size_t idlist_addr;
> +	Elf_Data *symbols;
> +	Elf_Data *idlist;
> +	size_t strtabidx;
> +	Elf *elf = NULL;
> +	GElf_Shdr shdr;
> +	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++;
> +		if (!gelf_getshdr(scn, &shdr)) {
> +			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;
> +			symscn = scn;
> +			symbols = data;
> +			strtabidx = shdr.sh_link;
> +		} else if (!strcmp(secname, BTF_IDS_SECTION)) {
> +			idlist_shndx = i;
> +			idlist_addr = shdr.sh_addr;
> +			idlist = data;
> +		}
> +	}
> +
> +	/* 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.
> +	 */
> +	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;
> +			if (ranges[j].start <= addr && addr < ranges[j].end) {
> +				found = true;
> +				break;
> +			}
> +		}
> +		if (!found)
> +			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;