Re: [PATCH dwarves v5 2/2] pahole: Inject kfunc decl tags into BTF

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Wed, Mar 20, 2024 at 12:54:11PM +0000, Alan Maguire wrote:
> On 15/03/2024 19:48, 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.
> > 
> > This feature is enabled with --btf_features=decl_tag,decl_tag_kfuncs.
> > 
> > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> 
> This is great work; a lot of steps needed to collect this info, but it's
> really valuable!
> 
> Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
> Tested-by: Alan Maguire <alan.maguire@oracle.com>
> 
> BTW we need something like the attached patch to switch to using
> --btf_features for pahole 1.26 and later; will I send it officially or
> do you have something that does the same that you want to roll into your
> bpf-next series? Let me know what works from your side. Thanks!
> 
> > ---
> >  btf_encoder.c | 368 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 368 insertions(+)
> > 
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index 850e36f..31848e2 100644
> > --- a/btf_encoder.c
> > +++ b/btf_encoder.c
> > @@ -34,6 +34,21 @@
> >  #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"
> > +
> > +/* Adapted from include/linux/btf_ids.h */
> > +struct btf_id_set8 {
> > +        uint32_t cnt;
> > +        uint32_t flags;
> > +        struct {
> > +                uint32_t id;
> > +                uint32_t flags;
> > +        } pairs[];
> > +};
> >  
> >  /* state used to do later encoding of saved functions */
> >  struct btf_encoder_state {
> > @@ -75,6 +90,7 @@ struct btf_encoder {
> >  			  verbose,
> >  			  force,
> >  			  gen_floats,
> > +			  skip_encoding_decl_tag,
> >  			  tag_kfuncs,
> >  			  is_rel;
> >  	uint32_t	  array_index_id;
> > @@ -94,6 +110,17 @@ 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;

Why size_t for addresses?

> > +};
> > +
> >  static LIST_HEAD(encoders);
> >  static pthread_mutex_t encoders__lock = PTHREAD_MUTEX_INITIALIZER;
> >  
> > @@ -1363,8 +1390,339 @@ 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)
> > +{
> > +	void *ptr = idlist->d_buf;
> > +	struct btf_id_set8 *set;
> > +	bool is_set8;
> > +	int off;
> > +
> > +	/* 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;


We have, as the kernel and tools/perf:

dutil.h: * strstarts - does @str start with @prefix?
dutil.h:static inline bool strstarts(const char *str, const char *prefix)

Can you please use it?

> > +
> > +	off = sym->st_value - idlist_addr;
> > +	if (off >= 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 */
> > +	set = ptr + off;
> > +	return set->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;

strstarts()

> > +	/* Strip prefix */
> > +	func = strdup(sym + sizeof(BTF_ID_FUNC_PFX) - 1);
> > +	if (strlen(func) < 2) {
> > +                free(func);
> > +                return NULL;
> > +        }

Can you add a comment as to why this exception? Does this limitation
matches one in the kernel? Where?

> > +
> > +	/* Strip suffix */
> > +	end = strrchr(func, '_');
> > +	if (!end || *(end - 1) != '_') {
> > +		free(func);
> > +		return NULL;
> > +	}
> > +	*(end - 1) = '\0';

So the suffix has to have __? Can you add an example of such a func name
as a comment above this chopping?

> > +	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)
> > +{
> > +	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);

Better to introcduce a 'gobuffer__sort(funcs, sizeof(struct btf_func), btf_func_cmp)' like we
have a gobuffer__compress(), please.

> > +	err = 0;
> > +out:
> > +	return err;
> > +}
> > +
> > +static int btf_encoder__tag_kfunc(struct btf_encoder *encoder, struct gobuffer *funcs, 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(funcs);
> > +	cnt = gobuffer__nr_entries(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 = {};
> > +	struct gobuffer btf_funcs = {};
> > +	Elf_Data *symbols = NULL;
> > +	Elf_Data *idlist = NULL;
> > +	Elf_Scn *symscn = NULL;
> > +	int symbols_shndx = -1;
> > +	size_t idlist_addr = 0;
> > +	int fd = -1, err = -1;
> > +	int idlist_shndx = -1;
> > +	size_t strtabidx = 0;
> > +	Elf_Scn *scn = NULL;
> > +	Elf *elf = NULL;
> > +	GElf_Shdr shdr;
> > +	size_t strndx;
> > +	char *secname;
> > +	int nr_syms;
> > +	int i = 0;

You did some coding on the Linux kernel net/  directory? 8-)

Good.

> > +	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;
> > +	}
> > +
> > +	/* Locate symbol table and .BTF_ids sections */
> > +	if (elf_getshdrstrndx(elf, &strndx) < 0)
> > +		goto out;
> > +	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, &btf_funcs);
> > +	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) {
> > +			free(func);
> > +			continue;
> > +		}
> > +
> > +		err = btf_encoder__tag_kfunc(encoder, &btf_funcs, 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_funcs);
> > +	__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)
> >  {
> > +	bool should_tag_kfuncs;
> >  	int err;
> >  
> >  	/* for single-threaded case, saved funcs are added here */
> > @@ -1377,6 +1735,15 @@ 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().
> > +	 */
> > +	should_tag_kfuncs = encoder->tag_kfuncs && !encoder->skip_encoding_decl_tag;
> > +	if (should_tag_kfuncs && 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;
> > @@ -1660,6 +2027,7 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
> >  		encoder->force		 = conf_load->btf_encode_force;
> >  		encoder->gen_floats	 = conf_load->btf_gen_floats;
> >  		encoder->skip_encoding_vars = conf_load->skip_encoding_btf_vars;
> > +		encoder->skip_encoding_decl_tag	 = conf_load->skip_encoding_btf_decl_tag;
> >  		encoder->tag_kfuncs	 = conf_load->btf_decl_tag_kfuncs;
> >  		encoder->verbose	 = verbose;
> >  		encoder->has_index_type  = false;