[dwarves,v6,3/3] pahole: Inject kfunc decl tags into BTF

Commit Message

Daniel Xu March 25, 2024, 5:53 p.m. UTC

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.

Acked-by: Jiri Olsa <[e-mail]>
Tested-by: Jiri Olsa <[e-mail]>
Reviewed-by: Alan Maguire <[e-mail]>
Tested-by: Alan Maguire <[e-mail]>
Signed-off-by: Daniel Xu <[e-mail]>
---
 btf_encoder.c | 372 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 372 insertions(+)

Comments

Alan Maguire April 17, 2024, 9:32 a.m. UTC | #1

On 25/03/2024 17:53, 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.
> 
> Acked-by: Jiri Olsa <[e-mail]>
> Tested-by: Jiri Olsa <[e-mail]>
> Reviewed-by: Alan Maguire <[e-mail]>
> Tested-by: Alan Maguire <[e-mail]>

I re-tested for both vmlinux and a module with a kfunc (nf_nat); both
generated decl_tags pointing at the kfunc functions so all working
great. Nice work!

> Signed-off-by: Daniel Xu <[e-mail]>
> ---
>  btf_encoder.c | 372 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 372 insertions(+)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 850e36f..d326404 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 {
> +	uint64_t start;
> +	uint64_t end;
> +};
> +
>  static LIST_HEAD(encoders);
>  static pthread_mutex_t encoders__lock = PTHREAD_MUTEX_INITIALIZER;
>  
> @@ -1363,8 +1390,343 @@ 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;
> +	int off;
> +
> +	/* kfuncs are only found in BTF_SET8's */
> +	if (!strstarts(name, BTF_ID_SET8_PFX))
> +		return false;
> +
> +	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;
> +
> +	/* Example input: __BTF_ID__func__vfs_close__1
> +	 *
> +	 * The goal is to strip the prefix and suffix such that we only
> +	 * return vfs_close.
> +	 */
> +
> +	if (!strstarts(sym, BTF_ID_FUNC_PFX))
> +		return NULL;
> +
> +	/* Strip prefix and handle malformed input such as  __BTF_ID__func___ */
> +	func = strdup(sym + sizeof(BTF_ID_FUNC_PFX) - 1);
> +	if (strlen(func) < 2) {
> +                free(func);
> +                return NULL;
> +        }
> +
> +	/* 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)
> +{
> +	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 */
> +	gobuffer__sort(funcs, sizeof(struct btf_func), btf_func_cmp);
> +
> +	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;
> +
> +	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 +1739,15 @@ 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 +2031,7 @@ 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;

Patch

diff --git a/btf_encoder.c b/btf_encoder.c
index 850e36f..d326404 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 {
+	uint64_t start;
+	uint64_t end;
+};
+
 static LIST_HEAD(encoders);
 static pthread_mutex_t encoders__lock = PTHREAD_MUTEX_INITIALIZER;
 
@@ -1363,8 +1390,343 @@  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;
+	int off;
+
+	/* kfuncs are only found in BTF_SET8's */
+	if (!strstarts(name, BTF_ID_SET8_PFX))
+		return false;
+
+	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;
+
+	/* Example input: __BTF_ID__func__vfs_close__1
+	 *
+	 * The goal is to strip the prefix and suffix such that we only
+	 * return vfs_close.
+	 */
+
+	if (!strstarts(sym, BTF_ID_FUNC_PFX))
+		return NULL;
+
+	/* Strip prefix and handle malformed input such as  __BTF_ID__func___ */
+	func = strdup(sym + sizeof(BTF_ID_FUNC_PFX) - 1);
+	if (strlen(func) < 2) {
+                free(func);
+                return NULL;
+        }
+
+	/* 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)
+{
+	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 */
+	gobuffer__sort(funcs, sizeof(struct btf_func), btf_func_cmp);
+
+	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;
+
+	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 +1739,15 @@  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 +2031,7 @@  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;