Re: [PATCH bpf-next v1 05/14] resolve_btfids: Index BTF ID symbols by address

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 6/16/26 11:45 AM, Emil Tsalapatis wrote:
> On Mon Jun 1, 2026 at 6:17 PM EDT, Ihor Solodrai wrote:
>> Keep an address-sorted index of parsed .BTF_ids symbols so code that
>> the original BTF_ID symbol name can be recovered from an entry
>> address.
>>
>> Use the index in find_kfunc_flags() to scan BTF_SET8_KFUNCS entries
>> directly and match each entry back to the requested kfunc.
>>
>> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> 
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> 
> Some notes on clarity below.
> 
>> ---
>>  tools/bpf/resolve_btfids/main.c | 103 +++++++++++++++++++++++++-------
>>  1 file changed, 80 insertions(+), 23 deletions(-)
>>
>> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
>> index f8a91fa7584f..43512af13148 100644
>> --- a/tools/bpf/resolve_btfids/main.c
>> +++ b/tools/bpf/resolve_btfids/main.c
>> @@ -119,6 +119,11 @@ struct btf_id {
>>  	Elf64_Addr	 addr[ADDR_CNT];
>>  };
>>  
>> +struct addr_sym {
>> +	Elf64_Addr	 addr;
>> +	const char	*name;
>> +};
>> +
>>  struct object {
>>  	const char *path;
>>  	const char *btf_path;
>> @@ -150,6 +155,10 @@ struct object {
>>  	int nr_structs;
>>  	int nr_unions;
>>  	int nr_typedefs;
>> +
>> +	struct addr_sym *addr_syms;
>> +	int nr_addr_syms;
>> +	int max_addr_syms;
>>  };
>>  
>>  #define KF_IMPLICIT_ARGS (1 << 16)
>> @@ -480,6 +489,49 @@ static int elf_collect(struct object *obj)
>>  	return 0;
>>  }
>>  
>> +static int push_addr_sym(struct object *obj, Elf64_Addr addr, const char *name)
>> +{
>> +	struct addr_sym *arr = obj->addr_syms;
>> +	int cap = obj->max_addr_syms;
>> +
>> +	if (obj->nr_addr_syms + 1 > cap) {
>> +		cap = max(cap + 256, cap * 2);
>> +		arr = realloc(arr, sizeof(*arr) * cap);
>> +		if (!arr)
>> +			return -ENOMEM;
>> +		obj->max_addr_syms = cap;
>> +		obj->addr_syms = arr;
>> +	}
>> +
>> +	obj->addr_syms[obj->nr_addr_syms].addr = addr;
>> +	obj->addr_syms[obj->nr_addr_syms].name = name;
>> +	obj->nr_addr_syms++;
>> +
>> +	return 0;
>> +}
>> +
>> +static int cmp_addr_sym(const void *a, const void *b)
>> +{
>> +	Elf64_Addr aa = ((const struct addr_sym *)a)->addr;
>> +	Elf64_Addr ab = ((const struct addr_sym *)b)->addr;
>> +
>> +	if (aa < ab)
>> +		return -1;
>> +	if (aa > ab)
>> +		return 1;
>> +	return 0;
>> +}
>> +
>> +static const char *find_name_by_addr(struct object *obj, Elf64_Addr addr)
>> +{
>> +	struct addr_sym key = { .addr = addr };
>> +	struct addr_sym *res;
>> +
>> +	res = bsearch(&key, obj->addr_syms, obj->nr_addr_syms,
>> +		      sizeof(*obj->addr_syms), cmp_addr_sym);
>> +	return res ? res->name : NULL;
>> +}
>> +
>>  static int symbols_collect(struct object *obj)
>>  {
>>  	Elf_Scn *scn = NULL;
>> @@ -573,8 +625,14 @@ static int symbols_collect(struct object *obj)
>>  			return -1;
>>  		}
>>  		id->addr[id->addr_cnt++] = sym.st_value;
>> +
>> +		if (push_addr_sym(obj, sym.st_value, id->name))
>> +			return -1;
>>  	}
>>  
>> +	qsort(obj->addr_syms, obj->nr_addr_syms, sizeof(*obj->addr_syms),
>> +	      cmp_addr_sym);
>> +
>>  	return 0;
>>  }
>>  
>> @@ -946,43 +1004,41 @@ static int collect_decl_tags(struct btf2btf_context *ctx)
>>  }
>>  
>>  /*
>> - * To find the kfunc flags having its struct btf_id (with ELF addresses)
>> - * we need to find the address that is in range of a set8.
>> - * If a set8 is found, then the flags are located at addr + 4 bytes.
>> + * To find kfunc flags, scan BTF_SET8_KFUNCS entries and use the entry
>> + * address to recover the corresponding BTF_ID symbol name.
>>   * Return 0 (no flags!) if not found.
>>   */
>>  static u32 find_kfunc_flags(struct object *obj, struct btf_id *kfunc_id)
>>  {
>>  	const u32 *elf_data_ptr = obj->efile.idlist->d_buf;
>> -	u64 set_lower_addr, set_upper_addr, addr;
>>  	struct btf_id *set_id;
>>  	struct rb_node *next;
>> -	u32 flags;
>> -	u64 idx;
>> +	u64 idx, set_addr;
>> +	u32 set_flags;
>>  
>>  	for (next = rb_first(&obj->sets); next; next = rb_next(next)) {
>>  		set_id = rb_entry(next, struct btf_id, rb_node);
>>  		if (set_id->kind != BTF_ID_KIND_SET8 || set_id->addr_cnt != 1)
>>  			continue;
>>  
>> -		set_lower_addr = set_id->addr[0];
>> -		set_upper_addr = set_lower_addr + set_id->cnt * sizeof(u64);
>> +		set_addr = set_id->addr[0];
>> +		idx = (set_addr - obj->efile.idlist_addr) / sizeof(u32) + 1;
> 
> Is the +1 because the set8-global flags are in &((char *)btf_id_set8)[4]?
> If that's the case, can you add a comment that you're checking the
> set-global flags to decide whether to skip all of it?

I explained this here:
https://lore.kernel.org/bpf/2d2329ac-5394-4eac-926a-990c83eabaa1@linux.dev/

I'll try to rework this to avoid mysterious +1 increments.

> 
>> +		set_flags = elf_data_ptr[idx];
>> +		if (!(set_flags & BTF_SET8_KFUNCS))
>> +			continue;
>>  
>> -		for (u32 i = 0; i < kfunc_id->addr_cnt; i++) {
>> -			addr = kfunc_id->addr[i];
>> -			/*
>> -			 * Lower bound is exclusive to skip the 8-byte header of the set.
>> -			 * Upper bound is inclusive to capture the last entry at offset 8*cnt.
>> -			 */
>> -			if (set_lower_addr < addr && addr <= set_upper_addr) {
>> -				pr_debug("found kfunc %s in BTF_ID_FLAGS %s\n",
>> -					 kfunc_id->name, set_id->name);
>> -				idx = addr - obj->efile.idlist_addr;
>> -				idx = idx / sizeof(u32) + 1;
>> -				flags = elf_data_ptr[idx];
>> -
>> -				return flags;
>> -			}
>> +		for (u32 i = 0; i < set_id->cnt; i++) {
>> +			Elf64_Addr addr = set_addr + sizeof(u64) * (i + 1);
> 
> Same here, can you point out that the +1 is to skip the set header?
> 
>> +			const char *name = find_name_by_addr(obj, addr);
>> +
>> +			if (!name || strcmp(name, kfunc_id->name) != 0)
>> +				continue;
>> +
>> +			pr_debug("found kfunc %s in BTF_ID_FLAGS %s\n",
>> +				 kfunc_id->name, set_id->name);
>> +
>> +			idx = (addr - obj->efile.idlist_addr) / sizeof(u32) + 1;
> 
> This seems to be the same calculation as we do for the whole set8 flag,
> but AFAICT the context is different and here we're indexing into the pair. 
> Can you add a comment for this?
> 
>> +			return elf_data_ptr[idx];
>>  		}
>>  	}
>>  
>> @@ -1575,6 +1631,7 @@ int main(int argc, const char **argv)
>>  	btf_id__free_all(&obj.typedefs);
>>  	btf_id__free_all(&obj.funcs);
>>  	btf_id__free_all(&obj.sets);
>> +	free(obj.addr_syms);
>>  	if (obj.efile.elf) {
>>  		elf_end(obj.efile.elf);
>>  		close(obj.efile.fd);
>