Re: [PATCH v2 dwarves 2/5] btf_encoder: Add true_signature feature support for "."-suffixed functions

[Matt Bobrowski <mattbobrowski@google.com>]

On Fri, Jan 23, 2026 at 05:26:47PM +0000, Alan Maguire wrote:
> Currently we collate function information by name and add functions
> provided there are no inconsistencies across various representations.
> 
> For true_signature support - where we wish to add the real signature
> of a function even if it differs from source level - we need to do
> a few things:
> 
> 1. For "."-suffixed functions, we need to match from DWARF->ELF;
>    we can do this via the address associated with the function.
>    In doing this, we can then be confident that the debug info
>    for foo.isra.0 is the right info for the function at that
>    address.
> 
> 2. When adding saved functions we need to look for such cases
>    and provided they do not violate other constraints around BTF
>    representation - unexpected reg usage for function, uncertain
>    parameter location or ambiguous address - we add them with
>    their "."-suffixed name.  The latter can be used as a signal
>    that the function is transformed from the original.
> 
> Doing this adds 500 functions to BTF.  These are traceable with
> their "."-suffix names and because we have excluded ambiguous
> address cases we know exactly which function address they refer
> to.
> 
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

Some minor nits, but apart from that it looks OK to me.

Acked-by: Matt Bobrowski <mattbobrowski@google.com>

> ---
>  btf_encoder.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++-----
>  dwarves.h     |  1 +
>  pahole.c      |  1 +
>  3 files changed, 71 insertions(+), 7 deletions(-)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 9a567e4..c1002c3 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -77,9 +77,16 @@ struct btf_encoder_func_annot {
>  	int16_t component_idx;
>  };
>  
> +struct elf_function_sym {
> +	const char *name;
> +	uint64_t addr;
> +};
> +
>  /* state used to do later encoding of saved functions */
>  struct btf_encoder_func_state {
>  	struct elf_function *elf;
> +	struct elf_function_sym *sym;
> +	uint64_t addr;
  		 ^
		 This appears to have leaked into wrong commit?
		 This member should've been introduced within patch
		 5/5.
		 
>  	uint32_t type_id_off;
>  	uint16_t nr_parms;
>  	uint16_t nr_annots;
> @@ -94,11 +101,6 @@ struct btf_encoder_func_state {
>  	struct btf_encoder_func_annot *annots;
>  };
>  
> -struct elf_function_sym {
> -	const char *name;
> -	uint64_t addr;
> -};
> -
>  struct elf_function {
>  	char		*name;
>  	struct elf_function_sym *syms;
> @@ -145,7 +147,8 @@ struct btf_encoder {
>  			  skip_encoding_decl_tag,
>  			  tag_kfuncs,
>  			  gen_distilled_base,
> -			  encode_attributes;
> +			  encode_attributes,
> +			  true_signature;
>  	uint32_t	  array_index_id;
>  	struct elf_secinfo *secinfo;
>  	size_t             seccnt;
> @@ -1270,14 +1273,36 @@ static int32_t btf_encoder__save_func(struct btf_encoder *encoder, struct functi
>  			goto out;
>  		}
>  	}
> +	if (encoder->true_signature && fn->lexblock.ip.addr) {
> +		int i;
> +
> +		for (i = 0; i < func->sym_cnt; i++) {
> +			if (fn->lexblock.ip.addr != func->syms[i].addr)
> +				continue;
> +			/* Only need to record address for '.'-suffixed
> +			 * functions, since we only currently need true
> +			 * signatures for them.
> +			 */
> +			if (!strchr(func->syms[i].name, '.'))
> +				continue;
> +			state->sym = &func->syms[i];
> +			break;
> +		}
> +	}
>  	state->inconsistent_proto = ftype->inconsistent_proto;
>  	state->unexpected_reg = ftype->unexpected_reg;
>  	state->optimized_parms = ftype->optimized_parms;
>  	state->uncertain_parm_loc = ftype->uncertain_parm_loc;
>  	state->reordered_parm = ftype->reordered_parm;
>  	ftype__for_each_parameter(ftype, param) {
> -		const char *name = parameter__name(param) ?: "";
> +		const char *name;
>  
> +		/* No location info/optimized + reordered means optimized out. */
> +		if (ftype->reordered_parm && (!param->has_loc || param->optimized)) {
> +			state->nr_parms--;
> +			continue;
> +		}
> +		name = parameter__name(param) ?: "";
>  		str_off = btf__add_str(btf, name);
>  		if (str_off < 0) {
>  			err = str_off;
> @@ -1366,6 +1391,9 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder,
>  
>  	btf_fnproto_id = btf_encoder__add_func_proto_for_state(encoder, state);
>  	name = func->name;
> +	if (encoder->true_signature && state->sym)
> +		name = state->sym->name;
> +
>  	if (btf_fnproto_id >= 0)
>  		btf_fn_id = btf_encoder__add_ref_type(encoder, BTF_KIND_FUNC, btf_fnproto_id,
>  						      name, false);
> @@ -1508,6 +1536,39 @@ static int btf_encoder__add_saved_funcs(struct btf_encoder *encoder, bool skip_e
>  		while (j < nr_saved_fns && saved_functions_combine(encoder, &saved_fns[i], &saved_fns[j]) == 0)
>  			j++;
>  
> +		/* Add true signatures for case where we have an exact
> +		 * symbol match by address from DWARF->ELF and have a
> +		 * "." suffixed name.
> +		 */
> +		if (encoder->true_signature) {
> +			bool true_added = false;
> +			int k;
> +
> +			for (k = i; k < nr_saved_fns; k++) {
> +				struct btf_encoder_func_state *true_state = &saved_fns[k];
> +
> +				if (state->elf != true_state->elf)
> +					break;
> +				if (!true_state->sym)
> +					continue;
> +				/* Unexpected reg, uncertain parm loc and
> +				 * ambiguous address mean we cannot trust fentry.
> +				 */
> +				if (true_state->unexpected_reg ||
> +				    true_state->uncertain_parm_loc ||
> +				    true_state->ambiguous_addr)
> +					continue;
> +				err = btf_encoder__add_func(encoder, true_state);
> +				if (err < 0)
> +					goto out;
> +				true_added = true;
> +				break;
> +			}
> +			/* If true symbol was added, skip the below. */
> +			if (true_added)
> +				continue;
> +		}
> +

I think this hunk should be factored out into a
helper. btf_encoder__add_saved_funcs() is starting to get hairy IMO.

>  		/* do not exclude functions with optimized-out parameters; they
>  		 * may still be _called_ with the right parameter values, they
>  		 * just do not _use_ them.  Only exclude functions with
> @@ -2584,6 +2645,7 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
>  		encoder->tag_kfuncs	 = conf_load->btf_decl_tag_kfuncs;
>  		encoder->gen_distilled_base = conf_load->btf_gen_distilled_base;
>  		encoder->encode_attributes = conf_load->btf_attributes;
> +		encoder->true_signature = conf_load->true_signature;
>  		encoder->verbose	 = verbose;
>  		encoder->has_index_type  = false;
>  		encoder->need_index_type = false;
> diff --git a/dwarves.h b/dwarves.h
> index 78bedf5..d7c6474 100644
> --- a/dwarves.h
> +++ b/dwarves.h
> @@ -101,6 +101,7 @@ struct conf_load {
>  	bool			btf_decl_tag_kfuncs;
>  	bool			btf_gen_distilled_base;
>  	bool			btf_attributes;
> +	bool			true_signature;
>  	uint8_t			hashtable_bits;
>  	uint8_t			max_hashtable_bits;
>  	uint16_t		kabi_prefix_len;
> diff --git a/pahole.c b/pahole.c
> index ef01e58..02a0d19 100644
> --- a/pahole.c
> +++ b/pahole.c
> @@ -1234,6 +1234,7 @@ struct btf_feature {
>  	BTF_NON_DEFAULT_FEATURE(global_var, encode_btf_global_vars, false),
>  	BTF_NON_DEFAULT_FEATURE_CHECK(attributes, btf_attributes, false,
>  				      attributes_check),
> +	BTF_NON_DEFAULT_FEATURE(true_signature, true_signature, false),
>  };
>  
>  #define BTF_MAX_FEATURE_STR	1024
> -- 
> 2.43.5
>