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

Matt Bobrowski
2026-01-26T11:18:47Z

On Mon, Jan 26, 2026 at 10:49:00AM +0000, Alan Maguire wrote:
> On 26/01/2026 09:52, Matt Bobrowski wrote:
> > 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.
> >
>
> It's used here too though; specifically in btf_encoder__save_func()
> below. We need to use addresses to map between a DWARF representation
> and the associated ELF function to ensure we're using debug info
> for the correct "."-suffixed function. We need to do this because
> the late DWARF generated after optimizations are applied uses
> the original name ("foo" rather than "foo.isra.0").

Sorry, I thought I only saw usage of member "addr" within
btf_encoder__save_func() against fn->lenblock.ip.addr which maps to
member "addr" within struct ip_tag and func->syms[i].addr which maps
to member "addr" within struct elf_function_sym. Member "addr" within
struct btf_encoder_func_state remains unused until patch 5/5 where we
assign it the value returned from function__addr().

With that said, within btf_encoder__save_func() why don't you do the
following instead:

state->addr = function__addr(fn);

...

if (encoder->true_signature && state->addr) {
if (state->addr != func->syms[i].addr) {
...
}
}

There's no need to open code fn->lexblock.ip.addr everywhere as it
deminishes readability.

> >> 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.
> >
>
> Yeah, I think that's reasonable. The combination of loops within loops,
> breaks and continues is getting hard to follow. I'll respin with a
> btf_encoder__add_true_signature() function replicating this logic.
>
>
> >> /* 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
> >>
>