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Message-ID: <20251205123608.3125691-1-sesse@google.com>
Date: Fri, 5 Dec 2025 13:36:08 +0100
From: "Steinar H. Gunderson" <sesse@...gle.com>
To: acme@...nel.org
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
irogers@...gle.com, "Steinar H. Gunderson" <sesse@...gle.com>
Subject: [PATCH] perf probe: Improve C++ support in arguments
For purposes of specifying arguments in tracepoints, C++ classes
work exactly like structs, and C++ references work almost exactly
like pointers (they are dereferenced with . instead of ->).
However, they use different tags in DWARF, which means you cannot
use them in arguments.
Implement support for both, based on the existing struct and pointer
support. This allows us to to something like this in Chromium:
perf probe --exec content_shell --add 'PerformanceMark=\
_ZN5blink15PerformanceMark6CreateEPNS_11ScriptStateERKNS_12\
AtomicStringEPNS_22PerformanceMarkOptionsERNS_14ExceptionStateE \
mark_name.string_.impl_.ptr_'
even though mark_name is an class AtomicString&.
We don't support advanced C++ features, such as operator overloading.
Unfortunately, we also don't support C++-style strings where there
is a separate length field instead of null-termination. We also cannot
go past ptr_ in the example above (e.g. add ->length_), due to
unrelated DWARF issues.
Signed-off-by: Steinar H. Gunderson <sesse@...gle.com>
Cc: Ian Rogers <irogers@...gle.com>
---
tools/perf/util/annotate-data.c | 3 ++-
tools/perf/util/dwarf-aux.c | 13 ++++++++++---
tools/perf/util/probe-finder.c | 24 +++++++++++++++++-------
3 files changed, 29 insertions(+), 11 deletions(-)
diff --git a/tools/perf/util/annotate-data.c b/tools/perf/util/annotate-data.c
index 07cf9c334be0..d97b5fc61e6f 100644
--- a/tools/perf/util/annotate-data.c
+++ b/tools/perf/util/annotate-data.c
@@ -459,7 +459,8 @@ static bool is_pointer_type(Dwarf_Die *type_die)
{
int tag = dwarf_tag(type_die);
- return tag == DW_TAG_pointer_type || tag == DW_TAG_array_type;
+ return tag == DW_TAG_pointer_type || tag == DW_TAG_reference_type ||
+ tag == DW_TAG_array_type;
}
static bool is_compound_type(Dwarf_Die *type_die)
diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
index 9267af204c7d..25bbc840d1a8 100644
--- a/tools/perf/util/dwarf-aux.c
+++ b/tools/perf/util/dwarf-aux.c
@@ -1112,6 +1112,8 @@ int die_get_typename_from_type(Dwarf_Die *type_die, struct strbuf *buf)
tag = dwarf_tag(type_die);
if (tag == DW_TAG_pointer_type)
tmp = "*";
+ else if (tag == DW_TAG_reference_type)
+ tmp = "&";
else if (tag == DW_TAG_array_type)
tmp = "[]";
else if (tag == DW_TAG_subroutine_type) {
@@ -1124,6 +1126,8 @@ int die_get_typename_from_type(Dwarf_Die *type_die, struct strbuf *buf)
tmp = "union ";
else if (tag == DW_TAG_structure_type)
tmp = "struct ";
+ else if (tag == DW_TAG_class_type)
+ tmp = "class ";
else if (tag == DW_TAG_enumeration_type)
tmp = "enum ";
else if (name == NULL)
@@ -2065,7 +2069,8 @@ Dwarf_Die *die_get_member_type(Dwarf_Die *type_die, int offset,
tag = dwarf_tag(type_die);
/* If it's not a compound type, return the type directly */
- if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type) {
+ if (tag != DW_TAG_structure_type && tag != DW_TAG_class_type &&
+ tag != DW_TAG_union_type) {
Dwarf_Word size;
if (dwarf_aggregate_size(type_die, &size) < 0)
@@ -2080,7 +2085,8 @@ Dwarf_Die *die_get_member_type(Dwarf_Die *type_die, int offset,
mb_type = *type_die;
/* TODO: Handle union types better? */
- while (tag == DW_TAG_structure_type || tag == DW_TAG_union_type) {
+ while (tag == DW_TAG_structure_type || tag == DW_TAG_class_type ||
+ tag == DW_TAG_union_type) {
member = die_find_child(&mb_type, __die_find_member_offset_cb,
(void *)(long)offset, die_mem);
if (member == NULL)
@@ -2091,7 +2097,8 @@ Dwarf_Die *die_get_member_type(Dwarf_Die *type_die, int offset,
tag = dwarf_tag(&mb_type);
- if (tag == DW_TAG_structure_type || tag == DW_TAG_union_type) {
+ if (tag == DW_TAG_structure_type || tag == DW_TAG_class_type ||
+ tag == DW_TAG_union_type) {
Dwarf_Word loc;
/* Update offset for the start of the member struct */
diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 5ffd97ee4898..ca4734c75bba 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -360,12 +360,20 @@ static int convert_variable_fields(Dwarf_Die *vr_die, const char *varname,
ref->offset += dwarf_bytesize(&type) * field->index;
ref->user_access = user_access;
goto next;
- } else if (tag == DW_TAG_pointer_type) {
+ } else if (tag == DW_TAG_pointer_type || tag == DW_TAG_reference_type) {
/* Check the pointer and dereference */
- if (!field->ref) {
- pr_err("Semantic error: %s must be referred by '->'\n",
- field->name);
- return -EINVAL;
+ if (tag == DW_TAG_pointer_type) {
+ if (!field->ref) {
+ pr_err("Semantic error: %s must be referred by '->'\n",
+ field->name);
+ return -EINVAL;
+ }
+ } else {
+ if (field->ref) {
+ pr_err("Semantic error: %s must be referred by '.'\n",
+ field->name);
+ return -EINVAL;
+ }
}
/* Get the type pointed by this pointer */
if (die_get_real_type(&type, &type) == NULL) {
@@ -374,7 +382,8 @@ static int convert_variable_fields(Dwarf_Die *vr_die, const char *varname,
}
/* Verify it is a data structure */
tag = dwarf_tag(&type);
- if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type) {
+ if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type &&
+ tag != DW_TAG_class_type) {
pr_warning("%s is not a data structure nor a union.\n",
varname);
return -EINVAL;
@@ -389,7 +398,8 @@ static int convert_variable_fields(Dwarf_Die *vr_die, const char *varname,
*ref_ptr = ref;
} else {
/* Verify it is a data structure */
- if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type) {
+ if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type &&
+ tag != DW_TAG_class_type) {
pr_warning("%s is not a data structure nor a union.\n",
varname);
return -EINVAL;
--
2.51.0
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