[PATCH v2 3/4] perf probe: Support DW_AT_const_value constant value

Masami Hiramatsu
2019-11-07T16:28:23Z

Support DW_AT_const_value for variable assignment instead of location.
Note that this requires ftrace supporting immediate value.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
tools/perf/util/probe-file.c | 7 +++++++
tools/perf/util/probe-file.h | 1 +
tools/perf/util/probe-finder.c | 11 +++++++++++
3 files changed, 19 insertions(+)

diff --git a/tools/perf/util/probe-file.c b/tools/perf/util/probe-file.c
index a63f1a19b0e8..5003ba403345 100644
--- a/tools/perf/util/probe-file.c
+++ b/tools/perf/util/probe-file.c
@@ -1008,6 +1008,7 @@ enum ftrace_readme {
FTRACE_README_UPROBE_REF_CTR,
FTRACE_README_USER_ACCESS,
FTRACE_README_MULTIPROBE_EVENT,
+ FTRACE_README_IMMEDIATE_VALUE,
FTRACE_README_END,
};

@@ -1022,6 +1023,7 @@ static struct {
DEFINE_TYPE(FTRACE_README_UPROBE_REF_CTR, "*ref_ctr_offset*"),
DEFINE_TYPE(FTRACE_README_USER_ACCESS, "*[u]<offset>*"),
DEFINE_TYPE(FTRACE_README_MULTIPROBE_EVENT, "*Create/append/*"),
+ DEFINE_TYPE(FTRACE_README_IMMEDIATE_VALUE, "*\\imm-value,*"),
};

static bool scan_ftrace_readme(enum ftrace_readme type)
@@ -1092,3 +1094,8 @@ bool multiprobe_event_is_supported(void)
{
return scan_ftrace_readme(FTRACE_README_MULTIPROBE_EVENT);
}
+
+bool immediate_value_is_supported(void)
+{
+ return scan_ftrace_readme(FTRACE_README_IMMEDIATE_VALUE);
+}
diff --git a/tools/perf/util/probe-file.h b/tools/perf/util/probe-file.h
index 850d1b52d60a..0dba88c0f5f0 100644
--- a/tools/perf/util/probe-file.h
+++ b/tools/perf/util/probe-file.h
@@ -72,6 +72,7 @@ bool kretprobe_offset_is_supported(void);
bool uprobe_ref_ctr_is_supported(void);
bool user_access_is_supported(void);
bool multiprobe_event_is_supported(void);
+bool immediate_value_is_supported(void);
#else /* ! HAVE_LIBELF_SUPPORT */
static inline struct probe_cache *probe_cache__new(const char *tgt __maybe_unused, struct nsinfo *nsi __maybe_unused)
{
diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 9ecea45da4ca..2e3a468c8350 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -177,6 +177,17 @@ static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
if (dwarf_attr(vr_die, DW_AT_external, &attr) != NULL)
goto static_var;

+ /* Constant value */
+ if (dwarf_attr(vr_die, DW_AT_const_value, &attr) &&
+ immediate_value_is_supported()) {
+ Dwarf_Sword snum;
+
+ dwarf_formsdata(&attr, &snum);
+ ret = asprintf(&tvar->value, "\\%ld", (long)snum);
+
+ return ret < 0 ? -ENOMEM : 0;
+ }
+
/* TODO: handle more than 1 exprs */
if (dwarf_attr(vr_die, DW_AT_location, &attr) == NULL)
return -EINVAL; /* Broken DIE ? */