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Subject[PATCH 3/3] perf probe: Add --range option to show variable location range
Date
It is not easy for users to get the accurate byte offset or the line
number where a local variable can be probed. With '--range' option,
local variables in scope of the probe point are showed with byte offset
range, and can be added according to this range information.

For example, there are some variables in function
generic_perform_write():

<generic_perform_write@mm/filemap.c:0>
0 ssize_t generic_perform_write(struct file *file,
1 struct iov_iter *i, loff_t pos)
2 {
3 struct address_space *mapping = file->f_mapping;
4 const struct address_space_operations *a_ops = mapping->a_ops;
...
42 status = a_ops->write_begin(file, mapping, pos, bytes, flags,
&page, &fsdata);
44 if (unlikely(status < 0))

But we got failed when we try to probe the variable 'a_ops' at line 42
or 44.

$ perf probe --add 'generic_perform_write:42 a_ops'
Failed to find the location of a_ops at this address.
Perhaps, it has been optimized out.

This is because source code do not match assembly, so a variable may not
be available in the sourcecode line where it presents. After this patch,
we can lookup the accurate byte offset range of a variable, 'INV'
indicates that this variable is not valid at the given point, but
available in scope:

$ perf probe --vars 'generic_perform_write:42' --range
Available variables at generic_perform_write:42
@<generic_perform_write+141>
[INV] ssize_t written [byte offset]: <324-331>
[INV] struct address_space_operations* a_ops [byte offset]: <55-61>,<170-176>,<223-246>
[VAL] (unknown_type) fsdata [byte offset]: <70-307>,<346-411>
[VAL] loff_t pos [byte offset]: <0-286>,<286-336>,<346-411>
[VAL] long int status [byte offset]: <83-342>,<346-411>
[VAL] long unsigned int bytes [byte offset]: <122-311>,<320-338>,<346-403>,<403-411>
[VAL] struct address_space* mapping [byte offset]: <35-344>,<346-411>
[VAL] struct iov_iter* i [byte offset]: <0-340>,<346-411>
[VAL] struct page* page [byte offset]: <70-307>,<346-411>

Then it is more clearly for us to do the probe with this variable:

$ perf probe --add 'generic_perform_write+170 a_ops'
Added new event:
probe:generic_perform_write (on generic_perform_write+170 with a_ops)

Signed-off-by: He Kuang <hekuang@huawei.com>
---
tools/perf/builtin-probe.c | 6 ++-
tools/perf/util/dwarf-aux.c | 103 +++++++++++++++++++++++++++++++++++++++++
tools/perf/util/dwarf-aux.h | 2 +
tools/perf/util/probe-finder.c | 69 ++++++++++++++++++++-------
tools/perf/util/probe-finder.h | 3 +-
5 files changed, 165 insertions(+), 18 deletions(-)

diff --git a/tools/perf/builtin-probe.c b/tools/perf/builtin-probe.c
index 19ebe93..1b20004 100644
--- a/tools/perf/builtin-probe.c
+++ b/tools/perf/builtin-probe.c
@@ -52,6 +52,7 @@ static struct {
bool list_events;
bool force_add;
bool show_ext_vars;
+ bool show_location_range;
bool uprobes;
bool quiet;
bool target_used;
@@ -375,6 +376,8 @@ __cmd_probe(int argc, const char **argv, const char *prefix __maybe_unused)
"Show accessible variables on PROBEDEF", opt_show_vars),
OPT_BOOLEAN('\0', "externs", &params.show_ext_vars,
"Show external variables too (with --vars only)"),
+ OPT_BOOLEAN('\0', "range", &params.show_location_range,
+ "Show variables location range in scope (with --vars only)"),
OPT_STRING('k', "vmlinux", &symbol_conf.vmlinux_name,
"file", "vmlinux pathname"),
OPT_STRING('s', "source", &symbol_conf.source_prefix,
@@ -479,7 +482,8 @@ __cmd_probe(int argc, const char **argv, const char *prefix __maybe_unused)
params.filter = strfilter__new(DEFAULT_VAR_FILTER,
NULL);

- var_flags = params.show_ext_vars << VAR_FLAGS_EXTERN;
+ var_flags = params.show_ext_vars << VAR_FLAGS_EXTERN |
+ params.show_location_range << VAR_FLAGS_LOCATION_RANGE;

ret = show_available_vars(params.events, params.nevents,
params.max_probe_points,
diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
index 75d264e..23053c0 100644
--- a/tools/perf/util/dwarf-aux.c
+++ b/tools/perf/util/dwarf-aux.c
@@ -897,3 +897,106 @@ int die_get_varname(Dwarf_Die *vr_die, struct strbuf *buf)
return 0;
}

+/**
+ * die_get_var_innermost_scope - Get innermost scope range of given variable DIE
+ * @sp_die: a subprogram DIE
+ * @vr_die: a variable DIE
+ * @buf: a strbuf for variable byte offset range
+ *
+ * Get the innermost scope range of @vr_die and stores it in @buf as
+ * "[byte offset]: <NN-NN>,<NN-NN>".
+ */
+static int die_get_var_innermost_scope(Dwarf_Die *sp_die, Dwarf_Die *vr_die,
+ struct strbuf *buf)
+{
+ Dwarf_Die *scopes;
+ int count;
+ size_t offset = 0;
+ Dwarf_Addr base;
+ Dwarf_Addr start, end;
+ Dwarf_Addr entry;
+ int ret;
+ bool first = true;
+
+ ret = dwarf_entrypc(sp_die, &entry);
+ if (ret)
+ return ret;
+
+ count = dwarf_getscopes_die(vr_die, &scopes);
+
+ /* (*SCOPES)[1] is the DIE for the scope containing that scope */
+ if (count <= 1) {
+ ret = -EINVAL;
+ goto end;
+ }
+
+ while ((offset = dwarf_ranges(&scopes[1], offset, &base,
+ &start, &end)) > 0) {
+ start -= entry;
+ end -= entry;
+
+ if (first) {
+ strbuf_addf(buf, "[byte offset]: <%ld-%ld>",
+ start, end);
+ first = false;
+ } else {
+ strbuf_addf(buf, ",<%ld-%ld>",
+ start, end);
+ }
+ }
+end:
+ free(scopes);
+ return ret;
+}
+
+/**
+ * die_get_var_range - Get byte offset range of given variable DIE
+ * @sp_die: a subprogram DIE
+ * @vr_die: a variable DIE
+ * @buf: a strbuf for type and variable name and byte offset range
+ *
+ * Get the byte offset range of @vr_die and stores it in @buf as
+ * "[byte offset]: <NN-NN>,<NN-NN>".
+ */
+int die_get_var_range(Dwarf_Die *sp_die, Dwarf_Die *vr_die, struct strbuf *buf)
+{
+ int ret = 0;
+ Dwarf_Addr base;
+ Dwarf_Addr start, end;
+ Dwarf_Addr entry;
+ Dwarf_Op *op;
+ size_t nops;
+ size_t offset = 0;
+ Dwarf_Attribute attr;
+ bool first = true;
+
+ ret = dwarf_entrypc(sp_die, &entry);
+ if (ret)
+ return ret;
+
+ if (dwarf_attr(vr_die, DW_AT_location, &attr) == NULL)
+ return -EINVAL;
+
+ while ((offset = dwarf_getlocations(
+ &attr, offset, &base,
+ &start, &end, &op, &nops)) > 0) {
+ if (start == 0) {
+ /* Single Location Descriptions */
+ ret = die_get_var_innermost_scope(sp_die, vr_die, buf);
+ } else {
+ /* Location Lists */
+ start -= entry;
+ end -= entry;
+ if (first) {
+ strbuf_addf(buf, "[byte offset]: <%ld-%ld>",
+ start, end);
+ first = false;
+ } else {
+ strbuf_addf(buf, ",<%ld-%ld>",
+ start, end);
+ }
+ }
+ }
+
+ return ret;
+}
diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
index c0d6173..8390cde 100644
--- a/tools/perf/util/dwarf-aux.h
+++ b/tools/perf/util/dwarf-aux.h
@@ -118,4 +118,6 @@ extern int die_get_typename(Dwarf_Die *vr_die, struct strbuf *buf);

/* Get the name and type of given variable DIE, stored as "type\tname" */
extern int die_get_varname(Dwarf_Die *vr_die, struct strbuf *buf);
+extern int die_get_var_range(Dwarf_Die *sp_die, Dwarf_Die *vr_die,
+ struct strbuf *buf);
#endif
diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 6bbdf3f..af0a7ea 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -43,6 +43,9 @@
/* Kprobe tracer basic type is up to u64 */
#define MAX_BASIC_TYPE_BITS 64

+/* Variable location invalid at addr but valid in scope */
+#define VARIABLE_LOCATION_INVALID_AT_ADDR -10000
+
/* Dwarf FL wrappers */
static char *debuginfo_path; /* Currently dummy */

@@ -167,7 +170,8 @@ static struct probe_trace_arg_ref *alloc_trace_arg_ref(long offs)
*/
static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
Dwarf_Op *fb_ops, Dwarf_Die *sp_die,
- struct probe_trace_arg *tvar)
+ struct probe_trace_arg *tvar,
+ unsigned long *var_flags)
{
Dwarf_Attribute attr;
Dwarf_Addr tmp = 0;
@@ -177,7 +181,7 @@ static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
Dwarf_Word offs = 0;
bool ref = false;
const char *regs;
- int ret;
+ int ret, ret2 = 0;

if (dwarf_attr(vr_die, DW_AT_external, &attr) != NULL)
goto static_var;
@@ -187,9 +191,20 @@ static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
return -EINVAL; /* Broken DIE ? */
if (dwarf_getlocation_addr(&attr, addr, &op, &nops, 1) <= 0) {
ret = dwarf_entrypc(sp_die, &tmp);
- if (ret || addr != tmp ||
- dwarf_tag(vr_die) != DW_TAG_formal_parameter ||
- dwarf_highpc(sp_die, &tmp))
+ if (ret)
+ return -ENOENT;
+
+ if (var_flags &&
+ test_bit(VAR_FLAGS_LOCATION_RANGE, var_flags) &&
+ (dwarf_tag(vr_die) == DW_TAG_variable)) {
+ ret2 = VARIABLE_LOCATION_INVALID_AT_ADDR;
+ } else if (addr != tmp ||
+ dwarf_tag(vr_die) != DW_TAG_formal_parameter) {
+ return -ENOENT;
+ }
+
+ ret = dwarf_highpc(sp_die, &tmp);
+ if (ret)
return -ENOENT;
/*
* This is fuzzed by fentry mcount. We try to find the
@@ -210,7 +225,7 @@ found:
if (op->atom == DW_OP_addr) {
static_var:
if (!tvar)
- return 0;
+ return ret2;
/* Static variables on memory (not stack), make @varname */
ret = strlen(dwarf_diename(vr_die));
tvar->value = zalloc(ret + 2);
@@ -220,7 +235,7 @@ static_var:
tvar->ref = alloc_trace_arg_ref((long)offs);
if (tvar->ref == NULL)
return -ENOMEM;
- return 0;
+ return ret2;
}

/* If this is based on frame buffer, set the offset */
@@ -250,7 +265,7 @@ static_var:
}

if (!tvar)
- return 0;
+ return ret2;

regs = get_arch_regstr(regn);
if (!regs) {
@@ -269,7 +284,7 @@ static_var:
if (tvar->ref == NULL)
return -ENOMEM;
}
- return 0;
+ return ret2;
}

#define BYTES_TO_BITS(nb) ((nb) * BITS_PER_LONG / sizeof(long))
@@ -516,7 +531,7 @@ static int convert_variable(Dwarf_Die *vr_die, struct probe_finder *pf)
dwarf_diename(vr_die));

ret = convert_variable_location(vr_die, pf->addr, pf->fb_ops,
- &pf->sp_die, pf->tvar);
+ &pf->sp_die, pf->tvar, NULL);
if (ret == -ENOENT || ret == -EINVAL)
pr_err("Failed to find the location of %s at this address.\n"
" Perhaps, it has been optimized out.\n", pf->pvar->var);
@@ -1105,7 +1120,7 @@ static int copy_variables_cb(Dwarf_Die *die_mem, void *data)
(tag == DW_TAG_variable && vf->vars)) {
if (convert_variable_location(die_mem, vf->pf->addr,
vf->pf->fb_ops, &pf->sp_die,
- NULL) == 0) {
+ NULL, NULL) == 0) {
vf->args[vf->nargs].var = (char *)dwarf_diename(die_mem);
if (vf->args[vf->nargs].var == NULL) {
vf->ret = -ENOMEM;
@@ -1254,11 +1269,33 @@ static int collect_variables_cb(Dwarf_Die *die_mem, void *data)
tag == DW_TAG_variable) {
ret = convert_variable_location(die_mem, af->pf.addr,
af->pf.fb_ops, &af->pf.sp_die,
- NULL);
- if (ret == 0) {
- ret = die_get_varname(die_mem, &buf);
- pr_debug2("Add new var: %s\n", buf.buf);
- if (ret == 0) {
+ NULL, &af->var_flags);
+ if (ret == 0 || ret == VARIABLE_LOCATION_INVALID_AT_ADDR) {
+ int ret2;
+ bool show_range = test_bit(VAR_FLAGS_LOCATION_RANGE,
+ &af->var_flags);
+ bool externs = !af->child;
+
+ if (show_range) {
+ if (!externs) {
+ if (ret)
+ strbuf_addf(&buf, "[INV]\t");
+ else
+ strbuf_addf(&buf, "[VAL]\t");
+ } else
+ strbuf_addf(&buf, "[EXT]\t");
+ }
+
+ ret2 = die_get_varname(die_mem, &buf);
+
+ if (!ret2 && show_range && !externs) {
+ strbuf_addf(&buf, "\t");
+ ret2 = die_get_var_range(&af->pf.sp_die,
+ die_mem, &buf);
+ }
+
+ pr_debug("Add new var: %s\n", buf.buf);
+ if (ret2 == 0) {
strlist__add(vl->vars,
strbuf_detach(&buf, NULL));
}
diff --git a/tools/perf/util/probe-finder.h b/tools/perf/util/probe-finder.h
index 95bb79d..4b30a9b 100644
--- a/tools/perf/util/probe-finder.h
+++ b/tools/perf/util/probe-finder.h
@@ -11,7 +11,8 @@
#define MAX_PROBE_ARGS 128

enum var_flags_bits {
- VAR_FLAGS_EXTERN
+ VAR_FLAGS_EXTERN,
+ VAR_FLAGS_LOCATION_RANGE
};

#define PROBE_ARG_VARS "$vars"
--
1.8.5.2


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