[PATCH -tip ] [BUGFIX] perf probe: Add a workaround for GCC -mfentry
From: Masami Hiramatsu
Date: Wed Oct 03 2012 - 08:19:03 EST
Add a workaround for GCC -mfentry option. This enables perf
probe to find function parameters(arguments) at given function
entry point again.
With -mfentry, GCC (I've found this with Red Hat 4.7.0-5 and
Red Hat 4.7.2-2 on Fedora 17) generates odd debuginfo about
variable locations.
Here are examples:
-----
<1><9a58>: Abbrev Number: 86 (DW_TAG_subprogram)
<9a59> DW_AT_external : 1
<9a59> DW_AT_name : (indirect string, offset: 0xd82): unregister_di
e_notifier
<9a5d> DW_AT_decl_file : 1
<9a5e> DW_AT_decl_line : 551
<9a60> DW_AT_prototyped : 1
<9a60> DW_AT_type : <0x7c>
<9a64> DW_AT_low_pc : 0x740
<9a6c> DW_AT_high_pc : 0x75a
<9a74> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa)
<9a76> DW_AT_GNU_all_call_sites: 1
<9a76> DW_AT_sibling : <0x9aac>
<2><9a7a>: Abbrev Number: 87 (DW_TAG_formal_parameter)
<9a7b> DW_AT_name : nb
<9a7e> DW_AT_decl_file : 1
<9a7f> DW_AT_decl_line : 551
<9a81> DW_AT_type : <0x2a96>
<9a85> DW_AT_location : 0x172f (location list)
...
0000172f 0000000000000745 0000000000000750 (DW_OP_reg5 (rdi))
0000172f 0000000000000750 0000000000000757 (DW_OP_reg4 (rsi))
0000172f 0000000000000757 000000000000075a (DW_OP_GNU_entry_value: (DW_OP_reg5 (rdi)); DW_OP_stack_value)
0000172f <End of list>
-----
As you can see, the location of the parameter "nb", starts from 0x745
but unregister_die_notifier() function itself starts from 0x740.
-----
<1><9aac>: Abbrev Number: 86 (DW_TAG_subprogram)
<9aad> DW_AT_external : 1
<9aad> DW_AT_name : (indirect string, offset: 0x321f): notify_die
<9ab1> DW_AT_decl_file : 1
<9ab2> DW_AT_decl_line : 530
<9ab4> DW_AT_prototyped : 1
<9ab4> DW_AT_type : <0x7c>
<9ab8> DW_AT_low_pc : 0xb0
<9ac0> DW_AT_high_pc : 0xe0
<9ac8> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa)
<9aca> DW_AT_GNU_all_call_sites: 1
<9aca> DW_AT_sibling : <0x9b6c>
<2><9ace>: Abbrev Number: 87 (DW_TAG_formal_parameter)
<9acf> DW_AT_name : val
<9ad3> DW_AT_decl_file : 1
<9ad4> DW_AT_decl_line : 530
<9ad6> DW_AT_type : <0x2b36>
<9ada> DW_AT_location : 0x177b (location list)
...
0000177b 00000000000000b0 00000000000000cd (DW_OP_reg5 (rdi))
0000177b 00000000000000cd 00000000000000dd (DW_OP_reg4 (rsi))
0000177b 00000000000000dd 00000000000000e0 (DW_OP_GNU_entry_value: (DW_OP_reg5 (rdi)); DW_OP_stack_value)
0000177b <End of list>
-----
However, without fentry call (notifiy_die is marked notrace),
the parameter "val" location start with 0xb0, which is same as
notify_die start address.
To avoid failing variable search at the function entry point,
this add a workaround which adjusts var-search address +5
only if all the below conditions are true;
- given variable is a formal_parameter
- the searching address is a function entry point
- the formal_parameter has no location at the function entry point
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Michal Marek <mmarek@xxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
tools/perf/util/probe-finder.c | 40 +++++++++++++++++++++++++++++++++-------
1 file changed, 33 insertions(+), 7 deletions(-)
diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 1daf5c1..84b3b5f 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -276,7 +276,7 @@ static struct probe_trace_arg_ref *alloc_trace_arg_ref(long offs)
* If tvar == NULL, this just checks variable can be converted.
*/
static int convert_variable_location(Dwarf_Die *vr_die, Dwarf_Addr addr,
- Dwarf_Op *fb_ops,
+ Dwarf_Op *fb_ops, bool fentry,
struct probe_trace_arg *tvar)
{
Dwarf_Attribute attr;
@@ -291,13 +291,31 @@ 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;
+ if (dwarf_attr(vr_die, DW_AT_location, &attr) == NULL)
+ return -ENOENT;
+
+ if (fentry && (dwarf_tag(vr_die) == DW_TAG_formal_parameter) &&
+ (dwarf_getlocation_addr(&attr, addr, &op, &nops, 1) == 0))
+ /*
+ * Workaround: GCC -mfentry option generates odd
+ * variable location entries which start from a
+ * function entry+5, even if it is a formal_parameter.
+ * On the other hand, for functions without fentry call
+ * (e.g. notrace function), the formal_parameter location
+ * starts from the function entry address.
+ * Here, if we find a formal_parameter which doesn't
+ * start from the function entry, but from function
+ * entry+5, it should be a buggy entry.
+ * We forcibly get the variable(parameter) location
+ * information from entry+5.
+ */
+ addr += 5;
+
/* TODO: handle more than 1 exprs */
- if (dwarf_attr(vr_die, DW_AT_location, &attr) == NULL ||
- dwarf_getlocation_addr(&attr, addr, &op, &nops, 1) <= 0 ||
- nops == 0) {
+ if (dwarf_getlocation_addr(&attr, addr, &op, &nops, 1) <= 0 ||
+ nops == 0)
/* TODO: Support const_value */
return -ENOENT;
- }
if (op->atom == DW_OP_addr) {
static_var:
@@ -595,13 +613,19 @@ next:
static int convert_variable(Dwarf_Die *vr_die, struct probe_finder *pf)
{
Dwarf_Die die_mem;
+ Dwarf_Addr eaddr;
+ bool fentry;
int ret;
pr_debug("Converting variable %s into trace event.\n",
dwarf_diename(vr_die));
+ /* GCC -mfentry workaround */
+ fentry = (dwarf_entrypc(&pf->sp_die, &eaddr) == 0) &&
+ (eaddr == pf->addr);
+
ret = convert_variable_location(vr_die, pf->addr, pf->fb_ops,
- pf->tvar);
+ fentry, pf->tvar);
if (ret == -ENOENT)
pr_err("Failed to find the location of %s at this address.\n"
" Perhaps, it has been optimized out.\n", pf->pvar->var);
@@ -1220,7 +1244,9 @@ static int collect_variables_cb(Dwarf_Die *die_mem, void *data)
if (tag == DW_TAG_formal_parameter ||
tag == DW_TAG_variable) {
ret = convert_variable_location(die_mem, af->pf.addr,
- af->pf.fb_ops, NULL);
+ af->pf.fb_ops,
+ vl->point.offset == 0,
+ NULL);
if (ret == 0) {
ret = die_get_varname(die_mem, buf, MAX_VAR_LEN);
pr_debug2("Add new var: %s\n", buf);
--
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