[PATCH] perf/probe: Search both .eh_frame and .debug_frame sections for probe location

[Hemant Kumar]

perf probe through debuginfo__find_probes() in util/probe-finder.c
checks for the functions' frame descriptions in either .eh_frame section
of an ELF or the .debug_frame. The check is based on whether either one
of these sections is present. But sometimes, it may happen that,
.eh_frame, even if present, may not be complete and may miss some
descriptions. For e.g., in powerpc, this may happen :
 $ gcc -g bin.c -o bin

 $ objdump --dwarf ./bin
 <1><145>: Abbrev Number: 7 (DW_TAG_subprogram)
    <146>   DW_AT_external    : 1
    <146>   DW_AT_name        : (indirect string, offset: 0x9e): main
    <14a>   DW_AT_decl_file   : 1
    <14b>   DW_AT_decl_line   : 39
    <14c>   DW_AT_prototyped  : 1
    <14c>   DW_AT_type        : <0x57>
    <150>   DW_AT_low_pc      : 0x100007b8

If the .eh_frame and .debug_frame are checked for the same binary, we
will find that, .eh_frame (although present) doesn't contain a
description for "main" function.
But, .debug_frame has a description :

000000d8 00000024 00000000 FDE cie=00000000 pc=100007b8..10000838
  DW_CFA_advance_loc: 16 to 100007c8
  DW_CFA_def_cfa_offset: 144
  DW_CFA_offset_extended_sf: r65 at cfa+16
...

Due to this (since, perf checks whether .eh_frame is present and goes on
searching for that address inside that frame), perf is unable to process
the probes :
 # perf probe -x ./bin main
Failed to get call frame on 0x100007b8
  Error: Failed to add events.

To avoid this issue, we need to check both the sections (.eh_frame and
.debug_frame), which is done in this patch.

Note that, we can always force everything into both .eh_frame and
.debug_frame by :
 $ gcc bin.c -fasynchronous-unwind-tables  -fno-dwarf2-cfi-asm -g -o bin

Signed-off-by: Hemant Kumar <hemant at linux.vnet.ibm.com>
---
 tools/perf/util/probe-finder.c | 59 +++++++++++++++++++++++++-----------------
 1 file changed, 35 insertions(+), 24 deletions(-)

diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 2da65a7..7ce02b9 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -1022,9 +1022,8 @@ static int pubname_search_cb(Dwarf *dbg, Dwarf_Global *gl, void *data)
 	return DWARF_CB_OK;
 }
 
-/* Find probe points from debuginfo */
-static int debuginfo__find_probes(struct debuginfo *dbg,
-				  struct probe_finder *pf)
+static int debuginfo__find_probe_location(struct debuginfo *dbg,
+					  struct probe_finder *pf)
 {
 	struct perf_probe_point *pp = &pf->pev->point;
 	Dwarf_Off off, noff;
@@ -1032,27 +1031,6 @@ static int debuginfo__find_probes(struct debuginfo *dbg,
 	Dwarf_Die *diep;
 	int ret = 0;
 
-#if _ELFUTILS_PREREQ(0, 142)
-	Elf *elf;
-	GElf_Ehdr ehdr;
-	GElf_Shdr shdr;
-
-	/* Get the call frame information from this dwarf */
-	elf = dwarf_getelf(dbg->dbg);
-	if (elf == NULL)
-		return -EINVAL;
-
-	if (gelf_getehdr(elf, &ehdr) == NULL)
-		return -EINVAL;
-
-	if (elf_section_by_name(elf, &ehdr, &shdr, ".eh_frame", NULL) &&
-	    shdr.sh_type == SHT_PROGBITS) {
-		pf->cfi = dwarf_getcfi_elf(elf);
-	} else {
-		pf->cfi = dwarf_getcfi(dbg->dbg);
-	}
-#endif
-
 	off = 0;
 	pf->lcache = intlist__new(NULL);
 	if (!pf->lcache)
@@ -1115,6 +1093,39 @@ found:
 	return ret;
 }
 
+/* Find probe points from debuginfo */
+static int debuginfo__find_probes(struct debuginfo *dbg,
+				  struct probe_finder *pf)
+{
+	int ret = 0;
+
+#if _ELFUTILS_PREREQ(0, 142)
+	Elf *elf;
+	GElf_Ehdr ehdr;
+	GElf_Shdr shdr;
+
+	/* Get the call frame information from this dwarf */
+	elf = dwarf_getelf(dbg->dbg);
+	if (elf == NULL)
+		return -EINVAL;
+
+	if (gelf_getehdr(elf, &ehdr) == NULL)
+		return -EINVAL;
+
+	if (elf_section_by_name(elf, &ehdr, &shdr, ".eh_frame", NULL) &&
+	    shdr.sh_type == SHT_PROGBITS) {
+		pf->cfi = dwarf_getcfi_elf(elf);
+		ret = debuginfo__find_probe_location(dbg, pf);
+		if (ret >= 0)
+			return ret;
+	}
+	pf->cfi = dwarf_getcfi(dbg->dbg);
+#endif
+
+	ret = debuginfo__find_probe_location(dbg, pf);
+	return ret;
+}
+
 struct local_vars_finder {
 	struct probe_finder *pf;
 	struct perf_probe_arg *args;
-- 
1.9.3