[Libunwind-devel] perf: callchain feature on ARM - question about libunwind API

[Jean Pihet]

Hi!

Here is a message that I sent to the LKML to request help on the libunwind API.
Can you please check if I am using the API correctly or just abusing it?

Cheers,
Jean

---------- Forwarded message ----------
From: Jean Pihet <address@hidden>
Date: 27 August 2013 13:58
Subject: Fwd: perf: callchain feature on ARM - question about libunwind API
To: address@hidden


Hi!

I am looking for info and help on the libunwind API.

What I am trying to achieve is to allow the callchain feature on perf
for ARM (v7 to start with, then ARMv8), from the DWARF info found in
the .debug_frame section.

>From the source code in tools/perf I have added the call to
dwarf_find_debug_frame to load and parse the debug info from
.debug_frame. This works correctly, all IP ranges are found from the
debug code (test program and libraries). Then at some point I get an
assertion: 'perf: dwarf/Gparser.c:459: fetch_proc_info: Assertion
`c->pi.unwind_info' failed.' At that point c->pi is NULL. Cf. below
for more info.

It looks like I am not using the libunwind API as it should, so that
the perf code apparently builds a list of IP ranges to resolve but
cannot use it later on to gather statistics on the runtime info.

Any idea on how to progress? I am also looking at the *_proc_info API
of libunwind.

Here is the patch I am using below.

Cheers,
Jean

---
Debug log:

perf report --sort symbol --call-graph --stdio

_Uarm_step: calling dwarf_step(ip=0x8464, c->address@hidden,
c->address@hidden)
unwind: find_proc_info dso /home/linaro/perf-libunwind/
test_app/stress_bt
read_unwind_spec_debug_frame: .debug_frame offset=9304
opened file '/home/linaro/perf-libunwind/test_app/stress_bt'. Section
header at offset 18152
read 3160 bytes of .debug_frame from offset 9304
...
_ULarm_step: calling dwarf_step(ip=0xb6e6e764, c->address@hidden,
c->address@hidden)
_ULarm_find_proc_info: looking for IP=0xb6e6e763
opened file '/usr/lib/arm-linux-gnueabihf/libunwind.so.8'. Section
header at offset 513260
read 17072 bytes of .debug_frame from offset 451836
_ULarm_dwarf_search_unwind_table: looking for IP=0xb6e6e763
_ULarm_find_proc_info returns 0
...
perf: dwarf/Gparser.c:459: fetch_proc_info: Assertion
`c->pi.unwind_info' failed.


---
Patch to the kernel source:

diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c
index 958723b..dd97688 100644
--- a/tools/perf/util/unwind.c
+++ b/tools/perf/util/unwind.c
@@ -39,6 +39,14 @@ UNW_OBJ(dwarf_search_unwind_table) (unw_addr_space_t as,

 #define dwarf_search_unwind_table UNW_OBJ(dwarf_search_unwind_table)

+extern int
+UNW_OBJ(dwarf_find_debug_frame) (int found, unw_dyn_info_t *di_debug,
+                unw_word_t ip, unw_word_t segbase,
+                const char* obj_name, unw_word_t start,
+                unw_word_t end);
+
+#define dwarf_find_debug_frame UNW_OBJ(dwarf_find_debug_frame)
+
 #define DW_EH_PE_FORMAT_MASK   0x0f    /* format of the encoded value */
 #define DW_EH_PE_APPL_MASK 0x70    /* how the value is to be applied */

@@ -245,8 +253,9 @@ static int unwind_spec_ehframe(struct dso *dso,
struct      machine *machine,
    return 0;
 }

-static int read_unwind_spec(struct dso *dso, struct machine *machine,
-               u64 *table_data, u64 *segbase, u64 *fde_count)
+static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
+                    u64 *table_data, u64 *segbase,
+                    u64 *fde_count)
 {
    int ret = -EINVAL, fd;
    u64 offset;
@@ -255,18 +264,40 @@ static int read_unwind_spec(struct dso *dso,
struct       machine *machine,
    if (fd < 0)
        return -EINVAL;

+   /* Check the .eh_frame section for unwinding info */
    offset = elf_section_offset(fd, ".eh_frame_hdr");
    close(fd);

-   if (offset)
+   if (offset) {
+       fprintf(stderr, "%s: .eh_frame offset=%llu\n", __func__, offset);
        ret = unwind_spec_ehframe(dso, machine, offset,
                      table_data, segbase,
                      fde_count);
+   }

-   /* TODO .debug_frame check if eh_frame_hdr fails */
    return ret;
 }

+static int read_unwind_spec_debug_frame(struct dso *dso,
+                   struct machine *machine,
+                   u64 *segbase)
+{
+   int fd = dso__data_fd(dso, machine);
+   if (fd < 0)
+       return -EINVAL;
+
+   /* Check the .debug_frame section for unwinding info */
+   *segbase = elf_section_offset(fd, ".debug_frame");
+   close(fd);
+
+   fprintf(stderr, "%s: .debug_frame offset=%llu\n", __func__, *segbase);
+
+   if (!segbase)
+       return -EINVAL;
+
+   return 0;
+}
+
 static struct map *find_map(unw_word_t ip, struct unwind_info *ui)
 {
    struct addr_location al;
@@ -289,22 +320,32 @@ find_proc_info(unw_addr_space_t as, unw_word_t
ip,        unw_proc_info_t *pi,
    if (!map || !map->dso)
        return -EINVAL;

-   pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
+   fprintf(stderr, "unwind: find_proc_info dso %s\n", map->dso->name);
+
+   /* Check the .eh_frame section for unwinding info */
+   if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
+                      &table_data, &segbase, &fde_count)) {
+       memset(&di, 0, sizeof(di));
+       di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
+       di.start_ip = map->start;
+       di.end_ip   = map->end;
+       di.u.rti.segbase    = map->start + segbase;
+       di.u.rti.table_data = map->start + table_data;
+       di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
+                 / sizeof(unw_word_t);
+       return dwarf_search_unwind_table(as, ip, &di, pi,
+                        need_unwind_info, arg);
+   }

-   if (read_unwind_spec(map->dso, ui->machine,
-                &table_data, &segbase, &fde_count))
-       return -EINVAL;
+   /* Check the .debug_frame section for unwinding info */
+   if (!read_unwind_spec_debug_frame(map->dso, ui->machine, &segbase)) {
+       memset(&di, 0, sizeof(di));
+       if (dwarf_find_debug_frame(0, &di, ip, segbase, map->dso->name,
+                      map->start, map->end))
+           return 0;
+   }

-   memset(&di, 0, sizeof(di));
-   di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
-   di.start_ip = map->start;
-   di.end_ip   = map->end;
-   di.u.rti.segbase    = map->start + segbase;
-   di.u.rti.table_data = map->start + table_data;
-   di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
-                 / sizeof(unw_word_t);
-   return dwarf_search_unwind_table(as, ip, &di, pi,
-                    need_unwind_info, arg);
+   return -EINVAL;
 }

 static int access_fpreg(unw_addr_space_t __maybe_unused as,