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SubjectRe: Broken dwarf unwinding - wrong stack pointer register value?
Date
On Mittwoch, 24. Oktober 2018 16:48:18 CET Andi Kleen wrote:
> > Can someone at least confirm whether unwinding from a function prologue
> > via
> > .eh_frame (but without .debug_frame) should actually be possible?
>
> Yes it should be possible. Asynchronous unwind tables should work
> from any instruction.

I'm still trying to figure out what's going on here... I have remembered, that
libunwind has a quite helpful debug mode, which I further extended to generate
a more detailed report of the failed unwinding attempt. For one of the broken
samples, I see:

```
$ UNW_DEBUG_LEVEL=15 perf script -v
...
cpp-inlining 30227 18486.357148: 788066 cycles:ppp:
>_Ux86_64_init_remote: (cursor=0x7fffafa55c10)
unwind: reg 16, val 7f91345d77b4
unwind: reg 7, val 7ffd1e276f18
>_Ux86_64_step: (cursor=0x7fffafa55c10, ip=0x00007f91345d77b4,
cfa=0x00007ffd1e276f18)
unwind: find_proc_info dso /usr/lib/libm-2.28.so
>_Ux86_64_dwarf_search_unwind_table: ip=0x7f91345d77b4,
start_ip=0xffffffffffeb3fbc
>_Ux86_64_dwarf_search_unwind_table: e->fde_offset = 4794, segbase =
7f91347236a4, debug_frame_base = 0, fde_addr = 7f9134727e38
>_Ux86_64_dwarf_extract_proc_info_from_fde: FDE @ 0x7f9134727e38
>_Ux86_64_dwarf_extract_proc_info_from_fde: looking for CIE at
address 7f91347252d8
>parse_cie: CIE parsed OK, augmentation = "zR", handler=0x0
>_Ux86_64_dwarf_extract_proc_info_from_fde: FDE covers IP
0x7f91345d7660-0x7f91345d79ce, LSDA=0x0
>_Ux86_64_fetch_frame: fetch frame ip=0x7f91345d77b4 cfa=0x7ffd1e276f18
format=0
>run_cfi_program: CFA_def_cfa r7+0x8
>run_cfi_program: CFA_offset r16 at cfa+0xfffffffffffffff8
>run_cfi_program: CFA_advance_loc to 0x7f91345d7666
>run_cfi_program: CFA_def_cfa_offset 0x10
>run_cfi_program: CFA_offset r13 at cfa+0xfffffffffffffff0
>run_cfi_program: CFA_advance_loc to 0x7f91345d7668
>run_cfi_program: CFA_def_cfa_offset 0x18
>run_cfi_program: CFA_offset r12 at cfa+0xffffffffffffffe8
>run_cfi_program: CFA_advance_loc to 0x7f91345d7669
>run_cfi_program: CFA_def_cfa_offset 0x20
>run_cfi_program: CFA_offset r6 at cfa+0xffffffffffffffe0
>run_cfi_program: CFA_advance_loc to 0x7f91345d766f
>run_cfi_program: CFA_def_cfa_offset 0x28
>run_cfi_program: CFA_offset r3 at cfa+0xffffffffffffffd8
>run_cfi_program: CFA_advance_loc to 0x7f91345d768c
>run_cfi_program: CFA_def_cfa_offset 0x50
>run_cfi_program: CFA_advance_loc2 to 0x7f91345d77af
>run_cfi_program: CFA_remember_state
>run_cfi_program: CFA_def_cfa_offset 0x28
>run_cfi_program: CFA_advance_loc to 0x7f91345d77b4
>run_cfi_program: CFA_def_cfa_offset 0x20
>run_cfi_program: CFA_advance_loc to 0x7f91345d77b5
>_Ux86_64_cache_frame: cache frame ip=0x7f91345d77b4 cfa=0x7ffd1e276f18
format=0
>_Ux86_64_reuse_frame: reuse frame ip=0x7f91345d77b4 cfa=0x7ffd1e276f18
format=0 addr=0x0 offset=+0
>apply_reg_state: CFA_REG_COLUMN[17] = 0x7,
DWARF_CFA_OFF_COLUMN[18] = 0x20
>apply_reg_state: CFA = 0x7ffd1e276f18 (pop) + 0x20 =
0x7ffd1e276f38
>apply_reg_state: 0: DWARF_WHERE_SAME = (0, 2)
>apply_reg_state: 1: DWARF_WHERE_SAME = (1, 2)
>apply_reg_state: 2: DWARF_WHERE_SAME = (2, 2)
>apply_reg_state: 3: DWARF_WHERE_CFAREL = 7ffd1e276f38 + -40 =
(7ffd1e276f10, 0)
>apply_reg_state: 4: DWARF_WHERE_SAME = (4, 2)
>apply_reg_state: 5: DWARF_WHERE_SAME = (5, 2)
>apply_reg_state: 6: DWARF_WHERE_CFAREL = 7ffd1e276f38 + -32 =
(7ffd1e276f18, 0)
>apply_reg_state: 7: DWARF_WHERE_SAME = (7, 2)
>apply_reg_state: 8: DWARF_WHERE_SAME = (8, 2)
>apply_reg_state: 9: DWARF_WHERE_SAME = (9, 2)
>apply_reg_state: 10: DWARF_WHERE_SAME = (a, 2)
>apply_reg_state: 11: DWARF_WHERE_SAME = (b, 2)
>apply_reg_state: 12: DWARF_WHERE_CFAREL = 7ffd1e276f38 + -24 =
(7ffd1e276f20, 0)
>apply_reg_state: 13: DWARF_WHERE_CFAREL = 7ffd1e276f38 + -16 =
(7ffd1e276f28, 0)
>apply_reg_state: 14: DWARF_WHERE_SAME = (e, 2)
>apply_reg_state: 15: DWARF_WHERE_SAME = (f, 2)
>apply_reg_state: 16: DWARF_WHERE_CFAREL = 7ffd1e276f38 + -8 =
(7ffd1e276f30, 0)
unwind: access_mem addr 0x7ffd1e276f30 val c0d885722245b5e4, offset 24
>apply_reg_state: ret_addr_column: 16, ip: c0d885722245b5e4
>_Ux86_64_step: dwarf_step returned 1
>_Ux86_64_step: returning 1
>_Ux86_64_step: (cursor=0x7fffafa55c10, ip=0xc0d885722245b5e4,
cfa=0x00007ffd1e276f38)
>_Ux86_64_step: dwarf_step returned -22
>_Ux86_64_step: returning -22
unwind: __hypot_finite:ip = 0x7f91345d77b4 (0x297b4)
unwind: '':ip = 0xc0d885722245b5e3 (0x0)

7f91345d77b4 __hypot_finite+0x154 (/usr/lib/libm-2.28.so)
c0d885722245b5e3 [unknown] ([unknown])
```


Now, I also tried the following:

```
$ perf probe -x /usr/lib/libm-2.28.so -a __hypot_finite+0x154
$ perf record -F 1000 --call-graph dwarf -e probe_libm:__hypot_finite ./cpp-
inlining
```

And all of the samples unwind correctly! This makes me believe that it's not
the .eh_frame information which is wrong - otherwise unwinding would always
fail from these locations, esp. when using the custom probe trace point. But
since this is not happening, what else could it be? I only see two
possibilities: the register values or the stack memory stored in in the sample
by perf.

The register values is unlikely, since I now understand how the .eh_frame
contents get analyzed. For __hypot_finite+0x154, we will always end up asking
for the address at SP+24. access_mem thus will always look at the address at
offset 24, independent of the actual value of SP.

So, what remains is that the stack dump is somehow wrong, i.e. its contents
are moved by some offset. Note how I can "fix" the unwinding for such broken
samples by manually applying some offset in access_mem. By looking at other
samples where unwinding works from __hypot_finite, I could figure out that the
correct address to be read for unwnding should be 7f91345bdaf8, e.g.:

```
7f91345d76ed __hypot_finite+0x8d (/usr/lib/libm-2.28.so)
7f91345bdaf8 hypotf32x+0x18 (/usr/lib/libm-2.28.so)
5620579cb128 main+0x88 (/home/milian/projects/kdab/rnd/hotspot/
build/tests/test-clients/cpp-inlining/cpp-inlining)
```

This address indeed occurs in the user stack dump (starting at 0xe0 in the raw
event data) for the broken sample, cf.:


```
. 00e0: 00 20 00 00 00 00 00 00 c0 b1 9c 57 20 56 00 00 . .........W V..
. 00f0: 70 70 27 1e fd 7f 00 00 f9 da 5b 34 91 7f 00 00 pp'.......[4....
. 0100: e4 b5 45 22 72 85 d8 c0 c0 1d 16 84 43 30 bb c0 ..E"r.......C0..
.
```

We can find `7f91345bdaf8+1 = 7f91345bdaf9" at offset 16 (search for "f9 da 5b
34 91 7f"). Using that address makes unwinding work for this sample. What
could be the reason for this shift?

Thanks

--
Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts[unhandled content-type:application/pkcs7-signature]
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