[Dwarf-Discuss] unwinding stack (CIE/FDE)

Tue Aug 10 15:56:43 GMT 2010

Thanks John for taking the time and patience to answer all the question.? Thanks 
a lot!!!
?
Assuming I get to the current FDE and get to the upper function FDE ?using the 
return address. How would I be able to change register states to the one of 
upper frame? The FDE/CIE instructions of upper frame wouldn?t provide right 
address when?the registers are still holding values of?current frame. You said I 
need to maintain states, can you explain a little more?

My apologies if?I am?asking something that has already been discussed here.

________________________________
From: John DelSignore <John.DelSignore@roguewave.com>
To: Saurabh <saurabhcv at yahoo.com>
Cc: dwarf info <dwarf-discuss at lists.dwarfstd.org>
Sent: Mon, August 9, 2010 6:41:51 AM
Subject: Re: [Dwarf-Discuss] unwinding stack (CIE/FDE)

Saurabh wrote:
> Thanks for the enlightenment!!!
> 
> I was wondering? ... isn't the instruction " DW_CFA_def_cfa_offset: 16"
> redundant?

No, it means that, at offset +1 into the function, the cfa is r7+16, instead of 
r7+8.

> The location for r6 is easily described by DW_CFA_offset. All
> dumps I have seen have this instruction.
> 00000018 0000001c 00000000 FDE cie=00000000 pc=00000000..00000070
>? DW_CFA_advance_loc: 1 to 00000001
>? DW_CFA_def_cfa_offset: 16
>? DW_CFA_offset: r6 at cfa-16

I believe that the above means that, at offset +1 into the function, r6 is 
stored at cfa-16. I think readelf is probably multiplying the offset by the 
"Data alignment factor" before printing it out, so the LEB128 factored offset in 
the CFA was probably 2.

>? DW_CFA_advance_loc: 3 to 00000004
>? DW_CFA_def_cfa_reg: r6

The above means that, at offset +4 into the function, the cfa register is now r6 
and the offset stays the same, so the cfa is r6+16.

Cheers, John D.

> 
> 
> 
> ------------------------------------------------------------------------
> *From:* John DelSignore <John.DelSignore at roguewave.com>
> *To:* Saurabh <saurabhcv at yahoo.com>
> *Cc:* "dwarf-discuss at lists.dwarfstd.org" <dwarf-discuss at lists.dwarfstd.org>
> *Sent:* Sun, August 8, 2010 3:08:28 PM
> *Subject:* Re: [Dwarf-Discuss] unwinding stack (CIE/FDE)
> 
> Saurabh wrote:
>> Sorry guys,
>> I think the discussion got diverted and I am still unclear on how to
>> find the return address. The example in the Dwarf Appendix is confusing
>> and varying compiler implementations have corroded my understanding
> further.
>> 
>> These are my questions.
>> 1) I have not been able to fully interpret CFA instructions, so I am
>> having trouble finding where the return address is saved. From what I
>> understand CFA is same as SP at entry of a function. Which means that
>> the CFA is what is pointing to the return address.
> 
> Not necessarily true. This may be true on all x86, but other
> architectures like Power, the return address is stored in a register.
> 
>> 2) I understand "advance loc" is used to add a line to the reg rule
>> table. I have noticed that sometimes the prologoue can be missing, but
>> that shouldn't stop me from getting the return address(CFA)
>> 
>> 3) I need to know which entry would tell me where the return address is
>> saved or where CFA for the FDE is?
> 
> Look at an example using gcc on Linux x86_64 dumped with readelf:
> 
> "
> The section .debug_frame contains:
> 
> 00000000 00000014 ffffffff CIE
>? Version:? ? ? ? ? ? ? 1
>? Augmentation:? ? ? ? ? ""
>? Code alignment factor: 1
>? Data alignment factor: -8
>? Return address column: 16
> 
>? DW_CFA_def_cfa: r7 ofs 8
>? DW_CFA_offset: r16 at cfa-8
> 
> 00000018 0000001c 00000000 FDE cie=00000000 pc=00000000..00000070
>? DW_CFA_advance_loc: 1 to 00000001
>? DW_CFA_def_cfa_offset: 16
>? DW_CFA_offset: r6 at cfa-16
>? DW_CFA_advance_loc: 3 to 00000004
>? DW_CFA_def_cfa_reg: r6
> "
> 
> The CIE line "Return address column" tells you the column 16 is where
> the return address is stored. In this case, column 16 is a virtual
> register "r16", not an actual hardware register.
> 
> The CIE line "DW_CFA_offset: r16 at cfa-8" tells you that r16 (column
> 16) is at "cfa-8".
> 
>> 4) Which register is the one pointing to CFA?
> 
> The line before says the cfa is r7+8.
> 
>> 4) Where are the arguments being saved?
> 
> The unwind information doesn't tell you about the arguments. The DIEs
> do. In my example above, they are in the .debug_info section:
> 
> <1><3e5>: Abbrev Number: 14 (DW_TAG_subprogram)
>? ? DW_AT_sibling? ? : <42c>
>? ? DW_AT_external? ? : 1
>? ? DW_AT_name? ? ? ? : main
>? ? DW_AT_decl_file? : 1
>? ? DW_AT_decl_line? : 4
>? ? DW_AT_prototyped? : 1
>? ? DW_AT_type? ? ? ? : <a8>
>? ? DW_AT_low_pc? ? ? : 0 0
>? ? DW_AT_high_pc? ? : 0xb8 184
>? ? DW_AT_frame_base? : 1 byte block: 6f? ? ? (DW_OP_reg31)
> <2><401>: Abbrev Number: 15 (DW_TAG_formal_parameter)
>? ? DW_AT_name? ? ? ? : argc
>? ? DW_AT_decl_file? : 1
>? ? DW_AT_decl_line? : 3
>? ? DW_AT_type? ? ? ? : <a8>
>? ? DW_AT_location? ? : 2 byte block: 91 8? ? (DW_OP_fbreg: 8)
> <2><410>: Abbrev Number: 15 (DW_TAG_formal_parameter)
>? ? DW_AT_name? ? ? ? : argv
>? ? DW_AT_decl_file? : 1
>? ? DW_AT_decl_line? : 3
>? ? DW_AT_type? ? ? ? : <42c>
>? ? DW_AT_location? ? : 2 byte block: 91 c? ? (DW_OP_fbreg: 12)
> 
>> 5) How to get to the CFA at next level, in order to make a tree. Do I
>> simply use the return address to find the corrsponding FDE using
>> start/end PC and repeat.
> 
> Yes, but you may also need to maintain register state as you go.
> 
> Good luck.
> 
> Cheers, John D.
> 
> 
>> Below are dumps from dwarfdump and readelf of a sample run that you can
>> refer to.
>> 
>> I have got the libunwinder library, but it has too much detail and I
>> just need to understand the unlying basic logic.
>> 
>> fde:
>> <? 0><0x0:0x6><><fde offset 0x4c length: 0x10><eh offset none>
>>? ? 0x00000000:
>> *** DWARF CHECK: An fde low pc of 0x0 is not the first fde with that pc.
>> The first is not named. ***
>> <? 0><0x0:0x6><><fde offset 0x60 length: 0x10><eh offset none>
>>? ? 0x00000000:
>> *** DWARF CHECK: An fde low pc of 0x0 is not the first fde with that pc.
>> The first is not named. ***
>> <? 0><0x0:0x6><><fde offset 0x74 length: 0x10><eh offset none>
>>? ? 0x00000000:
>> *** DWARF CHECK: An fde low pc of 0x0 is not the first fde with that pc.
>> The first is not named. ***
>> <? 0><0x0:0x1e><><fde offset 0x88 length: 0x1c><eh offset none>
>>? ? 0x00000000:
>>? ? 0x00000002: <off r13=-4(cfa) >
>>? ? 0x00000004: <off r13=-4(cfa) > <off r31=r13 >
>> =================
>> 0000004c 00000010 00000000 FDE cie=00000000 pc=00000000..00000006
>>? DW_CFA_advance_loc: 6 to 00000006
>>? DW_CFA_nop
>>? DW_CFA_nop
>>? DW_CFA_nop
>> 00000060 00000010 00000000 FDE cie=00000000 pc=00000008..0000000e
>>? DW_CFA_advance_loc: 6 to 0000000e
>>? DW_CFA_nop
>>? DW_CFA_nop
>>? DW_CFA_nop
>> 00000074 00000010 00000000 FDE cie=00000000 pc=00000010..00000016
>>? DW_CFA_advance_loc: 6 to 00000016
>>? DW_CFA_nop
>>? DW_CFA_nop
>>? DW_CFA_nop
>> 00000088 0000001c 00000000 FDE cie=00000000 pc=00000018..00000036
>>? DW_CFA_advance_loc: 2 to 0000001a
>>? DW_CFA_def_cfa_offset: 4
>>? DW_CFA_offset: r13 at cfa-4
>>? DW_CFA_advance_loc: 2 to 0000001c
>>? DW_CFA_register: r31 in r13
>>? DW_CFA_advance_loc: 26 to 00000036
>>? DW_CFA_restore: r13
>>? DW_CFA_def_cfa_offset: 0
>>
>> ------------------------------------------------------------------------
>> *From:* Mark Wielaard <mjw at redhat.com <mailto:mjw at redhat.com>>
>> *To:* Jason Molenda <jmolenda at apple.com <mailto:jmolenda at apple.com>>
>> *Cc:* Frank Ch. Eigler <fche at redhat.com <mailto:fche at redhat.com>>;
> Saurabh <saurabhcv at yahoo.com <mailto:saurabhcv at yahoo.com>>;
>> dwarf-discuss at lists.dwarfstd.org <mailto:dwarf-discuss at lists.dwarfstd.org>
>> *Sent:* Wed, August 4, 2010 1:06:08 AM
>> *Subject:* Re: [Dwarf-Discuss] unwinding stack (CIE/FDE)
>>
>> On Tue, 2010-08-03 at 17:37 -0700, Jason Molenda wrote:
>>> On Aug 3, 2010, at 5:31 PM, Frank Ch. Eigler wrote:
>>>
>>> > Hi -
>>> >
>>> > On Tue, Aug 03, 2010 at 04:55:23PM -0700, Jason Molenda wrote:
>>> >> [...]
>>> >> Looking at the debug_frame that my compiler (gcc-4.2 in this case)
>> outputs,
>>> >> [...]
>>> >> There's nothing about that stack movement in the FDE.
>>> >
>>> > Try again with CFLAGS+=-fasynchronous-unwind-tables.
>>>
>>> I'm not making this stuff up. :)? And the next question you'll ask
>>> yourself is why -fasynchronous-unwind-tables isn't the default output
>>> style for the debug_frame section in the first place.? But as you can
>>> see, it doesn't make any difference - at least with gcc-4.2 with the
>>> Apple local patches.
>>
>> You might want to upgrade your compiler. gcc 4.2 is fairly old (2007,
>> hasn't seen a point upgrade since 2008). Current is gcc 4.5 (which will
>> output unwind info not just for function prologues, but also for
>> epilogues). On my fedora machine -fasynchronous-unwind-tables is the
>> default (although I couldn't replicate your situation there, since newer
>> gcc seems to not generate that push/pop/call dance for me). You might
>> also want to look at both the .debug_frame and the .eh_frame. If a
>> location is described in one, the compiler might not also output it in
>> the other.
>>
>> Cheers,
>>
>> Mark
>>
>>
> 
> 

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