[Dwarf-Discuss] _starting address_ in eh_frame FDE encoding
Francesco Zappa Nardelli
francesco.zappa.nardelli at gmail.com
Thu Mar 31 01:43:45 PDT 2016
> I am still confused though, can we check this example together?
Sorry for the noise, I made a silly mistake and the addresses in the
hexdump, which are off by 0x10. Problem solved, thank you for the help!
> Here is an annotated hexdump for the first CIE and FDE of an eh_frame
> $ hexdump -C -s 0x580 -n 658 foo2
> 00000580 14 00 00 00 00 00 00 00 01 7a 52 00 01 78 10 01
> length | ID | v aug | af df rr la
> 00000590 1b 0c 07 08 90 01 07 10 14 00 00 00 1c 00 00 00
> *| opcds . . || length | ptr to CIE|
> *FDE encoding (aug = R) : 1b = PC_REL, sdata4
> 000005a0 00 fe ff ff 2a 00 00 00 00 00 00 00 00 00 00 00
> strt_addr*| ||
> The CIE augmentation data state states:
> DW_EH_PE_pcrel, DW_EH_PE_sdata4
> The untranslated starting address in the FDE (noted strt_addr) is
> fffffe00 (that is -200 in hex, -512 in dec)
> which, according to readelf must be translated to
> The offset since the beginning of the eh_frame of the strt_addr field is
> 32, and
> $ readelf -S foo2
> There are 34 section headers, starting at offset 0xc20:
> Section Headers:
> [Nr] Name Type Address Offset
> Size EntSize Flags Link Info Align
>  .eh_frame PROGBITS 0000000000400590 00000590
> 0000000000000114 0000000000000000 A 0 0 8
> In hex, 400590 + 20 = 4005b0, is this the “PC address” that we should
> consider when we parse the PCREL start address? If yes, 4005b0 - 200 =
> 4003b0, but we should have got 4003a0. Can you point me to where I am
> Thank you again.
> On Thu, Mar 24, 2016 at 3:47 PM, Cary Coutant <ccoutant at gmail.com> wrote:
>> > I have a question about the FDE encoding in eh_frame (and not
>> > tables.
>> > This document (I could not find a better description of eh_frame)
>> > http://www.airs.com/blog/archives/460
>> The official description of the .eh_frame section, with a description
>> of the augmentation string and what the various DW_EH_PE values mean,
>> can be found in the relevant psABI document for your architecture.
>> > states that:
>> > "An FDE starts with the length and ID described above, and then
>> > as follows. 1 The _starting address_ to which this FDE applies. This
>> > encoded using the FDE encoding specified by the associated CIE.”
>> > In a sample elf file, the associated CIE augmentation data state
>> > DW_EH_PE_pcrel, DW_EH_PE_sdata4
>> > and the untranslated starting address value in the FDE is fffffe88
>> > (according to readelf this should be translated to 4004d0). I do not
>> > understand what “value is PC relative” means here, as we are actually
>> > defining the range of PC values.
>> > Can somebody explain me how to interpret this _starting address_ field?
>> PC-relative simply means that the value is not an absolute address,
>> but rather an offset relative to itself (i.e., the address of the
>> field that contains the value. The value you read there was
>> 0xfffffe88, which, according to the augmentation string, is a signed
>> 4-byte value, or -0x178. If you add that to the address of the
>> starting address field itself, you'll get the actual starting address,
>> which readelf has computed as 0x4004d0. Working backwards, that
>> implies that your FDE started at 400640, with the starting address
>> field at 0x400648.
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