Cary, thanks for your reply! I am still confused though, can we check this
example together?
Here is an annotated hexdump for the first CIE and FDE of an eh_frame table:
$ 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
pc=00000000004003a0
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
...
[16] .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
wrong?
Thank you again.
Best
-francesco
On Thu, Mar 24, 2016 at 3:47 PM, Cary Coutant <ccout...@gmail.com> wrote:
> > I have a question about the FDE encoding in eh_frame (and not
> dwarf_frame)
> > 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
> continues
> > as follows. 1 The _starting address_ to which this FDE applies. This is
> > encoded using the FDE encoding specified by the associated CIE.”
> >
> > In a sample elf file, the associated CIE augmentation data state reports:
> >
> > 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.
>
> -cary
>
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