FDEs in .eh_frame section

Ian Lance Taylor iant@google.com
Fri Sep 17 05:34:00 GMT 2010


Martin Kirchengast <martin_kir_@hotmail.com> writes:

> what is the data format of "initial_location" and "address_range" in
> the .eh_frame section of an ELF64 binary? I read the DWARF
> specification which says:
>
> "initial_location (target address): An addressing-unit sized constant indicating the address of the first location associated with this table entry."
>
> and
>
> "address_range (target address): An addressing unit sized constant indicating the number of bytes of program instructions described by this entry."
>
> Because I'm building for x86-64 I thought that must be 64 bit, but
> that seems to be wrong.

Yes.  It's a 32-bit value for most targets.  Note that the value is
normally PC relative--it's the offset from the FDE to the address which
it covers.  So 32 bits is normally sufficient.  The number of bytes is
also normally 32 bits.


> 00000020 0000001c 00000024 FDE cie=00000000 pc=000000ec..000000f6

> Contents of section .eh_frame:
>  3b78 1c000000 00000000 017a5052 00017810  .........zPR..x.
>  3b88 0a005423 00000000 00001b0c 07089001  ..T#............
>  3b98 1c000000 24000000 4cc5ffff 0a000000  ....$...L.......
>  3ba8 00410e10 4386020d 06450c07 08000000  .A..C....E......
>
> According to the above output of readelf the value of address_range
> should be 0xf6 - 0xec = 0xa. This value is at address 0x3ba4 and seems
> to be 32 bit wide. initial_location hast to be at address 0x3ba0, is
> 32 bit wide too and has a quite weird value. So please tell what am I
> doing wrong here?

The initial_location value is at 0x3ba0 is 0xffffc54c.  0x3ba0 +
0xffffc54c == 0xec, which is the value you want.

Ian



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