Re: debug problem with prelinked libraries

[Andrew Stubbs <ams@codesourcery.com>]

On 05/05/10 15:32, Daniel Jacobowitz wrote:
> On Wed, May 05, 2010 at 01:48:21PM +0100, Andrew Stubbs wrote:
>> Hi all,
>>
>> I'm trying to get to the bottom of a problem debugging prelinked
>> libraries. I've fixed a few aspects of the problem, but the further I
>> get into it the more I think I must be missing something. I mean,
>> debugging prelinked libraries is supposed to Just Work, right? But I
>> can't see how it could ever have worked.
>
> It definitely works.  GDB is even supposed to automatically handle
> libraries that are prelinked to a different location on disk than they
> were at runtime, although that's relatively recent, IIRC.

Yes, I see messages about it fixing these up (libc is different, but the 
interesting libraries are not).

>> Upon closer inspection, I find that the psymtab has the textlow and
>> texthigh addresses as the original file-offsets, before relocation.
>> This appears to be because it calculates the section offset as the
>> difference between the actual address and the ELF VMA, but the file
>> is prelinked, so the offset is zero, and the debug info and symbols
>> are then not relocated.
>
> What do you mean by "before relocation"?

The textlow and texthigh values get set to small values, such as 0x6c0, 
rather than the run-time relocated values, which are rather larger 
numbers. The numbers can be corrected by adding the correct offset.

> Are the libraries used to create the core dump prelinked in exactly
> the same way as the libraries on the host during debug, or not?  It
> should work either way, but they're different cases.

They are the exact same binaries. I copied them to the target myself.

> If both libraries are identically prelinked, then I would have
> expected the prelinked libraries to not require relocation.  prelink
> includes code to manipulate the contents of the debug info.

Hmmm, well maybe that's it then. Maybe prelink has failed to relocate 
the debug info in this case?

> I don't know if prelink follows things across to separate debug info
> files.  It doesn't look like it.  So if you have that, too, then the
> separate debug info file should be treated as a not-prelinked copy of
> the shared library.

The debug info is not separated, in this case.

Andrew