debug problem with prelinked libraries
Andrew Stubbs
ams@codesourcery.com
Wed May 5 14:59:00 GMT 2010
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
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