Re: [PATCH 2/3] relocate the entry point addess when used

Tom Tromey
2014-01-13T20:46:19Z

>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:

Tom> I think the existing code here is wrong. It computes the entry point
Tom> address directly from the BFD, not applying any runtime offsets.
Tom> However, then objfile_relocate1 passes this address to find_pc_section
Tom> -- which does use the offsets . So, it seems to me that the current
Tom> code can only find the correct address by luck.

Pedro> It's twisted, but I don't think it's luck. You can convince yourself
Pedro> it works by debugging a PIE, and trying a backtrace past main
Pedro> ("set backtrace past-main" will then trigger the code to stop the
Pedro> backtrace at the entry point), or doing "info files" (shows the entry).

Well, what I don't understand is that most addresses in dwarf2read.c are
offset:

baseaddr = ANOFFSET (objfile->section_offsets, SECT_OFF_TEXT (objfile));

... however this is not done for the entry point, which comes directly
from the BFD:

objfile->per_bfd->ei.entry_point = bfd_get_start_address (objfile->obfd);

I suppose there is some other invariant ensuring that the entry point is
only computed when all the objfile offsets are zero. This part is not
obvious to me.

[...]
Pedro> This is assuming osect ends up NULL after iterating over all.
Pedro> It's violating the abstraction of the macro. And, actually,
Pedro> it's wrong, showing exactly why such assumptions are a bad idea:

Sorry about that, I'll fix it up locally.

Tom