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Re: [PATCH 2/3] relocate the entry point addess when used
- From: Tom Tromey <tromey at redhat dot com>
- To: Pedro Alves <palves at redhat dot com>
- Cc: gdb-patches at sourceware dot org
- Date: Mon, 13 Jan 2014 13:46:15 -0700
- Subject: Re: [PATCH 2/3] relocate the entry point addess when used
- Authentication-results: sourceware.org; auth=none
- References: <1389040247-2620-1-git-send-email-tromey at redhat dot com> <1389040247-2620-3-git-send-email-tromey at redhat dot com> <52CD5129 dot 7090003 at redhat dot com>
>>>>> "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