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Re: [PATCH 2/2] Documentation and testcase
- From: Pedro Alves <palves at redhat dot com>
- To: Sergio Durigan Junior <sergiodj at redhat dot com>
- Cc: GDB Patches <gdb-patches at sourceware dot org>
- Date: Tue, 24 Mar 2015 23:48:15 +0000
- Subject: Re: [PATCH 2/2] Documentation and testcase
- Authentication-results: sourceware.org; auth=none
- References: <1426807358-18295-1-git-send-email-sergiodj at redhat dot com> <1426807358-18295-3-git-send-email-sergiodj at redhat dot com> <550C7905 dot 9090501 at redhat dot com> <87mw37wfd6 dot fsf at redhat dot com> <550C9A7C dot 90705 at redhat dot com> <87wq283gmx dot fsf at redhat dot com> <5510773D dot 4010107 at redhat dot com> <87wq274e1w dot fsf at redhat dot com> <55109C7E dot 2040504 at redhat dot com> <87619q2961 dot fsf at redhat dot com> <55113880 dot 4060707 at redhat dot com> <87egoe2dij dot fsf at redhat dot com>
On 03/24/2015 11:15 PM, Sergio Durigan Junior wrote:
> And I am back, with new investigations and results. This whole thing is
> starting to driving me crazy.
> What I found is interesting. When a memory mapping is file-backed, but
> contains Anonymous: pages, the Linux kernel dumps this region when:
> - The user asks for anonymous pages, *OR*
> - The user asks for file-backed pages
> Yes, it dumps the mapping in *both* scenarios. It is like a
> "file-backed anonymous" mapping...
It makes sense to me.
> I adjusted my patch to mimic this behavior. However, there is one more
> For some reason, when I run the binary inside GDB, the .text segment
> *contains* Anonymous: pages. However, when I run the binary outside
> GDB, the Anonymous: counter is always zero. This means that, inside
> GDB, when we ask for a corefile that excludes file-backed private
> mappings (i.e., the .text segment), according to the Linux kernel's
> rules, the .text segment *still* should be dumped because it contains
> Anonymous: pages.
I'd guess those were pages gdb poked/wrote to? Breakpoints, most
likely. That should trigger a COW of the poked page.
> Unfortunately, I was not able to generate a binary whose .text segment
> contained Anonymous: pages outside GDB. However, I made a binary
> that has Anonymous: pages on a file-backed mapping, and I made the Linux
> kernel generate a corefile for it while asking it to exclude file-backed
> mappings, and I could confirm that the Linux kernel indeed includes this
> mapping in the corefile.
> My final proposal, which will be reflected in a patch that will be
> submitted soon, is to relax the test of the the disassembly of main.
> This way, we can still mimic what the Linux kernel does and make GDB
> compatible with it.
Not sure what relax means, but I'll just wait for the patch. :-)