Re: Concerns regarding e17bebd049 ("dump: Set correct vaddr for ELF dump")

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On Thu, 21 Sept 2023 at 01:45, Stephen Brennan
<[email protected]> wrote:
>
> Stephen Brennan <[email protected]> writes:
> > Hi Jon,
> >
> > Jon Doron <[email protected]> writes:
> >> Hi Stephen,
> >> Like you have said the reason is as I wrote in the commit message,
> >> without "fixing" the vaddr GDB is messing up mapping and working with
> >> the generated core file.
> >
> > For the record I totally love this workaround :)
> >
> > It's clever and gets the job done and I would have done it in a
> > heartbeat. It's just that it does end up making vmcores that have
> > incorrect data, which is a pain for debuggers that are actually designed
> > to look at kernel core dumps.
> >
> >> This patch is almost 4 years old, perhaps some changes to GDB has been
> >> introduced to resolve this, I have not checked since then.
> >
> > Program Headers:
> >   Type           Offset             VirtAddr           PhysAddr
> >                  FileSiz            MemSiz              Flags  Align
> >   NOTE           0x0000000000000168 0x0000000000000000 0x0000000000000000
> >                  0x0000000000001980 0x0000000000001980         0x0
> >   LOAD           0x0000000000001ae8 0x0000000000000000 0x0000000000000000
> >                  0x0000000080000000 0x0000000080000000         0x0
> >   LOAD           0x0000000080001ae8 0x0000000000000000 0x00000000fffc0000
> >                  0x0000000000040000 0x0000000000040000         0x0
> >
> > (gdb) info files
> > Local core dump file:
> >         `/home/stepbren/repos/test_code/elf/dumpfile', file type elf64-x86-64.
> >         0x0000000000000000 - 0x0000000080000000 is load1
> >         0x0000000000000000 - 0x0000000000040000 is load2
> >
> > $ gdb --version
> > GNU gdb (GDB) Red Hat Enterprise Linux 10.2-10.0.2.el9
> > Copyright (C) 2021 Free Software Foundation, Inc.
> > License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> > This is free software: you are free to change and redistribute it.
> > There is NO WARRANTY, to the extent permitted by law.
> >
> >
> > It doesn't *look like* anything has changed in this version of GDB. But
> > I'm not really certain that GDB is expected to use the physical
> > addresses in the load segments: it's not a kernel debugger.
> >
> > I think hacking the p_vaddr field _is_ the way to get GDB to behave in
> > the way you want: allow you to read physical memory addresses.
> >
> >> As I'm no longer using this feature and have not worked and tested it
> >> in a long while, so I have no obligations to this change, but perhaps
> >> someone else might be using it...
> >
> > I definitely think it's valuable for people to continue being able to
> > use QEMU vmcores generated with paging=off in GDB, even if GDB isn't
> > desgined for it. It seems like a useful hack that appeals to the lowest
> > common denominator: most people have GDB and not a purpose-built kernel
> > debugger. But maybe we could point to a program like the below that will
> > tweak the p_paddr field after the fact, in order to appeal to GDB's
> > sensibilities?
>
> And of course I sent the wrong copy of the file. Attached is the program
> I intended to send (which properly handles endianness and sets the vaddr
> as expected).
>