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

[Stephen Brennan]

Stephen Brennan <stephen.s.brennan@oracle.com> writes:
> Hi Jon,
>
> Jon Doron <arilou@gmail.com> 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).

phys2virt.c
Description: phys2virt.c