Re: [Qemu-devel] [PATCH] linux-user: Support gdb 'qOffsets' query for ELF

[Josh Kunz]

The `Data` and `Code` flags in `qOffsets` are actually section offsets
rather than segment offsets. GDB relocates the symbols in those sections
relative to their location in the binary. So we have to use `load_bias`.

See here for a more detailed description:
https://sourceware.org/gdb/onlinedocs/gdb/General-Query-Packets.html#General-Query-Packets

On Mon, Aug 26, 2019 at 1:29 AM Laurent Vivier <address@hidden> wrote:

> Le 17/08/2019 à 01:34, Josh Kunz via Qemu-devel a écrit :
> > This is needed to support debugging PIE ELF binaries running under QEMU
> > user mode. Currently, `code_offset` and `data_offset` remain unset for
> > all ELF binaries, so GDB is unable to correctly locate the position of
> > the binary's text and data.
> >
> > The fields `code_offset`, and `data_offset` were originally added way
> > back in 2006 to support debugging of bFMT executables (978efd6aac6),
> > and support was just never added for ELF. Since non-PIE binaries are
> > loaded at exactly the address specified in the binary, GDB does not need
> > to relocate any symbols, so the buggy behavior is not normally observed.
> >
> > Buglink: https://bugs.launchpad.net/qemu/+bug/1528239
> > Signed-off-by: Josh Kunz <address@hidden>
> > ---
> >  linux-user/elfload.c | 2 ++
> >  1 file changed, 2 insertions(+)
>
> As it seems they are text and data segment offsets, why it's not based
> on info->start_code and info->start_data?
>
> Thanks,
> Laurent
>