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Re: [Qemu-trivial] [Qemu-devel] [PATCH] linux-user: Support gdb 'qOffset

From: Josh Kunz
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] linux-user: Support gdb 'qOffsets' query for ELF
Date: Tue, 3 Sep 2019 12:19:45 -0700

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?


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