Re: [PATCH 3/3] gdbserver: pass osabi to GDB in target description

[Luis Machado <luis.machado@arm.com>]

On 10/6/24 19:37, Andrew Burgess wrote:
> On a Windows machine I built gdbserver, configured for the target
> 'x86_64-w64-mingw32', then on a GNU/Linux machine I built GDB with
> support for all target (--enable-targets=all).
> 
> On the Windows machine I start gdbserver with a small test binary:
> 
>   $ gdbserver 192.168.129.25:54321 C:\some\directory\executable.exe
> 
> On the GNU/Linux machine I start GDB without the test binary, and
> connect to gdbserver.
> 
> As I have not given GDB the test binary, my expectation is that GDB
> would connect to gdbserver and then download the file over the remote
> protocol, but instead I was presented with this message:
> 
>   (gdb) target remote 192.168.129.25:54321
>   Remote debugging using 192.168.129.25:54321
>   warning: C:\some\directory\executable.exe: No such file or directory.
>   0x00007ffa3e1e1741 in ?? ()
>   (gdb)
> 
> What I found is that if I told GDB where to find the binary, like
> this:
> 
>   (gdb) file target:C:/some/directory/executable.exe
>   A program is being debugged already.
>   Are you sure you want to change the file? (y or n) y
>   Reading C:/some/directory/executable.exe from remote target...
>   warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
>   Reading C:/some/directory/executable.exe from remote target...
>   Reading symbols from target:C:/some/directory/executable.exe...
>   (gdb)
> 
> then GDB would download the executable.
> 
> I eventually tracked the problem down to exec_file_find (solib.c).
> The remote target was passing an absolute Windows filename (beginning
> with "C:/" in this case), but in exec_file_find GDB was failing the
> IS_TARGET_ABSOLUTE_PATH call, and so was treating the filename as
> relative.
> 
> The IS_TARGET_ABSOLUTE_PATH call was failing because GDB thought that
> the file system kind was "unix", and as the filename didn't start with
> a "/" it assumed the filename was not absolute.
> 
> But I'm connecting to a Windows target, my 'target-file-system-kind'
> was set to "auto", so should be figuring out that my file-system is
> "dos-based".
> 
> Looking in effective_target_file_system_kind (filesystem.c), we find
> that the logic of "auto" is delegated to the current gdbarch.  However
> in windows-tdep.c we see:
> 
>   set_gdbarch_has_dos_based_file_system (gdbarch, 1);
> 
> So if we are using a Windows gdbarch we should have "dos-based"
> filesystems.  What this means is that after connecting to the remote
> target GDB has selected the wrong gdbarch.
> 
> What's happening is that the target description sent back by the
> remote target only includes the x86-64 registers.  There's no
> information about which OS we're on.  As a consequence, GDB picks the
> first x86-64 gdbarch which can handle the provided register set, which
> happens to be a GNU/Linux gdbarch.
> 
> And indeed, there doesn't appear to be anywhere in gdbserver that sets
> the osabi on the target descriptions, though some target descriptions
> do have their osabi set when the description is created, e.g. in:
> 
>   gdb/arch/amd64.c	- Sets GNU/Linux osabi when appropriate.
>   gdb/arch/i386.c	- Likewise.
>   gdb/arch/tic6x.c	- Always set GNU/Linux osabi.
> 
> Most target descriptions are created without an osabi, gdbserver does
> nothing to fix this, and the description is returned to GDB without an
> osabi included.
> 
> I propose that we always set the osabi name on the target descriptions
> returned from gdbserver.  We could try to do this when the description
> is first created, but that would mean passing extra flags into the
> tdesc creation code (or just passing the osabi string in), and I don't
> think that's really necessary.  If we consider the tdesc creation as
> being about figuring out which registers are on the target, then it
> makes sense that the osabi information is injected later.
> 
> So what I've done is require the osabi name to be passed to the
> init_target_desc function.  This is called, I believe, for all
> targets, in the gdbserver code.
> 
> Now when I connect to the Windows remote the target description
> returned includes the osabi name.  With this extra information GDB
> selects the correct gdbarch object, which means that GDB understands
> the target has a "dos-based" file-system.  With that correct GDB
> understands that the filename it was given is absolute, and so fetches
> the file from the remote as we'd like.
> ---
>  gdbserver/linux-aarch32-tdesc.cc |  2 +-
>  gdbserver/linux-aarch64-tdesc.cc |  3 ++-
>  gdbserver/linux-arc-low.cc       |  2 +-
>  gdbserver/linux-arm-tdesc.cc     |  2 +-
>  gdbserver/linux-csky-low.cc      |  2 +-
>  gdbserver/linux-loongarch-low.cc |  2 +-
>  gdbserver/linux-riscv-low.cc     |  2 +-
>  gdbserver/linux-tic6x-low.cc     |  2 +-
>  gdbserver/linux-x86-tdesc.cc     | 14 ++++++++++++--
>  gdbserver/netbsd-aarch64-low.cc  |  2 +-
>  gdbserver/netbsd-amd64-low.cc    |  2 +-
>  gdbserver/netbsd-i386-low.cc     |  2 +-
>  gdbserver/tdesc.cc               |  7 ++++++-
>  gdbserver/tdesc.h                |  5 +++--
>  gdbserver/win32-i386-low.cc      |  4 ++--
>  gdbserver/win32-low.h            |  7 +++++++
>  16 files changed, 42 insertions(+), 18 deletions(-)
> 
> diff --git a/gdbserver/linux-aarch32-tdesc.cc b/gdbserver/linux-aarch32-tdesc.cc
> index b8987752b9f..bff671b4f1e 100644
> --- a/gdbserver/linux-aarch32-tdesc.cc
> +++ b/gdbserver/linux-aarch32-tdesc.cc
> @@ -34,7 +34,7 @@ aarch32_linux_read_description ()
>        tdesc_aarch32 = aarch32_create_target_description (false);
>  
>        static const char *expedite_regs[] = { "r11", "sp", "pc", 0 };
> -      init_target_desc (tdesc_aarch32, expedite_regs);
> +      init_target_desc (tdesc_aarch32, expedite_regs, "GNU/Linux");


These changes sound OK to me, but wouldn't it be cleaner to pass, say, an enum for the
osabi, and then later get that translated to the proper XML/string entry?

Maybe share the osabi naming with what gdb has.