GDB server for PCSXR

[Xavi92]

Based on this fork of pcsxr and this dependency-free GDB stub implementation, I have implemented a GDB server for PCSXR to be used with mipsel-unknown-elf-gdb. This allows source-level debugging and usual commands such as breakpoints, reading/writing variables, step into, step over, continue, break, etc.



Download
https://github.com/XaviDCR92/pcsxr

Installation

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git clone https://github.com/XaviDCR92/pcsxr
cd pcsxr/
mkdir build && cd build
cmake -G Ninja -DCMAKE_BUILD_TYPE=Debug .. && cmake --build .

Now you can run pcsxr with:

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cd gui && ./pcsxr

How to use
pcsxr now has a "-gdb" command line flag that starts the GDB server on port 3333. The same effect can be achieved by starting pcsxr with no flags and selecting "Configuration->CPU->Enable GDB Server". Ensure "Enable Interpreter CPU" is enabled and "Enable Debugger" (the telnet-based server) remains disabled. Once configured, "GDB server started on port 3333." should appear on stdout. Finally, load the .bin/.cue/.exe file that is attached to your .elf file.

Next, execute mipsel-unknown-elf-gdb using a valid .elf file with debug symbol information (available when compiling with mipsel-unknown-elf-gcc with the "-g" or "-ggdb" flags). If using the command line:

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mipsel-unknown-elf-gdb your-game.elf --tui # --tui not needed, but helpful nonetheless
target remote 3333
b main
c

Now pcsxr should stop at main() and source level should be displayed.

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(gdb) target remote :3333
Remote debugging using :3333
0xbfc00000 in ?? ()
(gdb) b main
Breakpoint 1 at 0x80020000
(gdb) c
Continuing.

Program stopped.
0x80020000 in main ()

For Visual Studio Code (or its fully FOSS variant VSCodium) users, the NativeDebug extension can be easily configured to support mipsel-unknown-elf-gdb.

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{
    "version": "0.2.0",
    "configurations":
    [
        {
            "type": "gdb",
            "request": "launch",
            "name": "Launch Program",
            "target": "Set application name e.g.: ${workspaceFolder}/my-game.elf",
            "cwd": "${workspaceFolder}",
            "gdbpath": "mipsel-unknown-elf-gdb",
            "autorun":
            [
                "target remote :3333",
                "b main",
                "c",
                "d"
            ],
            "valuesFormatting": "prettyPrinters"
        }
    ]
}

Known issues
- Porting to Windows is not done yet, but should not be too hard. Only Linux (using Kubuntu 20.04 LTS) has been tested so far.
- Running the GDB server might block the GUI, so if you want to close pcsxr please kill the process if blocked.
- CPU usage is high when waiting for continue/break signals as it must look up two different sockets simultaneously. Hint: I think this could be fixed by using select().
- Warning: PCSXr already had a Telnet-based server which I had to do some breaking changes to. If you find any bug, feel free to create a pull request.

[ackerman]

Congratulations, that's interesting.

[Xavi92]

New bugfixes have been pushed into the repository. Please check them out!

[algebro]

Thanks for sharing this! I'm excited to try it but I'm having a couple of issues and I was hoping you could help.

First, the gdbstub revision pinned for the submodule no longer exists--it looks like it was removed with a force push or something. I updated it to the latest `master` and now the project checks out and builds OK.

Second, after starting the gdbserver with `-gdb` and trying to connect with gdb-multiarch, I'm getting a remote communication error:

Client:

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(gdb) set arch mips
The target architecture is set to "mips".
(gdb) set endian little
The target is set to little endian.
(gdb) set debug remote 1
(gdb) target extended-remote :3333
Remote debugging using :3333
Sending packet: $qSupported:multiprocess+;swbreak+;hwbreak+;qRelocInsn+;fork-events+;vfork-events+;exec-events+;vContSupported+;QThreadEvents+;no-resumed+;xmlRegisters=i386#6a...Remote communication error.  Target disconnected.: Connection reset by peer.

pcsxr server:

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./pcsxr -gdb
GDB server started on port 3333.
Accepted gdb connection
pthread_join(): No such file or directory
Terminated active gdb connection
Closed gdb server

Any idea how to troubleshoot?

[Xavi92]

First, the gdbstub revision pinned for the submodule no longer exists--it looks like it was removed with a force push or something. I updated it to the latest `master` and now the project checks out and builds OK.

More like I forgot to push the commits stored in my computer. I have just pushed them - could you please try it again?

[algebro]

Thanks! That fixes the build problems but I'm still not having any luck with remote debugging:

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./pcsxr -gdb
GDB server started on port 3333.
Accepted gdb connection
send():: Broken pipe
send():: Broken pipe
send():: Broken pipe
send():: Broken pipe
send():: Broken pipe
send():: Broken pipe
send():: Broken pipe

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gdb-multiarch 
Exception caught while booting Guile.
Error in function "make_objcode_from_file":
bad header on object file: "\x7fELF\x02\x01\x01ÿ\x00\x00\x00\x00\x00\x00\x00\x00"
gdb-multiarch: warning: Could not complete Guile gdb module initialization from:
/usr/share/gdb/guile/gdb/boot.scm.
Limited Guile support is available.
Suggest passing --data-directory=/path/to/gdb/data-directory.

[...]

(gdb) set arch mips
The target architecture is set to "mips".
(gdb) set endian little
The target is set to little endian.
(gdb) target remote localhost:3333
Remote debugging using localhost:3333
Ignoring packet error, continuing...
warning: unrecognized item "timeout" in "qSupported" response
Ignoring packet error, continuing...
Remote replied unexpectedly to 'vMustReplyEmpty': timeout
(gdb) disass *main
No symbol table is loaded.  Use the "file" command.
(gdb) c
The program is not being run.

Any ideas on what could be wrong? Looking at the call stack, the broken pipe error occurs in dbg_sys_send_packet when called from the beginning of dbg_main (dbg_send_signal_packet):

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[#0] 0x451c9b → WriteSocket(client_socket=0xc, buffer=0x7fffe7afe3c4, len=0x1)
[#1] 0x491bac → dbg_sys_putchar(ch=0x24)
[#2] 0x492e27 → dbg_send_packet(pkt_data=0x7fffe7afe4d0 "S00F", pkt_len=0x3)
[#3] 0x493a1a → dbg_send_signal_packet(buf=0x7fffe7afe4d0 "S00F", buf_len=0x258, signal=0x0)
[#4] 0x493b5f → dbg_main(state=0x7fffe7afe760)
[#5] 0x492771 → loop(args=0x0)
[#6] 0x7ffff71ca3e9 → start_thread()
[#7] 0x7ffff5358293 → clone()

[Xavi92]

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gdb-multiarch 
Exception caught while booting Guile.
Error in function "make_objcode_from_file":
bad header on object file: "\x7fELF\x02\x01\x01ÿ\x00\x00\x00\x00\x00\x00\x00\x00"
gdb-multiarch: warning: Could not complete Guile gdb module initialization from:
/usr/share/gdb/guile/gdb/boot.scm.
Limited Guile support is available.
Suggest passing --data-directory=/path/to/gdb/data-directory.

That bit worries me. I have followed the same steps you described and none of these error messages appeared at all. Are you sure all dependencies needed by gdb are satisfied?

[algebro]

They should be, I'm using https://aur.archlinux.org/packages/gdb-multiarch/ on Arch Linux. What version of gdb are you testing with? Sometimes arch has newer versions of packages that break things (for example, gcc10 forces pcsxr to be compiled with -fcommon because -fno-common became the default)

update: i recompiled gdb-multiarch with `--with-guile` instead of `--with-guile=2.0` and that solved the guile errors, but I'm still getting the broken pipe errors when connecting to localhost:3333.

another update: looks like compiling pcsxr with gcc 10.2 instead of clang solved the problem. I'll reach out if I have more questions--thanks!

[NDR008]

FYI... I'll throw this out there:
https://github.com/NDR008/VSCodePSX