Support Remote File I/O

[daniel5151]

See https://sourceware.org/gdb/onlinedocs/gdb/File_002dI_002fO-Overview.html#File_002dI_002fO-Overview

The File I/O remote protocol extension (short: File-I/O) allows the target to use the host’s file system and console I/O to perform various system calls. System calls on the target system are translated into a remote protocol packet to the host system, which then performs the needed actions and returns a response packet to the target system. This simulates file system operations even on targets that lack file systems.

~Additionally, it would also be nice to implement Host I/O Packets at the same time (See https://sourceware.org/gdb/onlinedocs/gdb/Host-I_002fO-Packets.html#Host-I_002fO-Packets)~

This has been implemented in #66. See update below...

The Host I/O packets allow GDB to perform I/O operations on the far side of a remote link. For example, Host I/O is used to upload and download files to a remote target with its own filesystem. Host I/O uses the same constant values and data structure layout as the target-initiated File-I/O protocol.

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Skimming through the spec, it looks like implementing these extensions can be done in a totally backwards-compatible manner, simply by adding some new extension traits to Target, and shouldn't require any breaking API changes.

There's also some prior art from https://github.com/luser/rust-gdb-remote-protocol/pull/60, which could be a good source of inspiration.

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Update 8/20/2021: #66 has been merged, adding support for Host I/O packets!

In hindsight, this issue should probably have been split into two separate ones, as while there is some overlap between File I/O and Host I/O, the two are actually very different features.

[luser]

There's also some prior art from luser/rust-gdb-remote-protocol#60, which could be a good source of inspiration.

I hadn't seen this project before but it looks very cool! My crate has never gotten to a true working state, so if you have a more full-featured implementation feel free to take anything you think is useful from mine! My original goal was to see if I could write a tool that could be run against minidump files so developers could use gdb/lldb to debug minidumps from Firefox crash reports the same way that Microsoft's debuggers natively support postmortem debugging of minidumps. Given that I left Mozilla a while ago I'm not terribly invested in that anymore, although I still think it would be useful. I did cheat a bit and nerd-snipe @tromey (an actual GDB developer) into contributing, though. :)

[daniel5151]

Hey @luser, thanks for the heads up! Glad to know that when the time comes, gdbstub can ~crib your code with impunity~ reuse some of your excellent work :wink:

Yeah, I started working on gdbstub while working on some emulators and realizing that single-stepping through raw assembly wasn't going to cut it. Over time, gdbstub's scope has expanded past that initial emulation use case, and I'm hoping to support all the major GDB RSP operating modes (I've even got a rough 1.0.0 roadmap, though that milestone is certainly still a ways away). While I don't have as much time to work on the project as I used to, I still try to find some time to hack away at gdbstub, polishing up an API here, adding a new protocol extension there, etc... Plus, it looks like some other folks both big and small have started depending on gdbstub, so it seems right to keep working on the project from time to time :)

[Sladuca]

Hi! I recently did a very brief internship at Solana where I was trying see how to attach GDB to a eBPF VM, which Solana uses as an execution environment for smart contracts. I was able to use this crate (it's awesome btw) to do basic debugging operations without any DWARF symbols, but I ended up running into a lot of linker issues surrounding relocations that don't entirely make sense in the context of the manner in which we are using eBPF, which isn't really its intended use case (see the linked issue if you're curious).

Anyhow, if it's alright with you, I'm interested in attempting to implement Host I/O and File I/O packets as our VM has a dynamic loader that does perform relocations and it would make things much simpler if GDB could simply read executables after they have been loaded by the VM.

I should mention that I'm still rather new to open source contribution, so if I have bad etiquette or something please feel free to criticize me so that I can improve.

[daniel5151]

@Sladuca Go for it! Any and all contributions are always welcome.

If you ever need any guidance on how to structure the feature and/or how to tackle certain problems, feel free to open a draft PR with the work you've got. Alternatively, if you're more of a rapid-iteration kind of person, you can also reach me on Discord at daniel5151#4520.

And remember - gdbstub is a no_std library, so make sure that your code works without the std feature enabled!

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Unrelated to the specific issue of of Remote File I/O:

• It looks like you ended up writing your own eBPF arch definition as part of your integration. I'm always looking to include more built-in arch definitions, so if you're happy with the implementation, do consider upstreaming it!

• ... I ended up running into a lot of linker issues surrounding relocations ...

  The first thing that sprang to mind when I read through that issue is that this could be related to https://github.com/daniel5151/gdbstub/issues/20. Before diving into the Remote File I/O rabbit hole, you might want to explore how the GDB Remote Serial Protocol handles relocatable binaries, and see if implementing those bits of the protocol might fix your specific problem.