"Command 'context' failed to execute properly" on branching instructions while remote debugging MIPS

[amlamarra]

• Did you use the latest version of GEF from dev branch?
• Is your bug specific to GEF (not GDB)? - Try to reproduce it running gdb -nx
• Did you read the documentation first?
• Did you check issues (including
  the closed ones) - and the PR?

Step 1: Describe your environment

• Operating System / Distribution: Ubuntu 20.04.3 LTS
• Architecture: x86_64
• GEF version (including the Python library version) run version in GEF.

gef➤  version
GEF: (Standalone)
Blob Hash(/home/andrew/.gdbinit-gef.py): 2ad6e6e9ca1f68f9257b00baea569ae740dc85e0
SHA256(/home/andrew/.gdbinit-gef.py): de35aedca9670a0401e5072c9a63d711e5295fa7b730efbac872dac18f24b859
GDB: 9.2
GDB-Python: 3.8

Step 2: Describe your problem

Steps to reproduce

1. Run gdbserver on a remote MIPS machine (I only tested with mipseb, not mipsel) and either attach to a running process or start the program.
2. Run gdb and gef-remote to the target.
3. While stepping through the program, any branching instructions (not including jumps) end up looking like this:
   
4. I can still get the instructions manually:

gef➤  x/2i $pc
=> 0x77fe6288:	bnez	v1,0x77fe6284
   0x77fe628c:	addiu	v0,v0,4

Minimalist test case

// compile with gcc -fPIE -pic -o my_issue.out my_issue.c
int main(){ return 0; }

N/A

I can provide a mipseb binary if needed.

Observed Results

• What happened? This could be a description, log output, etc.

Expected results

• What did you expect to happen?

Traces

Here's a stacktrace:

   0x77fe627c                  sw     gp, 24(sp)
   0x77fe6280                  move   v0, s0
●  0x77fe6284                  lw     v1, 0(v0)

─────────────────────────────── Exception raised ───────────────────────────────
AssertionError: 
───────────────────────────── Detailed stacktrace ──────────────────────────────
↳ File "~/.gdbinit-gef.py", line 3047, in __get_register_for_selected_frame()
    →             assert regname[0] == "$"

↳ File "~/.gdbinit-gef.py", line 3039, in get_register()
    →         return __get_register_for_selected_frame(regname, key)

↳ File "~/.gdbinit-gef.py", line 2707, in is_branch_taken()
    →                 taken, reason = get_register(ops[0]) != 0, "{0[0]} != 0".format(ops)

↳ File "~/.gdbinit-gef.py", line 8541, in context_code()
    →                             is_taken, reason = current_arch.is_branch_taken(insn)

↳ File "~/.gdbinit-gef.py", line 8372, in do_invoke()
    →                     display_pane_function()

↳ File "~/.gdbinit-gef.py", line 2854, in wrapper()
    →                 return f(*args, **kwargs)

↳ File "~/.gdbinit-gef.py", line 244, in wrapper()
    →                 rv = f(*args, **kwargs)

↳ File "~/.gdbinit-gef.py", line 4619, in invoke()
    →                 bufferize(self.do_invoke)(argv)

─────────────────────────────────── Version ────────────────────────────────────
GEF: (Standalone)
Blob Hash(/home/andrew/.gdbinit-gef.py): 2ad6e6e9ca1f68f9257b00baea569ae740dc85e0
SHA256(/home/andrew/.gdbinit-gef.py): de35aedca9670a0401e5072c9a63d711e5295fa7b730efbac872dac18f24b859
GDB: 9.2
GDB-Python: 3.8
Loaded commands: $, aliases, aliases add, aliases ls, aliases rm, aslr, assemble, canary, capstone-disassemble, checksec, context, dereference, edit-flags, elf-info, entry-break, format-string-helper, functions, gef-remote, got, heap, heap arenas, heap bins, heap bins fast, heap bins large, heap bins small, heap bins tcache, heap bins unsorted, heap chunk, heap chunks, heap set-arena, heap-analysis-helper, hexdump, hexdump byte, hexdump dword, hexdump qword, hexdump word, highlight, highlight add, highlight clear, highlight list, highlight remove, hijack-fd, ida-interact, is-syscall, ksymaddr, memory, memory list, memory reset, memory unwatch, memory watch, name-break, nop, patch, patch byte, patch dword, patch qword, patch string, patch word, pattern, pattern create, pattern search, pcustom, pcustom edit, pcustom list, pcustom show, pie, pie attach, pie breakpoint, pie delete, pie info, pie remote, pie run, print-format, process-search, process-status, registers, reset-cache, ropper, scan, search-pattern, set-permission, shellcode, shellcode get, shellcode search, stub, syscall-args, theme, trace-run, unicorn-emulate, version, vmmap, xfiles, xinfo, xor-memory, xor-memory display, xor-memory patch
───────────────────────────── Last 10 GDB commands ─────────────────────────────
  253  gef-remote 192.168.1.10:1234
  254  b main
  255  ni
  256  x/2i $pc
  257  file ~/routers/K3C/root/bin/sh
  258  gef-remote 192.168.1.10:1234
  259  b *0x77fe6284
  260  gef config gef.debug 1
  261  c
  262  ni
───────────────────────────── Runtime environment ──────────────────────────────
* GDB: 9.2
* Python: 3.8.10 - final
* OS: Linux - 5.4.0-90-generic (x86_64)
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 20.04.3 LTS
Release:	20.04
Codename:	focal
────────────────────────────────────────────────────────────────────────────────

[hugsy]

I can provide a mipseb binary if needed.

Please do, upload it here

[amlamarra]

Here's one.
busybox.zip

[hugsy]

Thanks for the report. I confirm and can reproduce it, it's a strange bug. We're examining a proper fix but in the meantime you can quick patch locally by replacing the assert here by a simple if:

        if regname[0] == "$":
            regname = regname[1:]

[amlamarra]

Sure thing. I'm visiting family for the holiday, but I'll try it when I get back.