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delay slot note

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xenia 2020-07-26 03:18:08 -04:00
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2020/3kctf/babym1ps/README.md
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@ -80,7 +80,11 @@ stored in a register `ra` rather than directly on the stack. so instead of most
epilogue being able to work as a ROP gadget, only epilogues that pop `ra` from the stack and then
return are applicable. there are also some gadgets involving the temp register `t9` - which is used
by MIPS compilers to load certain library function calls from `gp` or other registers. so it's
really a mix of both return- and call-oriented programming.
really a mix of both return- and call-oriented programming. another important thing about MIPS is
that each branch/jump has a _delay slot_, the instruction directly after the branch gets executed
before the branch/jump gets taken, and also if it's not taken. the delay slots are prefixed in
ghidra with `_`. this means useful gadget operations can actually come after the corresponding
`jalr`, for example.
it turns out pwntools is fairly useless for MIPS ROP, and i also tried a port of some IDA scripts to
ghidra <https://github.com/tacnetsol/ghidra_scripts> but these didn't really turn up good results,