This C/asm project implements a register+stack sandbox of sorts, so that arbitrary snippets of machine code can be run and amended.
The program takes one commandline argument, the file from which it reads bytes, it can be a regular file, fifo, socket, whatever.
The bytes are assembled into simple packets: The first byte is the length of the data in the packet, and then follows that amount of bytes of data.
Maximum packet length is the maximum of a byte, 0xFF, however, a second packet may follow which will continue execution.
Sending the byte 0x00 as a length terminates the program, freeing mmapped memory.
There is no guarantee that the code will remain in the same place between packets, use only position independent machine code to be completely safe.
Code is appended into a growing mmapped region, you may work with code from previous packets (loops larger than 0xFF bytes, etc.)
The stack has a fixed maximum size of 2048 bytes and resides in non-dynamic memory.
The program itself is probably position-dependent, because I haven't writen the asm to be position-independent. But there is a chance it could run as is when compiled as pic/pie, feel free to test.
You can't define local variables or constants, but a workaround is shown in pkt/datastore.pkt.
Examples:
cat pkt/datastore.pkt pkt/stop.pkt | ./execasm /dev/fd/0cmake .
makeecho -n 001234ff | ./unhex is a simple (perl) oneliner that translates hex pairs into bytes.
Defuse.ca Online x86 assembler is a very useful tool for these things. (not affiliated)
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