Reverse Debugging

Radare2 has reverse debugger, that can seek the program counter backward. (e.g. reverse-next, reverse-continue in gdb) Firstly you need to save program state at the point that you want to start recording. The syntax for recording is:

[0x004028a0]> dts+

You can use dts commands for recording and managing program states. After recording the states, you can seek pc back and forth to any points after saved address. So after recording, you can try single step back:

[0x004028a0]> 2dso
[0x004028a0]> dr rip
0x004028ae
[0x004028a0]> dsb
continue until 0x004028a2
hit breakpoint at: 4028a2
[0x004028a0]> dr rip
0x004028a2

When you run dsb, reverse debugger restore previous recorded state and execute program from it until desired point.

Or you can also try continue back:

[0x004028a0]> db 0x004028a2
[0x004028a0]> 10dso
[0x004028a0]> dr rip
0x004028b9
[0x004028a0]> dcb
[0x004028a0]> dr rip
0x004028a2

dcb seeks program counter until hit the latest breakpoint. So once set a breakpoint, you can back to it any time.

You can see current recorded program states using dts:

[0x004028a0]> dts
session: 0   at:0x004028a0   ""
session: 1   at:0x004028c2   ""

NOTE: Program records can be saved at any moments. These are diff style format that save only different memory area from previous. It saves memory space rather than entire dump.

And also can add comment:

[0x004028c2]> dtsC 0 program start
[0x004028c2]> dtsC 1 decryption start
[0x004028c2]> dts
session: 0   at:0x004028a0   "program start"
session: 1   at:0x004028c2   "decryption start"

You can leave notes for each records to keep in your mind. dsb and dcb commands restore the program state from latest record if there are many records.

Program records can exported to file and of course import it. Export/Import records to/from file:

[0x004028c2]> dtst records_for_test
Session saved in records_for_test.session and dump in records_for_test.dump
[0x004028c2]> dtsf records_for_test
session: 0, 0x4028a0 diffs: 0
session: 1, 0x4028c2 diffs: 0

Moreover, you can do reverse debugging in ESIL mode. In ESIL mode, program state can be managed by aets commands.

[0x00404870]> aets+

And step back by aesb:

[0x00404870]> aer rip
0x00404870
[0x00404870]> 5aeso
[0x00404870]> aer rip
0x0040487d
[0x00404870]> aesb
[0x00404870]> aer rip
0x00404879

In addition to the native reverse debugging capabilities in radare2, it's also possible to use gdb's remote protocol to reverse debug a target gdbserver that supports it. =!dsb and =!dcb are available as dsb and dcb replacementments for this purpose, see remote gdb's documentation for more information.

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