2.2.2.4. Your first Binutils hack
Have you ever felt that a single inc instruction was not enough? Really? Me too!
So let’s hack the GNU GAS assembler, which is part of GNU Binutils, to add a new shiny version of inc called… myinc!
GCC uses GNU GAS as its backend, so we will test out new mnemonic with an GCC inline assembly test program: userland/arch/x86_64/binutils_hack.c, which is just a copy of userland/arch/x86_64/binutils_nohack.c but with myinc instead of inc.
The inline assembly is disabled with an #ifdef, so first modify the source to enable that.
Then, try to build userland:
./build-userland
and watch it fail with:
binutils_hack.c:8: Error: no such instruction: `myinc %rax'
Now, edit the file
vim submodules/binutils-gdb/opcodes/i386-tbl.h
and add a copy of the "inc" instruction just next to it, but with the new name "myinc":
diff --git a/opcodes/i386-tbl.h b/opcodes/i386-tbl.h
index af583ce578..3cc341f303 100644
--- a/opcodes/i386-tbl.h
+++ b/opcodes/i386-tbl.h
@@ -1502,6 +1502,19 @@ const insn_template i386_optab[] =
{ { { 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0,
1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0 } } } },
+ { "myinc", 1, 0xfe, 0x0, 1,
+ { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } },
+ { 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 1, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0 },
+ { { { 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0,
+ 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0 } } } },
{ "sub", 2, 0x28, None, 1,
{ { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
Finally, rebuild Binutils, userland and test our program with User mode simulation:
./build-buildroot -- host-binutils-rebuild ./build-userland --static ./run --static --userland userland/arch/x86_64/binutils_hack.c
and we se that myinc worked since the assert did not fail!
Tested on b60784d59bee993bf0de5cde6c6380dd69420dda + 1.