The 9 byte nop is a suffix of the 10 byte nop, and we need at most 6
bytes.
ntdll's version of strcpy is written in assembly and is very clever.
strcat tail calls strcpy but with a slightly different arrangement of
argument registers at an alternate entry point. It looks like this:
ntdll!strcpy:
00007ffd`64e8a7a0 4c8bd9 mov r11,rcx
ntdll!__entry_from_strcat_in_strcpy:
00007ffd`64e8a7a3 482bca sub rcx,rdx
00007ffd`64e8a7a6 f6c207 test dl,7
If we overwrite more than two bytes in our interceptor, that label will
no longer be a valid instruction boundary.
By recognizing the 9 byte nop, we use the two byte backwards branch to
start our trampoline, avoiding this issue.
Fixes https://github.com/google/sanitizers/issues/829
Patch by David Major
llvm-svn: 310419
c3fc3bc5af