Messages in this thread | | | Date | Tue, 18 Oct 2022 21:48:42 -0700 | From | Joao Moreira <> | Subject | Re: [PATCH] x86/ibt: Implement FineIBT |
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>> Is it useful to get the compiler to emit 0xcc with >> -fpatchable-function-entry under any circumstance? I can probably >> change >> that quickly if needed/useful. > > Having it emit 0xcc for the bytes in front of the symbol might be > interesting. It would mean a few kernel changes, but nothing too hard. > > That is, -fpatchable-function-entry=N,M gets us N-M bytes in at the > start of the symbol and M bytes in front of it. The N-M bytes at the > start of the function *are* executed and should obviously not become > 0xcc (GCC keeps them 0x90 while LLVM makes them large NOPs).
Uhum, all makes sense. I drafted something here:
https://github.com/lvwr/llvm-project/commits/joao/int3
Let me know if this works for you or if there is something that should be tweaked, like adding a specific flag and such. This currently emits 0xcc instead of 0x90 for the nops before the function entry symbol for kernel code on x86-64. It seems to be working (see generated snippet below), but let me know otherwise:
Generated with -fpatchable-function-entry=10,5
Disassembly of section .text:
0000000000000000 <save_processor_state-0x5>: 0: cc int3 1: cc int3 2: cc int3 3: cc int3 4: cc int3
0000000000000005 <save_processor_state>: 5: 0f 1f 44 00 08 nopl 0x8(%rax,%rax,1) a: 41 57 push %r15 c: 41 56 push %r14 ...
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