Messages in this thread | | | Date | Tue, 17 May 2022 11:38:36 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG |
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On Tue, May 17, 2022 at 08:48:41AM +0000, David Laight wrote: > From: Peter Zijlstra > > Sent: 17 May 2022 09:41 > ... > > > If we use %eax instead of %r10d for the hash transfer (as per Joao), and > > > use int3 instead of ud2, then we can shrink the fineibt sequence to: > > > > > > __cfi_\func: > > > endbr # 4 > > > xorl $0x12345678, %eax # 5 > > > jz 1f # 2 > > > int3 # 1 > > > \func: > > > ... > > > > > > Which is 12 bytes, and needs a larger preamble (up from 9 in the current > > > proposal). > > > > On all that; perhaps it would be good to have a compiler option to > > specify the preamble size. It can enforce the minimum at 7 to have at > > least the required: > > > > movl $0x12345678, %eax > > int3 > > int3 > > > > but any larger number will just increase the preamble with int3 padding > > at the top. > > > > That can go right along with the option to supress endbr when preamble > > :-) > > You also need a compiler option to specify the register. > While (I think) %eax is usable in kernel, it isn't in userspace. > It is used in varargs calls to pass (IIRC) the number of fp > args that are passed in registers.
You're mistaken, the compiler doesn't emit the FineIBT code *at*all*. That's all patched in later. For kCFI the mov is never executed and is only there to make it a valid instruction.
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