RE: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

[David Laight <David.Laight@ACULAB.COM>]

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.

(I can't remember which registers userspace has reserved for
the PLT code? - That might include r10??)

	David

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