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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Peter Zijlstra' <peterz@infradead.org>,
        Sami Tolvanen <samitolvanen@google.com>
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Subject: RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation
Thread-Topic: [patch 00/38] x86/retbleed: Call depth tracking mitigation
Thread-Index: AQHYnH2Hi03gNC/QIkePUsvbFv4MOq2Ie8qw
Date:   Thu, 21 Jul 2022 08:21:52 +0000
Message-ID: <8177b804cfcb461d9f1f40d242cd84db@AcuMS.aculab.com>
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From: Peter Zijlstra
> Sent: 20 July 2022 22:13
...
> The prettiest option to obscure the immediate at the callsite I could
> conjure up is something like:
> 
> kcfi_caller_linus:
> 	movl	$0x12345600, %r10d
> 	movb	$0x78, %r10b
> 	cmpl	%r10d, -OFFSET(%r11)
> 	je	1f
> 	ud2
> 1:	call	__x86_thunk_indirect_r11
> 
> Which comes to around 22 bytes (+5 over the original).

You'd be better doing:
	movl $0x12345678-0xaa, %r10d
	addl $0xaa, %r10d
so that the immediate is obscured even if the low bits are zero.

	David

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