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Date: Mon, 29 Jan 2018 15:23:39 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Will Deacon' <>, Andy Lutomirski <>
CC: Linus Torvalds <>, Al Viro
	<>, Alan Cox <>, "the arch/x86
 maintainers" <>, LKML <>, "Greg
 Kroah-Hartman" <>, Jann Horn <>,
	Samuel Neves <>, Dan Williams
	<>, Kernel Hardening
	<>, Borislav Petkov <>
Subject: RE: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

From: Will Deacon
> Sent: 29 January 2018 13:20
> Another issue with this style of macro definition exists on architectures
> where the calling convention needs you to carry state around depending on
> how you packed the previous parameters. For example, on 32-bit ARM, 64-bit
> values are passed in adjacent pairs of registers but the low numbered
> register needs to be even. This is what stopped me from trying to use
> existing helpers such as syscall_get_arguments to unpack the pt_regs
> and it generally means that anything that says "get me argument n" is going
> to require constructing arguments 0..n-1 first.
> To do this properly I think we'll either need to pass back the size and
> current register offset to the arch code, or just allow the thing to be
> overridden per syscall (the case above isn't especially frequent).

If you generate a structure from the argument list that might work
'by magic'.
Certainly you can add explicit pads to any structure.



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