Re: [PATCHv3] x86/selftests: add clobbers for int80 on x86_64

From: Andy Lutomirski
Date: Mon Feb 13 2017 - 14:06:38 EST

On Mon, Feb 13, 2017 at 2:13 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote:
> Kernel erases R8..R11 registers prior returning to userspace
> from int80:
> GCC can reuse this registers and doesn't expect them to change
> during syscall invocation. I met this kind of bug in CRIU once
> gcc 6.1 and clang stored local variables in those registers
> and the kernel zerofied them during syscall:
> By that reason I suggest to add those registers to clobbers
> in selftests. Also, as notted by Andy - removed unneeded clobber
> for flags in int 0x80 inline asm.

Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>

Although these days we could preserve these regs in the kernel at the
cost of (I think) 4 bytes of text and zero runtime overhead.