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

From: Dmitry Safonov
Date: Fri Feb 10 2017 - 11:51:11 EST

On 02/10/2017 07:13 PM, Andy Lutomirski wrote:
On Fri, Feb 10, 2017 at 3:52 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.

Seems reasonable, but presumably INT80_CLOBBERS should be defined the
same way in all the tests. IOW, if the "flags" clobber is actually
needed, it should be "flags", INT80_CLOBBERS (possibly without the
comma if it's problematic).

Well, that was my initial attempt: I've defined it as:
+# define INT80_CLOBBERS , "r8", "r9", "r10", "r11"

But that hanging comma looks awful, so I added "flags" there.
And if I do define it without coma and leave it in asm statement,
32-bit version would be unhappy.
So, I found that it's easier to define it with flags included.