Re: [PATCH 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

[Andy Lutomirski <luto@kernel.org>]

On Mon, Feb 5, 2018 at 7:48 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
>> On Mon, Feb 5, 2018 at 1:29 PM, Ingo Molnar <mingo@kernel.org> wrote:
>> >
>> > * Andy Lutomirski <luto@kernel.org> wrote:
>> >
>> >> [...] Clearing R10 is mostly useless in the syscall path because we'll just
>> >> unconditionally reload it in do_syscall_64().
>> >
>> > AFAICS do_syscall_64() doesn't touch R10 at all. So how does it reload R10?
>> >
>> > In fact do_syscall_64() as a C function does not touch R10, R11, R12, R13, R14,
>> > R15 - it passes their values through.
>> >
>> > What am I missing?
>>
>> The syscall ABI uses R10 for the 4th argument instead of RCX, because
>> RCX gets clobbered by the SYSCALL instruction for RIP.
>
> But we only reload the syscall-entry value of R10 it into RCX (4th C function
> argument):
>
>                 regs->ax = sys_call_table[nr](
>                         regs->di, regs->si, regs->dx,
>                         regs->r10, regs->r8, regs->r9);
>
> while RCX is a clobbered register, so in practice, while it will be briefly
> present in do_syscall_64() and the high level syscall functions, the value in RCX
> will be cleared from RCX in the overwhelming majority of cases.
>
> But the real R10 will survive much longer, because it's only used in a very small
> minority of the C functions!

Yes, indeed, brain fart on my part.