Re: [PATCH 8/8] x86: Use rd/wr fs/gs base in arch_prctl

[Andy Lutomirski]

On Fri, Apr 10, 2015 at 8:50 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> Convert arch_prctl to use the new instructions to
> change fs/gs if available, instead of using MSRs.
>
> This is merely a small performance optimization,
> no new functionality.
>
> With the new instructions the syscall is really obsolete,
> as everything can be set directly in ring 3. But the syscall
> is widely used by existing software, so we still support it.

It's also necessary on my poor obsolete Sandy Bridge machines :)

>
> The syscall still enforces that the addresses are not
> in kernel space, even though that is not needed more.
> This is mainly so that the programs written for new CPUs
> do not suddenly fail on old CPUs.
>
> With the new instructions available it prefers to use
> them in the context switch, instead of using the old
> "use GDT segment rewrite" trick.
>
> v2: Make kprobes safe
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/process_64.c | 48 ++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 40 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 3019c51..1fe4d79 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -534,20 +534,38 @@ unsigned long get_wchan(struct task_struct *p)
>         return 0;
>  }
>
> +static noinline __kprobes void reload_user_gs(unsigned long addr)
> +{
> +       local_irq_disable();
> +       swapgs();
> +       loadsegment(gs, 0);
> +       wrgsbase(addr);
> +       swapgs();
> +       local_irq_enable();
> +}

These names are terrifying.  How about
write_user_gsbase_and_set_gs_to_zero?  (Yes, it's a mouthful.)  Or
write_user_gs_and_gsbase(unsigned short gs, unsigned long gsbase),
which we'll need if we make __switch_to fully correct.

Also, at some point we should fix the fsindex vs fs crap in
thread_struct.  They should be fsindex and fsbase.

--Andy
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