Re: [PATCH 6/9] x86: Use rd/wr fs/gs base in arch_prctl
From: Andy Lutomirski
Date: Mon Mar 21 2016 - 14:18:18 EST
On Mon, Mar 21, 2016 at 9:16 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.
> 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.
> v2: Make kprobes safe
> v3: Rename things.
> 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 53fa839..5f40517 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -530,20 +530,38 @@ void set_personality_ia32(bool x32)
> +static noinline __kprobes void reload_user_gs(unsigned long addr)
> + local_irq_disable();
> + swapgs();
> + loadsegment(gs, 0);
> + wrgsbase(addr);
> + swapgs();
> + local_irq_enable();
The actual operation this does is to set the selector to zero and the
base to the specified value. Can you give it a name that makes it
clear (e.g. zero_user_gs_and_set_base)?
I'm also wondering whether it would make sense to move the cpu_has
into these helpers rather than putting it in the callers.