linux-kernel - Re: [PATCH v2 08/10] x86-64: Emulate vsyscalls

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Date:	Mon, 30 May 2011 08:51:09 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Ingo Molnar" <mingo@...e.hu>, "Andy Lutomirski" <luto@....EDU>
Cc:	"Borislav Petkov" <bp@...en8.de>, "Jesper Juhl" <jj@...osbits.net>,
	"richard -rw- weinberger" <richard.weinberger@...il.com>,
	"Arjan van de Ven" <arjan@...radead.org>,
	"Mikael Pettersson" <mikpe@...uu.se>, <x86@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 08/10] x86-64: Emulate vsyscalls

>>> On 30.05.11 at 05:48, Andy Lutomirski <luto@....EDU> wrote:
> This causes vsyscalls to be a little more expensive than real
> syscalls.  Fortunately sensible programs don't use them.

Hmm - weren't vsyscalls there for performance reasons?

Besides that, just a mostly cosmetic remark:

> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1650,6 +1650,23 @@ config COMPAT_VDSO
>  
>  	  If unsure, say Y.
>  
> +config UNSAFE_VSYSCALLS
> +	def_bool y
> +	prompt "Unsafe fast legacy vsyscalls"
> +	depends on X86_64
> +	---help---
> +	  Legacy user code expects to be able to issue three syscalls
> +	  by calling fixed addresses in kernel space.  If you say N,
> +	  then the kernel traps and emulates these calls.  If you say
> +	  Y, then there is actual executable code at a fixed address
> +	  to implement these calls efficiently.
> +
> +	  On a system with recent enough glibc (probably 2.14 or
> +	  newer) and no static binaries, you can say N without a
> +	  performance penalty to improve security
> +
> +	  If unsure, say Y.
> +
>  config CMDLINE_BOOL
>  	bool "Built-in kernel command line"
>  	---help---
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -42,6 +42,9 @@ obj-$(CONFIG_X86_32)	+= probe_roms_32.o
>  obj-$(CONFIG_X86_32)	+= sys_i386_32.o i386_ksyms_32.o
>  obj-$(CONFIG_X86_64)	+= sys_x86_64.o x8664_ksyms_64.o
>  obj-$(CONFIG_X86_64)	+= syscall_64.o vsyscall_64.o vread_tsc_64.o
> +ifndef CONFIG_UNSAFE_VSYSCALLS
> +	obj-$(CONFIG_X86_64)	+= vsyscall_emu_64.o
> +endif

With the Kconfig dependency on X86_64 above and the new
variable being a boolean one, these three lines can be written
as just

obj-$(CONFIG_UNSAFE_VSYSCALLS) += vsyscall_emu_64.o

Jan

>  obj-y			+= bootflag.o e820.o
>  obj-y			+= pci-dma.o quirks.o topology.o kdebugfs.o
>  obj-y			+= alternative.o i8253.o pci-nommu.o hw_breakpoint.o


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