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    SubjectRe: [PATCH v2 08/10] x86-64: Emulate vsyscalls
    >>> On 30.05.11 at 05:48, Andy Lutomirski <luto@MIT.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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