Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 28 Oct 2014 11:09:53 -0700 | Subject | Re: [PATCH 3/3] x86_64,vsyscall: Make vsyscall emulation configurable |
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On Tue, Oct 28, 2014 at 10:57 AM, Josh Triplett <josh@joshtriplett.org> wrote: > On Tue, Oct 28, 2014 at 10:22:28AM -0700, Andy Lutomirski wrote: >> This adds CONFIG_X86_VSYSCALL_EMULATION, guarded by CONFIG_EXPERT. >> Turning it off completely disables vsyscall emulation, saving ~3.5k >> for vsyscall_64.c, 4k for vsyscall_emu_64.S (the fake vsyscall >> page), some tiny amount of core mm code that supports a gate area, >> and possibly 4k for a wasted pagetable. The latter is because the >> vsyscall addresses are misaligned and fit poorly in the fixmap. >> >> Signed-off-by: Andy Lutomirski <luto@amacapital.net> > > One minor nit below, but with or without that change, > Reviewed-by: Josh Triplett <josh@joshtriplett.org> > >> --- a/arch/x86/xen/mmu.c >> +++ b/arch/x86/xen/mmu.c >> @@ -1456,11 +1456,13 @@ static int xen_pgd_alloc(struct mm_struct *mm) >> user_pgd = (pgd_t *)__get_free_page(GFP_KERNEL | __GFP_ZERO); >> page->private = (unsigned long)user_pgd; >> >> +#ifdef CONFIG_X86_VSYSCALL_EMULATION >> if (user_pgd != NULL) { >> user_pgd[pgd_index(VSYSCALL_ADDR)] = >> __pgd(__pa(level3_user_vsyscall) | _PAGE_TABLE); >> ret = 0; >> } >> +#endif > > Could you instead make the if use IS_ENABLED? > > if (IS_ENABLED(CONFIG_X86_VSYSCALL_EMULATION) && user_pgd != NULL) > > That has the advantage of ensuring that the code continues to compile. > (Given that you haven't removed level3_user_vsyscall, that should work.)
I need the ret = 0, I think, so I'll resend.
I think I'd rather use #ifdef here, since I think it would be great if the Xen people could clean this up further. With this change, under some configurations, there should be no user-accessible kernel addresses at all. (Also, is there some PV mechanism that I'm not thinking of that will break with this change? I know I've tripped over Xen pagetable and fixmap oddities before.)
--Andy
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