From 60a0e2039e3df6c0a2b896bd78af36ff36fb629c Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 4 Apr 2016 08:46:22 -0700 Subject: x86/extable: Add a comment about early exception handlers Borislav asked for a comment explaining why all exception handlers are allowed early. Signed-off-by: Andy Lutomirski Reviewed-by: Borislav Petkov Acked-by: Linus Torvalds Cc: Andrew Morton Cc: Arjan van de Ven Cc: Boris Ostrovsky Cc: Borislav Petkov Cc: KVM list Cc: Paolo Bonzini Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: xen-devel Link: http://lkml.kernel.org/r/5f1dcd6919f4a5923959a8065cb2c04d9dac1412.1459784772.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/mm/extable.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'arch/x86') diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c index fd9eb98c4f58..aaeda3ffaafe 100644 --- a/arch/x86/mm/extable.c +++ b/arch/x86/mm/extable.c @@ -125,6 +125,20 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr) if (regs->cs != __KERNEL_CS) goto fail; + /* + * The full exception fixup machinery is available as soon as + * the early IDT is loaded. This means that it is the + * responsibility of extable users to either function correctly + * when handlers are invoked early or to simply avoid causing + * exceptions before they're ready to handle them. + * + * This is better than filtering which handlers can be used, + * because refusing to call a handler here is guaranteed to + * result in a hard-to-debug panic. + * + * Keep in mind that not all vectors actually get here. Early + * fage faults, for example, are special. + */ if (fixup_exception(regs, trapnr)) return; -- cgit v1.2.3