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 <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: KVM list <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel <Xen-devel@lists.xen.org>
Link: http://lkml.kernel.org/r/5f1dcd6919f4a5923959a8065cb2c04d9dac1412.1459784772.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index fd9eb98c4f5811f5710e5627f8f148c556800d22..aaeda3ffaafef445a13111afb67aaa20d2bc2037 100644 (file)
--- 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;