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[Xen-devel] [tip:x86/asm] x86/extable: Add a comment about early exception handlers

Commit-ID:  60a0e2039e3df6c0a2b896bd78af36ff36fb629c
Gitweb:     http://git.kernel.org/tip/60a0e2039e3df6c0a2b896bd78af36ff36fb629c
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Mon, 4 Apr 2016 08:46:22 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 13 Apr 2016 11:37:47 +0200

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@xxxxxxxxxx>
Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: KVM list <kvm@xxxxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: xen-devel <Xen-devel@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
 arch/x86/mm/extable.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index fd9eb98..aaeda3f 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))

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