linux-kernel - [PATCH] x86/extable: Add a comment about early exception handlers

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Date:	Mon,  4 Apr 2016 08:46:22 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	X86 ML <x86@...nel.org>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KVM list <kvm@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] 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@...nel.org>
---
 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 98b5f45d9d79..36fe03bc81ee 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -132,6 +132,20 @@ 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;
 
-- 
2.5.5