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

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Message-ID: <tip-60a0e2039e3df6c0a2b896bd78af36ff36fb629c@git.kernel.org>
Date:	Wed, 13 Apr 2016 04:46:52 -0700
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	hpa@...or.com, pbonzini@...hat.com, bp@...e.de, mingo@...nel.org,
	Xen-devel@...ts.xen.org, kvm@...r.kernel.org,
	arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, tglx@...utronix.de,
	peterz@...radead.org, boris.ostrovsky@...cle.com,
	torvalds@...ux-foundation.org, luto@...nel.org, bp@...en8.de
Subject: [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@...nel.org>
AuthorDate: Mon, 4 Apr 2016 08:46:22 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
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@...nel.org>
Reviewed-by: Borislav Petkov <bp@...e.de>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: KVM list <kvm@...r.kernel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: xen-devel <Xen-devel@...ts.xen.org>
Link: http://lkml.kernel.org/r/5f1dcd6919f4a5923959a8065cb2c04d9dac1412.1459784772.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...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 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))
 		return;