linux-kernel - Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

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Date:   Fri, 5 Nov 2021 21:26:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        mark.rutland@....com, dvyukov@...gle.com, pbonzini@...hat.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

On Fri, Nov 05, 2021 at 09:15:57PM +0100, Peter Zijlstra wrote:

> That's really unfortunate that is... 6.4.4.1 item 5 of the C99 spec
> covers this gem :-( I suppose I'll go stick that (int) cast in the
> EX_IMM_MASK definition or something.

--- a/arch/x86/include/asm/extable_fixup_types.h
+++ b/arch/x86/include/asm/extable_fixup_types.h
@@ -2,10 +2,15 @@
 #ifndef _ASM_X86_EXTABLE_FIXUP_TYPES_H
 #define _ASM_X86_EXTABLE_FIXUP_TYPES_H
 
+/*
+ * Our IMM is signed, as such it must live at the top end of the word. Also,
+ * since C99 hex constants are of ambigious type, force cast the mask to 'int'
+ * so that FIELD_GET() will DTRT and sign extend the value when it extracts it.
+ */
 #define EX_DATA_TYPE_MASK		0x000000FF
 #define EX_DATA_REG_MASK		0x00000F00
 #define EX_DATA_FLAG_MASK		0x0000F000
-#define EX_DATA_IMM_MASK		0xFFFF0000
+#define EX_DATA_IMM_MASK		((int)0xFFFF0000)
 
 #define EX_DATA_REG_SHIFT		8
 #define EX_DATA_FLAG_SHIFT		12