linux-kernel - [PATCH v4 02/10] x86: tss: Eliminate fragile calculation of TSS segment limit

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Date:	Sun, 2 Nov 2014 09:32:07 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, x86@...nel.org,
	xen-devel@...ts.xenproject.org
Subject: [PATCH v4 02/10] x86: tss: Eliminate fragile calculation of TSS
 segment limit

__set_tss_desc has a complex calculation of the TSS segment limit,
duplicating the quirky details of the I/O bitmap array length, and
requiring a complex comment to explain.  Replace that calculation with a
simpler one based on the offsetof the "stack" field that follows the
array.

That then removes the last use of IO_BITMAP_OFFSET, so delete it.

Signed-off-by: Josh Triplett <josh@...htriplett.org>
Acked-by: Alexander van Heukelum <heukelum@...tmail.fm>
---
 arch/x86/include/asm/desc.h      | 11 +----------
 arch/x86/include/asm/processor.h |  4 +++-
 2 files changed, 4 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 50d033a..f8dc455 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -177,16 +177,7 @@ static inline void __set_tss_desc(unsigned cpu, unsigned int entry, void *addr)
 	struct desc_struct *d = get_cpu_gdt_table(cpu);
 	tss_desc tss;
 
-	/*
-	 * sizeof(unsigned long) coming from an extra "long" at the end
-	 * of the iobitmap. See tss_struct definition in processor.h
-	 *
-	 * -1? seg base+limit should be pointing to the address of the
-	 * last valid byte
-	 */
-	set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS,
-			      IO_BITMAP_OFFSET + IO_BITMAP_BYTES +
-			      sizeof(unsigned long) - 1);
+	set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS, TSS_LIMIT);
 	write_gdt_entry(d, entry, &tss, DESC_TSS);
 }
 
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index eb71ec7..76751cd 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -258,9 +258,11 @@ struct x86_hw_tss {
 #define IO_BITMAP_BITS			65536
 #define IO_BITMAP_BYTES			(IO_BITMAP_BITS/8)
 #define IO_BITMAP_LONGS			(IO_BITMAP_BYTES/sizeof(long))
-#define IO_BITMAP_OFFSET		offsetof(struct tss_struct, io_bitmap)
 #define INVALID_IO_BITMAP_OFFSET	0x8000
 
+/* Segment limits point to the last valid byte, hence the -1 */
+#define TSS_LIMIT	(offsetof(struct tss_struct, stack) - 1)
+
 struct tss_struct {
 	/*
 	 * The hardware state:
-- 
2.1.1

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