linux-kernel - [PATCH 1/3] x86,vdso: Change the PER_CPU segment to use struct desc

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Message-Id: <066c1ca913eb96f6324e8558f321e6f5b010d0d3.1406336986.git.luto@amacapital.net>
Date:	Fri, 25 Jul 2014 18:12:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Andy Lutomirski <luto@...capital.net>
Subject: [PATCH 1/3] x86,vdso: Change the PER_CPU segment to use struct desc_struct

This makes it easier to see what's going on.  It produces exactly
the same segment descriptor as the old code.

Signed-off-by: Andy Lutomirski <luto@...capital.net>
---
 arch/x86/kernel/vsyscall_64.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index e1e1e80..158cdff 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -289,7 +289,7 @@ sigsegv:
  */
 static void vsyscall_set_cpu(int cpu)
 {
-	unsigned long d;
+	struct desc_struct d;
 	unsigned long node = 0;
 #ifdef CONFIG_NUMA
 	node = cpu_to_node(cpu);
@@ -298,13 +298,17 @@ static void vsyscall_set_cpu(int cpu)
 		write_rdtscp_aux((node << 12) | cpu);
 
 	/*
-	 * Store cpu number in limit so that it can be loaded quickly
-	 * in user space in vgetcpu. (12 bits for the CPU and 8 bits for the node)
+	 * Store cpu number in limit so that it can be loaded
+	 * quickly in user space in vgetcpu. (12 bits for the CPU
+	 * and 8 bits for the node)
 	 */
-	d = 0x0f40000000000ULL;
-	d |= cpu;
-	d |= (node & 0xf) << 12;
-	d |= (node >> 4) << 48;
+	memset(&d, 0, sizeof(d));
+	d.limit0 = cpu | ((node & 0xf) << 12);
+	d.limit = node >> 4;
+	d.type = 4;		/* RO data, expand down */
+	d.dpl = 3;		/* Visible to user code */
+	d.s = 1;		/* Non a system segment */
+	d.p = 1;		/* Present */
 
 	write_gdt_entry(get_cpu_gdt_table(cpu), GDT_ENTRY_PER_CPU, &d, DESCTYPE_S);
 }
-- 
1.9.3

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