Introduce a write acessor for updating the current LDT. This is required for
hypervisors like Xen that do not allow LDT pages to be directly written.
Testing - here's a fun little LDT test that can be trivially modified to test
limits as well.
/*
* Copyright (c) 2005, Zachary Amsden ([EMAIL PROTECTED])
* This is licensed under the GPL.
*/
#include <stdio.h>
#include <signal.h>
#include <asm/ldt.h>
#include <asm/segment.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/mman.h>
#define __KERNEL__
#include <asm/page.h>
void main(void)
{
struct user_desc desc;
char *code;
unsigned long long tsc;
code = (char *)mmap(0, 8192, PROT_EXEC|PROT_READ|PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
desc.entry_number = 0;
desc.base_addr = code;
desc.limit = 1;
desc.seg_32bit = 1;
desc.contents = MODIFY_LDT_CONTENTS_CODE;
desc.read_exec_only = 0;
desc.limit_in_pages = 1;
desc.seg_not_present = 0;
desc.useable = 1;
if (modify_ldt(1, &desc, sizeof(desc)) != 0) {
perror("modify_ldt");
}
printf("code base is 0x%08x\n", (unsigned)code);
code[0x0ffe] = 0x0f; /* rdtsc */
code[0x0fff] = 0x31;
code[0x1000] = 0xcb; /* lret */
__asm__ __volatile("lcall $7,$0xffe" : "=A" (tsc));
printf("TSC is 0x%016llx\n", tsc);
}
Signed-off-by: Zachary Amsden <[EMAIL PROTECTED]>
Index: linux-2.6.13/arch/i386/kernel/ldt.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/ldt.c 2005-08-03 15:44:24.000000000
-0700
+++ linux-2.6.13/arch/i386/kernel/ldt.c 2005-08-03 15:48:53.000000000 -0700
@@ -177,7 +177,7 @@
static int write_ldt(void __user * ptr, unsigned long bytecount, int oldmode)
{
struct mm_struct * mm = current->mm;
- __u32 entry_1, entry_2, *lp;
+ __u32 entry_1, entry_2;
int error;
struct user_desc ldt_info;
@@ -205,8 +205,6 @@
goto out_unlock;
}
- lp = (__u32 *) ((ldt_info.entry_number << 3) + (char *)
mm->context.ldt);
-
/* Allow LDTs to be cleared by the user. */
if (ldt_info.base_addr == 0 && ldt_info.limit == 0) {
if (oldmode || LDT_empty(&ldt_info)) {
@@ -223,8 +221,7 @@
/* Install the new entry ... */
install:
- *lp = entry_1;
- *(lp+1) = entry_2;
+ write_ldt_entry(mm->context.ldt, ldt_info.entry_number, entry_1,
entry_2);
error = 0;
out_unlock:
Index: linux-2.6.13/include/asm-i386/desc.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/desc.h 2005-08-03 15:44:24.000000000
-0700
+++ linux-2.6.13/include/asm-i386/desc.h 2005-08-03 16:17:25.000000000
-0700
@@ -96,6 +96,13 @@
(info)->seg_not_present == 1 && \
(info)->useable == 0 )
+static inline void write_ldt_entry(void *ldt, int entry, __u32 entry_a, __u32
entry_b)
+{
+ __u32 *lp = (__u32 *)((char *)ldt + entry*8);
+ *lp = entry_a;
+ *(lp+1) = entry_b;
+}
+
#if TLS_SIZE != 24
# error update this code.
#endif
-
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