Commit f2f30ebc authored Sep 03, 2005 by Zachary Amsden Committed by Linus Torvalds Sep 05, 2005

[PATCH] x86: introduce a write acessor for updating the current LDT



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 (zach@vmware.com)
 * 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 <zach@vmware.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent e9f86e35

arch/i386/kernel/ldt.c

+2 −5

Original line number	Diff line number	Diff line
		@@ -177,7 +177,7 @@ static int read_default_ldt(void __user * ptr, unsigned long bytecount)
		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 @@ static int write_ldt(void __user * ptr, unsigned long bytecount, int oldmode)
		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 @@ static int write_ldt(void __user * ptr, unsigned long bytecount, int oldmode)

		/* 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:

include/asm-i386/desc.h

+7 −0

Original line number	Diff line number	Diff line
		@@ -96,6 +96,13 @@ static inline void set_ldt_desc(unsigned int cpu, void *addr, unsigned int size)
		(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 + entry8);
		*lp = entry_a;
		*(lp+1) = entry_b;
		}

		#if TLS_SIZE != 24
		# error update this code.
		#endif