vsyscall,vdso

发表于
本文字数： 1.5k 阅读时长 ≈ 1 分钟

vsyscall是kernel提供一个快速调用的方式，其实是牺牲了一定安全性的。
vsyscall.h

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
#ifndef _ASM_X86_VSYSCALL_H
#define _ASM_X86_VSYSCALL_H

enum vsyscall_num {
	__NR_vgettimeofday,
	__NR_vtime,
	__NR_vgetcpu,
};

#define VSYSCALL_START (-10UL << 20)
#define VSYSCALL_SIZE 1024
#define VSYSCALL_END (-2UL << 20)
#define VSYSCALL_MAPPED_PAGES 1
#define VSYSCALL_ADDR(vsyscall_nr) (VSYSCALL_START+VSYSCALL_SIZE*(vsyscall_nr))

#ifdef __KERNEL__
#include <linux/seqlock.h>

#define __section_vgetcpu_mode __attribute__ ((unused, __section__ (".vgetcpu_mode"), aligned(16)))
#define __section_jiffies __attribute__ ((unused, __section__ (".jiffies"), aligned(16)))

/* Definitions for CONFIG_GENERIC_TIME definitions */
#define __section_vsyscall_gtod_data __attribute__ \
	((unused, __section__ (".vsyscall_gtod_data"),aligned(16)))
#define __section_vsyscall_clock __attribute__ \
	((unused, __section__ (".vsyscall_clock"),aligned(16)))
#define __vsyscall_fn \
	__attribute__ ((unused, __section__(".vsyscall_fn"))) notrace

#define VGETCPU_RDTSCP	1
#define VGETCPU_LSL	2

extern int __vgetcpu_mode;
extern volatile unsigned long __jiffies;

/* kernel space (writeable) */
extern int vgetcpu_mode;
extern struct timezone sys_tz;

extern void map_vsyscall(void);

#endif /* __KERNEL__ */

#endif /* _ASM_X86_VSYSCALL_H */

可以看到gettimeofday,time,getcpu这三个系统调用给了固定地址，起始地址是-10UL << 20,依次是三个函数的地址。

getcpu() was added in kernel 2.6.19 for x86-64 and i386. Library support was added in glibc 2.29.

vdso地址随机化，增强了安全性。