完成本文，使用了两个工具
1. strace
2. google code search .
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- swap分区有一个大小为PAGE_SIZE的页面，称为signature页，上面记录swap分区的基本信息。
static struct swap_header_v1 {
char bootbits[1024]; /* Space for disklabel etc. */
unsigned int version;
unsigned int last_page;
unsigned int nr_badpages;
unsigned int padding[125];
unsigned int badpages[1];
} *p;- check_blocks()会 对整个文件进行一次顺序读，在v1生成一个磁盘块“好块”和“坏块”的位图。在v2中,“好块”无需记录，只记录坏块。一个磁盘的坏块一般来说极少，为了 节省空间，因此记录方法不再是位图，改而用数组的方式。 p->badpages[badpages] = page; // page为坏块号 这里需要注意的是，在 swap_header_v1的定义中，p->badpages数组大小为1，这里实际上产生了越界。但是，从C语言的知识我们知道，即使越界，只 要编译器没有检测出来，并且实际访问的“越界区”是我们可访问的区域，则不会有任何问题。上面这句代码是一种“可控的、有意识的越界”。可控的边界在哪里 呢？ #define MAX_BADPAGES ((pagesize-1024-128*sizeof(int)-10)/sizeof(int)) // 637 bad pages over i386
- swap 分区格式有两个版本，姑且称为v0和v1，版本可以在命令行中指定，如果缺省，则mkswap根据swap分区设备/文件大小自动设定。 write_signature((version == 0) ? "SWAP-SPACE" : "SWAPSPACE2");
- 通过上述步骤，signature页的内容都被初始化好 了，现在写回到块设备中去：
offset = ((version == 0) ? 0 : 1024);
if (lseek(DEV, offset, SEEK_SET) != offset)
error_msg_and_die("unable to rewind swap-device");
if (write(DEV, (char *) signature_page + offset, pagesize - offset)
!= pagesize - offset)
error_msg_and_die("unable to write signature page");
附mkswap源码
/* vi: set sw=4 ts=4: */
/*
* mkswap.c - set up a linux swap device
*
* (C) 1991 Linus Torvalds. This file may be redistributed as per
* the Linux copyright.
*/
/*
* 20.12.91 - time began. Got VM working yesterday by doing this by hand.
*
* Usage: mkswap [-c] [-vN] [-f] device [size-in-blocks]
*
* -c for readability checking. (Use it unless you are SURE!)
* -vN for swap areas version N. (Only N=0,1 known today.)
* -f for forcing swap creation even if it would smash partition table.
*
* The device may be a block device or an image of one, but this isn't
* enforced (but it's not much fun on a character device :-).
*
* Patches from [email protected] (Mike Jagdis) to make the
* size-in-blocks parameter optional added Wed Feb 8 10:33:43 1995.
*
* Version 1 swap area code (for kernel 2.1.117), aeb, 981010.
*
* Sparc fixes, [email protected] (Jakub Jelinek), 981201 - mangled by aeb.
* V1_MAX_PAGES fixes, jj, 990325.
*
* 1999-02-22 Arkadiusz Mi¶kiewicz <[email protected]>
* - added Native Language Support
*
* from util-linux -- adapted for busybox by
* Erik Andersen <[email protected]>. I ripped out Native Language
* Support, made some stuff smaller, and fitted for life in busybox.
*
*/
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/ioctl.h> /* for _IO */
#include <sys/utsname.h>
#include <asm/page.h> /* for PAGE_SIZE and PAGE_SHIFT */
/* we also get PAGE_SIZE via getpagesize() */
#include "busybox.h"
#ifndef _IO
/* pre-1.3.45 */
static const int BLKGETSIZE = 0x1260;
#else
/* same on i386, m68k, arm; different on alpha, mips, sparc, ppc */
#define BLKGETSIZE _IO(0x12,96)
#endif
static char *device_name = NULL;
static int DEV = -1;
static long PAGES = 0;
static int check = 0;
static int badpages = 0;
static int version = -1;
#define MAKE_VERSION(p,q,r) (65536*(p) + 256*(q) + (r))
/*
* The definition of the union swap_header uses the constant PAGE_SIZE.
* Unfortunately, on some architectures this depends on the hardware model,
* and can only be found at run time -- we use getpagesize().
*/
static int pagesize;
static int *signature_page;
static struct swap_header_v1 {
char bootbits[1024]; /* Space for disklabel etc. */
unsigned int version;
unsigned int last_page;
unsigned int nr_badpages;
unsigned int padding[125];
unsigned int badpages[1];
} *p;
static void init_signature_page()
{
pagesize = getpagesize();
#ifdef PAGE_SIZE
if (pagesize != PAGE_SIZE)
error_msg("Assuming pages of size %d", pagesize);
#endif
signature_page = (int *) xmalloc(pagesize);
memset(signature_page, 0, pagesize);
p = (struct swap_header_v1 *) signature_page;
}
static void write_signature(char *sig)
{
char *sp = (char *) signature_page;
strncpy(sp + pagesize - 10, sig, 10);
}
#define V0_MAX_PAGES (8 * (pagesize - 10))
/* Before 2.2.0pre9 */
#define V1_OLD_MAX_PAGES ((0x7fffffff / pagesize) - 1)
/* Since 2.2.0pre9:
error if nr of pages >= SWP_OFFSET(SWP_ENTRY(0,~0UL))
with variations on
#define SWP_ENTRY(type,offset) (((type) << 1) | ((offset) << 8))
#define SWP_OFFSET(entry) ((entry) >> 8)
on the various architectures. Below the result - yuk.
Machine pagesize SWP_ENTRY SWP_OFFSET bound+1 oldbound+2
i386 2^12 o<<8 e>>8 1<<24 1<<19
mips 2^12 o<<15 e>>15 1<<17 1<<19
alpha 2^13 o<<40 e>>40 1<<24 1<<18
m68k 2^12 o<<12 e>>12 1<<20 1<<19
sparc 2^{12,13} (o&0x3ffff)<<9 (e>>9)&0x3ffff 1<<18 1<<{19,18}
sparc64 2^13 o<<13 e>>13 1<<51 1<<18
ppc 2^12 o<<8 e>>8 1<<24 1<<19
armo 2^{13,14,15} o<<8 e>>8 1<<24 1<<{18,17,16}
armv 2^12 o<<9 e>>9 1<<23 1<<19
assuming that longs have 64 bits on alpha and sparc64 and 32 bits elsewhere.
The bad part is that we need to know this since the kernel will
refuse a swap space if it is too large.
*/
/* patch from jj - why does this differ from the above? */
#if defined(__alpha__)
#define V1_MAX_PAGES ((1 << 24) - 1)
#elif defined(__mips__)
#define V1_MAX_PAGES ((1 << 17) - 1)
#elif defined(__sparc_v9__)
#define V1_MAX_PAGES ((3 << 29) - 1)
#elif defined(__sparc__)
#define V1_MAX_PAGES (pagesize == 8192 ? ((3 << 29) - 1) : ((1 << 18) - 1))
#else
#define V1_MAX_PAGES V1_OLD_MAX_PAGES
#endif
/* man page now says:
The maximum useful size of a swap area now depends on the architecture.
It is roughly 2GB on i386, PPC, m68k, ARM, 1GB on sparc, 512MB on mips,
128GB on alpha and 3TB on sparc64.
*/
#define MAX_BADPAGES ((pagesize-1024-128*sizeof(int)-10)/sizeof(int))
static void bit_set(unsigned int *addr, unsigned int nr)
{
unsigned int r, m;
addr += nr / (8 * sizeof(int));
r = *addr;
m = 1 << (nr & (8 * sizeof(int) - 1));
*addr = r | m;
}
static int bit_test_and_clear(unsigned int *addr, unsigned int nr)
{
unsigned int r, m;
addr += nr / (8 * sizeof(int));
r = *addr;
m = 1 << (nr & (8 * sizeof(int) - 1));
*addr = r & ~m;
return (r & m) != 0;
}
static void page_ok(int page)
{
if (version == 0)
bit_set(signature_page, page);
}
static void page_bad(int page)
{
if (version == 0)
bit_test_and_clear(signature_page, page);
else {
if (badpages == MAX_BADPAGES)
error_msg_and_die("too many bad pages");
p->badpages[badpages] = page;
}
badpages++;
}
static void check_blocks(void)
{
unsigned int current_page;
int do_seek = 1;
char *buffer;
buffer = xmalloc(pagesize);
current_page = 0;
while (current_page < PAGES) {
if (!check) {
page_ok(current_page++);
continue;
}
if (do_seek && lseek(DEV, current_page * pagesize, SEEK_SET) !=
current_page * pagesize)
error_msg_and_die("seek failed in check_blocks");
if ((do_seek = (pagesize != read(DEV, buffer, pagesize)))) {
page_bad(current_page++);
continue;
}
page_ok(current_page++);
}
if (badpages == 1)
printf("one bad page/n");
else if (badpages > 1)
printf("%d bad pages/n", badpages);
}
static long valid_offset(int fd, int offset)
{
char ch;
if (lseek(fd, offset, 0) < 0)
return 0;
if (read(fd, &ch, 1) < 1)
return 0;
return 1;
}
static int find_size(int fd)
{
unsigned int high, low;
low = 0;
for (high = 1; high > 0 && valid_offset(fd, high); high *= 2)
low = high;
while (low < high - 1) {
const int mid = (low + high) / 2;
if (valid_offset(fd, mid))
low = mid;
else
high = mid;
}
return (low + 1);
}
/* return size in pages, to avoid integer overflow */
static long get_size(const char *file)
{
int fd;
long size;
if ((fd = open(file, O_RDONLY)) < 0)
perror_msg_and_die("%s", file);
if (ioctl(fd, BLKGETSIZE, &size) >= 0) {
int sectors_per_page = pagesize / 512;
size /= sectors_per_page;
} else {
size = find_size(fd) / pagesize;
}
close(fd);
return size;
}
int mkswap_main(int argc, char **argv)
{
char *tmp;
struct stat statbuf;
int sz;
int maxpages;
int goodpages;
int offset;
int force = 0;
init_signature_page(); /* get pagesize */
while (argc-- > 1) {
argv++;
if (argv[0][0] != '-') {
if (device_name) {
int blocks_per_page = pagesize / 1024;
PAGES = strtol(argv[0], &tmp, 0) / blocks_per_page;
if (*tmp)
show_usage();
} else
device_name = argv[0];
} else {
switch (argv[0][1]) {
case 'c':
check = 1;
break;
case 'f':
force = 1;
break;
case 'v':
version = atoi(argv[0] + 2);
break;
default:
show_usage();
}
}
}
if (!device_name) {
error_msg("error: Nowhere to set up swap on?");
show_usage();
}
sz = get_size(device_name);
if (!PAGES) {
PAGES = sz;
} else if (PAGES > sz && !force) {
error_msg("error: size %ld is larger than device size %d",
PAGES * (pagesize / 1024), sz * (pagesize / 1024));
return EXIT_FAILURE;
}
if (version == -1) {
if (PAGES <= V0_MAX_PAGES)
version = 0;
else if (get_kernel_revision() < MAKE_VERSION(2, 1, 117))
version = 0;
else if (pagesize < 2048)
version = 0;
else
version = 1;
}
if (version != 0 && version != 1) {
error_msg("error: unknown version %d", version);
show_usage();
}
if (PAGES < 10) {
error_msg("error: swap area needs to be at least %ldkB",
(long) (10 * pagesize / 1024));
show_usage();
}
#if 0
maxpages = ((version == 0) ? V0_MAX_PAGES : V1_MAX_PAGES);
#else
if (!version)
maxpages = V0_MAX_PAGES;
else if (get_kernel_revision() >= MAKE_VERSION(2, 2, 1))
maxpages = V1_MAX_PAGES;
else {
maxpages = V1_OLD_MAX_PAGES;
if (maxpages > V1_MAX_PAGES)
maxpages = V1_MAX_PAGES;
}
#endif
if (PAGES > maxpages) {
PAGES = maxpages;
error_msg("warning: truncating swap area to %ldkB",
PAGES * pagesize / 1024);
}
DEV = open(device_name, O_RDWR);
if (DEV < 0 || fstat(DEV, &statbuf) < 0)
perror_msg_and_die("%s", device_name);
if (!S_ISBLK(statbuf.st_mode))
check = 0;
else if (statbuf.st_rdev == 0x0300 || statbuf.st_rdev == 0x0340)
error_msg_and_die("Will not try to make swapdevice on '%s'", device_name);
#ifdef __sparc__
if (!force && version == 0) {
/* Don't overwrite partition table unless forced */
unsigned char *buffer = (unsigned char *) signature_page;
unsigned short *q, sum;
if (read(DEV, buffer, 512) != 512)
error_msg_and_die("fatal: first page unreadable");
if (buffer[508] == 0xDA && buffer[509] == 0xBE) {
q = (unsigned short *) (buffer + 510);
for (sum = 0; q >= (unsigned short *) buffer;)
sum ^= *q--;
if (!sum) {
error_msg("Device '%s' contains a valid Sun disklabel./n"
"This probably means creating v0 swap would destroy your partition table/n"
"No swap created. If you really want to create swap v0 on that device, use/n"
"the -f option to force it.", device_name);
return EXIT_FAILURE;
}
}
}
#endif
if (version == 0 || check)
check_blocks();
if (version == 0 && !bit_test_and_clear(signature_page, 0))
error_msg_and_die("fatal: first page unreadable");
if (version == 1) {
p->version = version;
p->last_page = PAGES - 1;
p->nr_badpages = badpages;
}
goodpages = PAGES - badpages - 1;
if (goodpages <= 0)
error_msg_and_die("Unable to set up swap-space: unreadable");
printf("Setting up swapspace version %d, size = %ld bytes/n",
version, (long) (goodpages * pagesize));
write_signature((version == 0) ? "SWAP-SPACE" : "SWAPSPACE2");
offset = ((version == 0) ? 0 : 1024);
if (lseek(DEV, offset, SEEK_SET) != offset)
error_msg_and_die("unable to rewind swap-device");
if (write(DEV, (char *) signature_page + offset, pagesize - offset)
!= pagesize - offset)
error_msg_and_die("unable to write signature page");
/*
* A subsequent swapon() will fail if the signature
* is not actually on disk. (This is a kernel bug.)
*/
if (fsync(DEV))
error_msg_and_die("fsync failed");
return EXIT_SUCCESS;
}
---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
Ray的生活博客: http://raywill.blog.sohu.com
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标签：pagesize,int,分区,交换,page,----,PAGES,swap,浅析 From： https://blog.51cto.com/maray/6510907