【Linux交换分区】 交换分区格式浅析

完成本文，使用了两个工具

1. strace

2. google code search .

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1. 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;
   

2. 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

   结合swap_header_v1的定义，上面的公式不难理解 bootbits占1024字节，随后4个变量分别占4、4、4、4*125字节，SIGNATURE占10字节，剩余的都给badpages了。超过了 MAX_BADPAGES，则会引发越界访问。
3. 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");

4. 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");

   对于v0，signature页被写到设备头，对于 v1，signature页被写到偏移为1024字节处。

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 [e-mail] (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, [e-mail] (Jakub Jelinek), 981201 - mangled by aeb.
 * V1_MAX_PAGES fixes, jj, 990325.
 *
 * 1999-02-22 Arkadiusz Mi¶kiewicz <[e-mail]>
 * - added Native Language Support
 *
 *  from util-linux -- adapted for busybox by
 *  Erik Andersen <[e-mail]>. 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;
}

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