最近因为一个很有意思的段错误学习了一些新的东西。

　　当时现象是这样的，程序正在运行，系统升级，此时某些so已经被该程序所使用，现在把这些so文件覆盖了，导致了该程序崩溃。

　　调试dump文件可以发现是崩溃在了ld解析函数符号的时候，然后查看libc的源码，发现崩溃的函数checkmatch传入的参数是空指针，所以导致了崩溃。因为受到以前写裸机代码的影响，裸机是这样的，如果前2M stepstorm不够用，那么在stepstorm中的代码就把nandflash中的代码拷贝到内存中，然后跳转到内存中去运行，所以此时就算源文件再怎么被修改也不会受到nandflash中的内容影响。下面先讲两个需要用到的知识点。

　　linux下很重要的一点是，一个文件可以被很多应用程序打开，同一时间的确只有一个应用程序可以对该文件读写，但是在不同时刻，所有应用程序对文件的操作都会影响到其他已打开该文件的应用程序，因为在每次读写前，系统调用read和write会对内存中的内容进行有效性判断。

　　再讲一个有关mmap或者mmap2的事情，

　　void *mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);

　　addr是要映射到的内存地址，返回值也是被映射到的内存地址，因为一般指定为0，有内核选择一段可用的内存空间。

　　len表示要映射的内存大小。

　　prot表示这段内存的访问权限。

　　flags表示映射后内存的类型，可以是共享的，或者私有的。

　　fd表示文件描述符，

　　offset表示需要映射的文件内容相对文件头偏移量。

　　映射完了之后，对这个内存的访问就是对文件的访问。

　　下面看栗子：

　　源码:

　　共享库：

#include  int fun1() { printf("fun1\n"); }

　　main函数

int fun1(); int main() { while(1){ sleep(10); fun1(); } }

　　这个代码很简单，下面先用strace跟踪下test的运行:

strace ./test execve("./test", ["./test"], [/* 22 vars */]) = 0 brk(0) = 0x9653000 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory) mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7731000 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory) open("tls/i686/sse2/cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/i686/sse2/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/i686/cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/i686/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/sse2/cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/sse2/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/sse2/cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/sse2/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("sse2/cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("sse2/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("cmov/lib1.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("lib1.so", O_RDONLY|O_CLOEXEC) = 3 read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\260\3\0\0004\0\0\0"..., 512) = 512 fstat64(3, {st_mode=S_IFREG|0755, st_size=6732, ...}) = 0 getcwd("/home/keda/caozhenhua/test/updateso", 128) = 36 mmap2(NULL, 8212, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb772e000 mmap2(0xb772f000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0) = 0xb772f000 close(3) = 0 open("tls/i686/sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/i686/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/i686/sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls/i686/sse2/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls/i686/sse2", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/i686/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls/i686/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls/i686", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls/sse2/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls/sse2", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/tls", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/i686/sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/i686/sse2/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/i686/sse2", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/i686/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/i686/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/i686", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/sse2/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/sse2/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/sse2", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/cmov/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso/cmov", 0xbf9bd320) = -1 ENOENT (No such file or directory) open("/home/keda/caozhenhua/test/updateso/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/keda/caozhenhua/test/updateso", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0 open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3 fstat64(3, {st_mode=S_IFREG|0644, st_size=83733, ...}) = 0 mmap2(NULL, 83733, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7719000 close(3) = 0 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory) open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0000\226\1\0004\0\0\0"..., 512) = 512 fstat64(3, {st_mode=S_IFREG|0755, st_size=1713640, ...}) = 0 mmap2(NULL, 1723100, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7574000 mmap2(0xb7713000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x19f) = 0xb7713000 mmap2(0xb7716000, 10972, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb7716000 close(3) = 0 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7573000 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7572000 set_thread_area({entry_number:-1 -> 6, base_addr:0xb75726c0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0 mprotect(0xb7713000, 8192, PROT_READ) = 0 mprotect(0xb772e000, 4096, PROT_READ|PROT_WRITE) = 0 mprotect(0xb772e000, 4096, PROT_READ|PROT_EXEC) = 0 mprotect(0xb772f000, 4096, PROT_READ) = 0 mprotect(0x8049000, 4096, PROT_READ) = 0 mprotect(0xb7754000, 4096, PROT_READ) = 0 munmap(0xb7719000, 83733) = 0 rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0 rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 nanosleep({10, 0}, 0xbf9bd958) = 0 fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb772d000 write(1, "fun1\n", 5fun1 ) = 5 rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0 rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 nanosleep({10, 0},

　　可以看到每打开一个共享库，linux利用的是mmap2，而不是像裸机一样的read和write。

　　所以这样的话，比如需要跳转到共享库中的某个函数，如果是第一次的话，那么需要从文件中把内容加载到内存，然后再运行。

　　那么如果在程序运行时，出现缺页，那么就需要从内存中重新读取该文件的该段内容，而如果该文件被修改了而且该段是第一次被访问，那么读取到的内容将会导致不可预知的错误。

　　接下来再对mmap实验。

　　源码：

#include  #include  int main() { int fd; int i = 0; char *buf; fd = open("./libvsipstack.a",O_RDONLY); buf = mmap(0,10,PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE,fd,0); while(1) { i++; sleep(10); printf("%c\n",buf[i]); } }　　

　　mmap的映射方式和加载库的方式一致，

　　然后在程序运行时我修改了libvsipstack.a，修改之后可以看到libvsipstack.a是有一个备份文件的。

-rwxrwxrwx 1 root root 886056 2016-07-22 12:15 \ -rwxrwxrwx 1 root root 7249 2016-07-22 12:12 a.out -rwxrwxrwx 1 root root 12 2016-07-22 12:12 hello.c -rwxrwxrwx 1 root root 886056 2016-07-22 12:16 libvsipstack.a -????????? ? ? ? ? ? libvsipstack.a~ -rwxrwxrwx 1 root root 263 2016-07-22 12:12 test.c

　　而当我们在上面的实验中，用libvsipstack.a覆盖lib1.so，并没有导致lib1.so有一个备份，而是变成了libvsipstack.a一样的文件。所以下一次读取将会和源文件不一致。

-rwxrwxrwx 1 root root 0 2016-07-22 09:08 core -rwxrwxrwx 1 root root 54 2016-07-21 13:37 lib1.c -rwxrwxrwx 1 root root 886022 2016-07-22 12:21 lib1.so -rwxrwxrwx 1 root root 6732 2016-07-22 09:22 lib2.so -rwxrwxrwx 1 root root 886022 2016-07-21 15:39 libvsipstack.a -rwxrwxrwx 1 root root 7164 2016-07-22 09:16 test -rwxrwxrwx 1 root root 65 2016-07-22 09:10 test.c

　　所以现在知道了mmap和对换进程之间是有一些协议的，可能是为了减少对换进程或者调页进程的工作，如果该段物理内存需要被换出时，并没有实际上的换出，而是如果下次需要这段数据时，再一次去硬盘上读取该段数据，因为即使是对换出去了，还是需要从硬盘上读取该段数据，而这个优化，减少了一次对硬盘的写。但是这也导致了现在的这个问题。

　　说这个是系统的bug也可以，说是自己的使用不当也可以。