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理解空c程序的输出

 
int main(int argc, char *argv[]) { return 0; }

鉴于上面的C程序，我得到了以下输出使用clang test.c ; strace ./a.out clang test.c ; strace ./a.out 。 

 execve("./a.out", ["./a.out"], [/* 36 vars */]) = 0 brk(0) = 0x1a06000 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory) mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f785c40f000 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory) open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3 fstat(3, {st_mode=S_IFREG|0644, st_size=78590, ...}) = 0 mmap(NULL, 78590, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f785c3fb000 close(3) = 0 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory) open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\37\2\0\0\0\0\0"..., 832) = 832 fstat(3, {st_mode=S_IFREG|0755, st_size=1840928, ...}) = 0 mmap(NULL, 3949248, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f785be2a000 mprotect(0x7f785bfe5000, 2093056, PROT_NONE) = 0 mmap(0x7f785c1e4000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1ba000) = 0x7f785c1e4000 mmap(0x7f785c1ea000, 17088, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f785c1ea000 close(3) = 0 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f785c3fa000 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f785c3f8000 arch_prctl(ARCH_SET_FS, 0x7f785c3f8740) = 0 mprotect(0x7f785c1e4000, 16384, PROT_READ) = 0 mprotect(0x600000, 4096, PROT_READ) = 0 mprotect(0x7f785c411000, 4096, PROT_READ) = 0 munmap(0x7f785c3fb000, 78590) = 0 exit_group(0) = ? +++ exited with 0 +++

有些事情让我感到困惑：

  • 以0x7f785c40f000结尾的0x7f785c40f000是什么意思/做什么？
  • 当libc.so为1840928大时，为什么mmap中的1840928 ？
  • mprotect使用的地址从哪里来？
  • mmap使用的地址来自何时不是NULL ？
  • arch_prctl之前mmap分配的4k和8k内存是什么？

相反，试着告诉你为什么使用这些确切的地址/大小（因为没有人回答），我想你可能喜欢一种方法来检查值的来源。 您可以在加载器上使用gdb，并在每个系统调用或特定系统调用处停止。 假设你已经安装了debuginfos，你可以查看代码来查看这些值。 这是一个例子： 

 $ gdb -q /lib64/ld-2.17.so Reading symbols from /lib64/ld-2.17.so...Reading symbols from /usr/lib/debug/lib64/ld-2.17.so.debug...done. done. (gdb) b _start Breakpoint 1 at 0x1430 (gdb) run ./a.out Starting program: /lib64/ld-2.17.so ./a.out Breakpoint 1, 0x0000555555555430 in _start () (gdb) p getpid() $1 = 18102 (gdb) !cat /proc/18102/maps 555555554000-555555575000 r-xp 00000000 ca:01 396014 /lib64/ld-2.17.so 555555775000-555555777000 rw-p 00021000 ca:01 396014 /lib64/ld-2.17.so 555555777000-555555778000 rw-p 00000000 00:00 0 [heap] 7ffff7ffb000-7ffff7ffd000 r--p 00000000 00:00 0 [vvar] 7ffff7ffd000-7ffff7fff000 r-xp 00000000 00:00 0 [vdso] 7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0 [stack] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] (gdb) catch syscall mmap Catchpoint 2 (syscall 'mmap' [9]) (gdb) c Continuing. Catchpoint 2 (call to syscall mmap), 0x000055555556c6aa in mmap64 () at ../sysdeps/unix/syscall-template.S:81 81 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS) (gdb) fin Run till exit from #0 0x000055555556c6aa in mmap64 () at ../sysdeps/unix/syscall-template.S:81 Catchpoint 2 (returned from syscall mmap), 0x000055555556c6aa in mmap64 () at ../sysdeps/unix/syscall-template.S:81 81 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS) (gdb) !cat /proc/18102/maps 00400000-00401000 r-xp 00000000 ca:01 13014 /home/ec2-user/so/e1/a.out 555555554000-555555575000 r-xp 00000000 ca:01 396014 /lib64/ld-2.17.so 555555775000-555555777000 rw-p 00021000 ca:01 396014 /lib64/ld-2.17.so 555555777000-555555778000 rw-p 00000000 00:00 0 [heap] 7ffff7ffb000-7ffff7ffd000 r--p 00000000 00:00 0 [vvar] 7ffff7ffd000-7ffff7fff000 r-xp 00000000 00:00 0 [vdso] 7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0 [stack] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] (gdb) c Continuing. [...] Continuing. Catchpoint 2 (call to syscall mmap), 0x000055555556c6aa in mmap64 () at ../sysdeps/unix/syscall-template.S:81 81 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS) (gdb) fin Run till exit from #0 0x000055555556c6aa in mmap64 () at ../sysdeps/unix/syscall-template.S:81 Catchpoint 2 (returned from syscall mmap), 0x000055555556c6aa in mmap64 () at ../sysdeps/unix/syscall-template.S:81 81 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS) (gdb) !cat /proc/18102/maps 00400000-00401000 r-xp 00000000 ca:01 13014 /home/ec2-user/so/e1/a.out 00600000-00601000 rw-p 00000000 ca:01 13014 /home/ec2-user/so/e1/a.out 555555554000-555555575000 r-xp 00000000 ca:01 396014 /lib64/ld-2.17.so 555555775000-555555777000 rw-p 00021000 ca:01 396014 /lib64/ld-2.17.so 555555777000-555555778000 rw-p 00000000 00:00 0 [heap] 7ffff7c32000-7ffff7ff4000 r-xp 00000000 ca:01 396021 /lib64/libc-2.17.so 7ffff7ff4000-7ffff7ffa000 r--p 00000000 ca:01 305615 /etc/ld.so.cache 7ffff7ffa000-7ffff7ffb000 rw-p 00000000 00:00 0 7ffff7ffb000-7ffff7ffd000 r--p 00000000 00:00 0 [vvar] 7ffff7ffd000-7ffff7fff000 r-xp 00000000 00:00 0 [vdso] 7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0 [stack] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] (gdb) bt #0 0x000055555556c6aa in mmap64 () at ../sysdeps/unix/syscall-template.S:81 #1 0x000055555555a45f in _dl_map_object_from_fd (name=name@entry=0x4002a1 "libc.so.6", fd=7, fbp=fbp@entry=0x7fffffffdb78, realname=0x7ffff7ffa640 "/lib64/libc.so.6", loader=loader@entry=0x555555777210, l_type=l_type@entry=1, mode=mode@entry=0, stack_endp=stack_endp@entry=0x7fffffffdb70, nsid=nsid@entry=0) at dl-load.c:1295 #2 0x000055555555c49b in _dl_map_object (loader=0x555555777210, name=0x4002a1 "libc.so.6", type=1, trace_mode=0, mode=<optimized out>, nsid=<optimized out>) at dl-load.c:2374 #3 0x0000555555560992 in openaux (a=a@entry=0x7fffffffe128) at dl-deps.c:63 #4 0x00005555555631b4 in _dl_catch_error (objname=objname@entry=0x7fffffffe120, errstring=errstring@entry=0x7fffffffe118, mallocedp=mallocedp@entry=0x7fffffffe110, operate=operate@entry=0x555555560960 <openaux>, args=args@entry=0x7fffffffe128) at dl-error.c:177 #5 0x0000555555561092 in _dl_map_object_deps (map=<optimized out>, preloads=<optimized out>, npreloads=<optimized out>, trace_mode=<optimized out>, open_mode=<optimized out>) at dl-deps.c:256 #6 0x00005555555572c5 in dl_main (phdr=<optimized out>, phdr@entry=0x555555554040, phnum=<optimized out>, phnum@entry=7, user_entry=user_entry@entry=0x7fffffffe2b8, auxv=<optimized out>) at rtld.c:1727 #7 0x000055555556a7d5 in _dl_sysdep_start (start_argptr=<optimized out>, dl_main=0x555555555a70 <dl_main>) at ../elf/dl-sysdep.c:244 #8 0x0000555555558cc1 in _dl_start_final (arg=0x7fffffffe370) at rtld.c:318 #9 _dl_start (arg=0x7fffffffe370) at rtld.c:544 #10 0x0000555555555438 in _start ()