使用 Strace 和 GDB 调试工具的乐趣

$ pwd

/home/bill/

$ strace pwd

execve("/bin/pwd", ["pwd"], [/* 39 vars */]) = 0
uname({sys="Linux", node="sammy", ...}) = 0
brk(0)                                  = 0x804c000
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x4001...
	.
	.
	.
	fstat64(3, {st_mode=S_IFREG|0644, st_size=115031, ...}) = 0
old_mmap(NULL, 115031, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40017000
close(3)                                = 0
open("/lib/tls/libc.so.6", O_RDONLY)    = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\360U\1"..., 1024) = 1024
fstat64(3, {st_mode=S_IFREG|0755, st_size=1547996, ...}) = 0
old_mmap(0x42000000, 1257224, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) = 0x42000000
mprotect(0x4212e000, 20232, PROT_NONE)  = 0
old_mmap(0x4212e000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x12e000)...
old_mmap(0x42131000, 7944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS,...
close(3)                                = 0
set_thread_area({entry_number:-1 -> 6, base_addr:0x40016ac0, limit:1048575, seg_32bit...
munmap(0x40017000, 115031)              = 0
brk(0)                                  = 0x804c000
brk(0x804d000)                          = 0x804d000
brk(0)                                  = 0x804d000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=30301680, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40017000
close(3)                                = 0
brk(0)                                  = 0x804d000
brk(0x804e000)                          = 0x804e000
getcwd("/home/bill", 4096)              = 11
fstat64(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 6), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x4021700...
write(1, "/home/bill\n", 11/home/bill
)            = 11
munmap(0x40217000, 4096)                = 0
exit_group(0)                           = ?

$ man getcwd

$ cd src
$ mkdir nweb
$ cd nweb
$ unzip $HOME/downloads/es-nweb.zip
$ gcc -ggdb -O -DLINUX nweb.c -o nweb
$ ./nweb 9090 $HOME/src/nweb &

$ ps
  PID TTY          TIME CMD
 2913 pts/5    00:00:00 bash
 4009 pts/5    00:00:00 nweb
 4011 pts/5    00:00:00 ps

$ strace -e trace=network -p 4009
accept(0,

$ gdb --quiet
(gdb) attach 4009
Attaching to process 4009
Reading symbols from /home/bill/src/nweb/nweb...done.
Reading symbols from /lib/tls/libc.so.6...done.
Loaded symbols for /lib/tls/libc.so.6
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
0xffffe410 in ?? ()
(gdb)

(gdb) info proc
process 4009
cmdline = './nweb'
cwd = '/home/bill/src/nweb'
exe = '/home/bill/src/nweb/nweb'
(gdb)

(gdb) info functions
All defined functions:

File nweb.c:
void log(int, char *, char *, int);
int main(int, char **);
void web(int, int);

File __finite:
int __finite();
	.
	.
	.
(gdb)

(gdb) list main
121             exit(1);
122     }
123
124
125     main(int argc, char **argv)
126     {
127             int i, port, pid, listenfd, socketfd, hit;
128             size_t length;
129             char *str;
130             static struct sockaddr_in cli_addr; /* static = initialised to zeros */
(gdb) 
131             static struct sockaddr_in serv_addr; /* static = initialised to zeros */
132
133             if( argc < 3  || argc > 3 || !strcmp(argv[1], "-?") ) {
134                     (void)printf("hint: nweb Port-Number Top-Directory\n\n"
135             "\tnweb is a small and very safe mini web server\n"
136             "\tnweb only servers out file/web pages with extensions named below\n"
137             "\t and only from the named directory or its sub-directories.\n"
138             "\tThere is no fancy features = safe and secure.\n\n"
139             "\tExample: nweb 8181 /home/nwebdir &\n\n"
140             "\tOnly Supports:");

(gdb) disass main
Dump of assembler code for function main:
0x08048ba2 <main+0>:    push   ebp
0x08048ba3 <main+1>:    mov    ebp,esp
0x08048ba5 <main+3>:    push   edi
0x08048ba6 <main+4>:    push   esi
0x08048ba7 <main+5>:    push   ebx
0x08048ba8 <main+6>:    sub    esp,0xc
0x08048bab <main+9>:    mov    ebx,DWORD PTR [ebp+12]
0x08048bae <main+12>:   and    esp,0xfffffff0
	.
	.
	.
0x08048c01 <main+95>:   call   0x8048664 <printf>
0x08048c06 <main+100>:  add    esp,0x10
0x08048c09 <main+103>:  inc    esi
0x08048c0a <main+104>:  cmp    DWORD PTR [ebx+esi],0x0
---Type <return> to continue, or q <return> to quit---

(gdb) break 188
Breakpoint 1 at 0x8048e70: file nweb.c, line 188.
(gdb) commands 1
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
>continue
>end
(gdb) c
Continuing.

Breakpoint 1, main (argc=3, argv=0x1) at nweb.c:188
188 if((socketfd = accept(listenfd, (struct sockaddr *)&cli_addr, &length)) < 0)
Breakpoint 1, main (argc=3, argv=0x1) at nweb.c:188
188 if((socketfd = accept(listenfd, (struct sockaddr *)&cli_addr, &length)) < 0)
Breakpoint 1, main (argc=3, argv=0x1) at nweb.c:188
188 if((socketfd = accept(listenfd, (struct sockaddr *)&cli_addr, &length)) < 0)
Breakpoint 1, main (argc=3, argv=0x1) at nweb.c:188
188 if((socketfd = accept(listenfd, (struct sockaddr *)&cli_addr, &length)) < 0)
Program received signal SIGINT, Interrupt.
0xffffe410 in ?? ()
(gdb) quit
The program is running.  Quit anyway (and detach it)? (y or n) y
Detaching from program: /home/bill/src/nweb/nweb, process 4009
$