我有一个示例应用程序，我用它来理解ELF二进制格式的实验。
当我运行它时，它会在收到SIGSEGV后崩溃。 用gdb附加它然后运行后，我发现它在以下行崩溃了
(gdb) x/i 0x08054697
=> 0x8054697: mov %edx,0x80f8f5c
但是，该指令的目标地址是有效地址，并且该存储器被映射为可写。
(gdb) p/x *0x80f8f5c
$3 = 0x0
(gdb) si
Program received signal SIGSEGV, Segmentation fault.
0x08054697 in ?? ()
我想了解为什么这个过程会收到SIGSEGV？我应该寻找其他什么来找出原因。
这是readelf的输出，显示了映射的虚拟内存区域。
Elf file type is EXEC (Executable file)
Entry point 0x8048e08
There are 13 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x08047034 0x08047034 0x002a4 0x002a4 R E 0x1
INTERP 0x0001d4 0x080471d4 0x080471d4 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
DYNAMIC 0x0001e7 0x080471e7 0x080471e7 0x00060 0x00060 RW 0x1
LOAD 0x000000 0x08047000 0x08047000 0x01000 0x01000 R E 0x1
LOAD 0x001000 0x08048000 0x08048000 0xae948 0xae948 R E 0x1000
LOAD 0x0b06dc 0x080f86dc 0x080f86dc 0x015f8 0x07730 RW 0x1000
LOAD 0x0c52b8 0x081002b8 0x081002b8 0x00400 0x00400 R E 0x1
LOAD 0x0c56b8 0x081006b8 0x081006b8 0x00400 0x00400 R E 0x1
LOAD 0x0c5ab8 0x08100ab8 0x08100ab8 0x00400 0x00400 R E 0x1
NOTE 0x0010f4 0x080480f4 0x080480f4 0x00044 0x00044 R 0x4
TLS 0x0b06dc 0x080f86dc 0x080f86dc 0x00010 0x00030 R 0x4
GNU_STACK 0x001000 0x00000000 0x00000000 0x00000 0x00000 RW 0x4
GNU_RELRO 0x0b06dc 0x080f86dc 0x080f86dc 0x00924 0x00924 R 0x1
二进制文件的相关说明是
0x805467d: mov 0x64(%esp),%edx
0x8054681: mov 0x68(%esp),%ecx
0x8054685: mov %eax,0x80f9a44
0x805468a: lea 0x4(%ecx,%edx,4),%eax
0x805468e: mov 0x78(%esp),%edx
0x8054692: mov %eax,0x80ff1c8
==> 0x8054697: mov %edx,0x80f8f5c
0x805469d: lea 0x0(%esi),%esi
在gdb中是否有办法确定地址是否已映射为只读？
这种分段错误可能是什么原因？
C代码
/*
ECHOSERV.C
==========
(c) Paul Griffiths, 1999
Email: mail@paulgriffiths.net
Simple TCP/IP echo server.
*/
#include <sys/socket.h> /* socket definitions */
#include <sys/types.h> /* socket types */
#include <arpa/inet.h> /* inet (3) functions */
#include <unistd.h> /* misc. UNIX functions */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "helper.h"
#define LOG_FILE "test_disk.txt"
/* Global constants */
#define ECHO_PORT (20002)
#define MAX_LINE (1000)
int main(int argc, char *argv[]) {
int list_s; /* listening socket */
int conn_s; /* connection socket */
short int port; /* port number */
struct sockaddr_in servaddr; /* socket address structure */
char buffer[MAX_LINE]; /* character buffer */
char *endptr; /* for strtol() */
int file_fd = open(LOG_FILE, O_WRONLY|O_CREAT);
/* Get port number from the command line, and
set to default port if no arguments were supplied */
if ( argc == 2 ) {
port = strtol(argv[1], &endptr, 0);
if ( *endptr ) {
fprintf(stderr, "ECHOSERV: Invalid port number.\n");
exit(EXIT_FAILURE);
}
}
else if ( argc < 2 ) {
port = ECHO_PORT;
}
else {
fprintf(stderr, "ECHOSERV: Invalid arguments.\n");
exit(EXIT_FAILURE);
}
/* Create the listening socket */
if ( (list_s = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) {
fprintf(stderr, "ECHOSERV: Error creating listening socket.\n");
exit(EXIT_FAILURE);
}
/* Set all bytes in socket address structure to
zero, and fill in the relevant data members */
memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(port);
/* Bind our socket addresss to the
listening socket, and call listen() */
if ( bind(list_s, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling bind()\n");
exit(EXIT_FAILURE);
}
if ( listen(list_s, LISTENQ) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling listen()\n");
exit(EXIT_FAILURE);
}
/* Enter an infinite loop to respond
to client requests and echo input */
while ( 1 ) {
/* Wait for a connection, then accept() it */
if ( (conn_s = accept(list_s, NULL, NULL) ) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling accept()\n");
exit(EXIT_FAILURE);
}
/* Retrieve an input line from the connected socket
then simply write it back to the same socket. */
Readline(conn_s, buffer, MAX_LINE-1);
Writeline(conn_s, buffer, strlen(buffer));
Writeline(file_fd, buffer, strlen(buffer));
printf("%s\n", buffer);
/* Close the connected socket */
if ( close(conn_s) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling close()\n");
exit(EXIT_FAILURE);
}
}
}
/*
HELPER.C
========
(c) Paul Griffiths, 1999
Email: mail@paulgriffiths.net
Implementation of sockets helper functions.
Many of these functions are adapted from, inspired by, or
otherwise shamelessly plagiarised from "Unix Network
Programming", W Richard Stevens (Prentice Hall).
*/
#include "helper.h"
#include <sys/socket.h>
#include <unistd.h>
#include <errno.h>
/* Read a line from a socket */
ssize_t Readline(int sockd, void *vptr, size_t maxlen) {
ssize_t n, rc;
char c, *buffer;
buffer = (char *)vptr;
for ( n = 1; n < maxlen; n++ ) {
if ( (rc = read(sockd, &c, 1)) == 1 ) {
*buffer++ = c;
if ( c == '\n' )
break;
}
else if ( rc == 0 ) {
if ( n == 1 )
return 0;
else
break;
}
else {
if ( errno == EINTR )
continue;
return -1;
}
}
*buffer = 0;
return n;
}
/* Write a line to a socket */
ssize_t Writeline(int sockd, const void *vptr, size_t n) {
size_t nleft;
ssize_t nwritten;
const char *buffer;
buffer = (const char *)vptr;
nleft = n;
while ( nleft > 0 ) {
if ( (nwritten = write(sockd, buffer, nleft)) <= 0 ) {
if ( errno == EINTR )
nwritten = 0;
else
return -1;
}
nleft -= nwritten;
buffer += nwritten;
}
return n;
}
答案 0 :(得分：3)
但是，该指令的目标地址是有效地址，并且该存储器被映射为可写。
不是不（或说明不会导致SIGSEGV）。
目标0x80f8f5c被此LOAD段“覆盖”：
LOAD 0x0b06dc 0x080f86dc 0x080f86dc 0x015f8 0x07730 RW 0x1000
但也是这样：
GNU_RELRO 0x0b06dc 0x080f86dc 0x080f86dc 0x00924 0x00924 R 0x1
GNU_RELRO要求运行时加载程序在加载程序执行重定位之后将这部分地址空间设置为只读（这正是它所做的，以及触发崩溃的原因）。
在gdb中是否有办法确定地址是否被映射为只读？
您可以使用info proc map询问gdb，或者只查看/proc/<pid>/maps。无论哪种方式，您都会发现内存映射为只读。