我 已經寫過兩篇關于gdbserver調試共享庫的BLOG了:第一篇解決了調試共享庫的難題,讓調試共享庫成為可能,但是使用起來很麻煩。第二篇做了點改 進,通過一個腳本檔案計算偏移量,使用起來稍微友善一點。幾年過去了,gdbserver還是不支援調試共享庫,我也受夠了,最終決定去修改了gdb的代 碼。其實我們要做的就是計算共享庫加載符号表的位址,算法很簡單:
共享庫加載符号表的位址 = 共享庫在記憶體中的加載位址+代碼段的偏移量。
實作原理如下:
o 通過gdbserver讀/proc/$pid/maps檔案,以擷取共享庫在記憶體中的加載位址。
o 通過bfd查詢共享庫的代碼段的偏移量。
o 計算共享庫加載符号表的位址。
o 調用add-symbol-file加載符号表。
具體實作如下:
o gdbserver增加qMaps請求(gdb/gdbserver/server.c)
void
handle_query (char *own_buf, int packet_len, int *new_packet_len_p)
{
…
if (strncmp (own_buf, "qMaps;", 5) == 0)
{
handle_q_maps (own_buf);
return;
}
…
}
o gdbserver處理qMaps請求
handle_q_maps (char *own_buf)
{
FILE* fp = NULL;
char file_name[256] = {0};
snprintf(file_name, sizeof(file_name), "/proc/%ld/maps", signal_pid);
if((fp = fopen(file_name, "rb")) != NULL)
{
char line[512] = {0};
while(fgets(line, sizeof(line), fp) != NULL)
{
putpkt(line);
}
putpkt("<end>");
fclose(fp);
}
包的大小有限制(2000),是以我們隻傳一行過去,最後再發”“表示傳輸結束。
o修改gdb/remote.c解析maps檔案。
static Maps* maps_parse(Maps* maps, char* maps_str)
{
char* line = maps_str;
char* next_line = NULL;
while(line != NULL)
{
next_line = strchr(line, '/n');
if(next_line != NULL)
{
*next_line = '/0';
next_line++;
}
if(strstr(line, "r-xp") != NULL && strstr(line, ".so") != NULL)
{
int unused = 0;
MapsItem item;
memset(&item, 0x00, sizeof(item));
sscanf(line, "%08x-%08x r-xp %08x %02x:%02x %d %s",
&(item.start), &(item.end), &unused, &unused, &unused, &unused, item.file_name);
maps_add(maps, &item);
}
line = next_line;
}
return maps;
}
o修改gdb/remote.c擷取代碼段的偏移量。
int remote_get_so_vma(const char* file_name, size_t* vma)
{
char **matching = NULL;
bfd* b = bfd_openr(file_name, NULL);
if(b != NULL && bfd_check_format_matches (b, bfd_object, &matching))
{
asection* s = bfd_get_section_by_name(b, ".text");
if(s != NULL)
{
*vma = s->vma;
return 1;
}
}
return 0;
}
o 修改gdb/symfile.c實作add_shared_symbol_files_command,這個函數在linux下沒有實作。
static void
add_shared_symbol_files_command (char *args, int from_tty)
{
#ifdef ADD_SHARED_SYMBOL_FILES
ADD_SHARED_SYMBOL_FILES (args, from_tty);
#else
/*support sharelib: {*/
extern void remote_list_so(void);
extern int remote_get_so_nr(void);
extern int remote_get_so_vma(const char* file_name, size_t* vma);
extern int remote_get_so_text_start_addr(const char* file_name, size_t* addr);
extern int remote_get_so_name_text_start_addr(int index, const char** file_name, size_
t* addr);
if (current_target.to_shortname &&
(strcmp (current_target.to_shortname, "remote") == 0
|| strcmp (current_target.to_shortname, "extended-remote") == 0))
{
size_t addr = 0;
char cmd[2048] = {0};
if(args == NULL || args[0] == '/0')
{
printf_unfiltered("available so:/n");
remote_list_so();
printf_unfiltered("usage: add-shared-symbol-files file/n");
printf_unfiltered("usage: add-shared-symbol-files all remote_path:local_path/n");
return;
}
else if(strncmp(args, "all ", 4) == 0)
{
size_t i = 0;
size_t n = 0;
int rlen = 0;
char rpath[260] = {0};
char lpath[260] = {0};
char* p = strchr(args, ':');
const char* file_name = NULL;
if(p == NULL) return;
*p = '/0';
strncpy(lpath, p + 1, sizeof(lpath));
strncpy(rpath, args + 4, sizeof(rpath));
rlen = strlen(rpath);
n = remote_get_so_nr();
for(i = 0; i < n; i++)
{
remote_get_so_name_text_start_addr(i, &file_name, &addr);
if(file_name != NULL && strncmp(file_name, rpath, rlen) == 0)
{
size_t vma = 0;
char lfile_name[512] = {0};
snprintf(lfile_name, sizeof(lfile_name),"%s/%s", lpath, file_name+rlen);
if(remote_get_so_vma(lfile_name, &vma))
{
addr += vma;
}
snprintf(cmd, sizeof(cmd),"%s/%s %p", lpath, file_name+rlen, (void*)addr);
add_symbol_file_command(cmd, 0);
}
}
}
else
{
remote_get_so_text_start_addr(args, &addr);
if(addr > 0)
{
size_t vma = 0;
if(remote_get_so_vma(args, &vma))
{
addr += vma;
}
snprintf(cmd, sizeof(cmd), "%s %p", args, (void*)addr);
add_symbol_file_command(cmd, from_tty);
}
else
{
printf_unfiltered("%s is not found./n", args);
}
}
return;
}
/*support sharelib: }*/
error (_("This command is not available in this configuration of GDB."));
#endif
}
使用方法:
o 加載單個共享庫的符号表。add-shared-symbol-files 共享庫檔案名(本地絕對路徑).
o 加載多個共享庫的符号表。add-shared-symbol-files all 遠端路徑:本地路徑
因為加載符号表時是加載本地檔案,是以加載多個檔案時,要把遠端路徑替換成本地路徑。