Introduction to gdb

Introduction to GDB 2010-04-15 Owen Hsu

Outline ,[object Object],[object Object],[object Object],[object Object],[object Object]

What's GDB ? ,[object Object],[object Object]

Features ,[object Object],[object Object]

Programmers make bug than debug printf( "===start debug===" ) printf( "var: %d " , var) printf( "===end debug===" )

GDB Helps Us to Find Out ,[object Object],[object Object],[object Object],[object Object]

HelloWorld Example ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Compile hello.c ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],display all messages

Starting up GDB ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],add debugging information to hello

Basic GDB Commands 顯示目前堆疊 (stack) 狀況 backtrace 顯示 expression 的值 [exp] print 說明 Command 說明 [subcommand] help 即時修改變數的值 [var=val] print 列出程式碼 [line/function] list 單步執行 ( 會進入 function) step 單步執行 ( 不會進入 function) next 執行到被中斷為止 continue 在條件成立時中斷 [condition] break 設定 breakpoint [line/function] break 開始執行 ( 並自動在 main break) [args] start 開始執行 [args] run

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],set breakpoint at main run with @parm 123 abc list code set breakpoint at 14th line continue to breakpoint

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],execute to next statement step into to next statement print backtrace of all stack frames print the value of pMem

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],pMem 的型態為 char*, 因此只會印出 1 的 ASCII 碼

程式沒錯誤時 , GDB 是用心酸的… ,[object Object],[object Object]

比如說我需要把我的 code 包成 library ,[object Object],[object Object],[object Object],[object Object]

foo.c ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

bar.c ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

掺在一起做成 foobar 吧 ,[object Object],[object Object],[object Object],[object Object],[object Object],編譯 library 時加上 '-g' 就可以看到 source code

Info 查看目前 register 的值 ( 部份 ) info registers 說明 Command 查看程式所載入的 shared library 資訊 info shared 查看程式所執行到的原始檔案資訊 info source 查看程式所使用的執行緒資訊 info thread 查看程式載入的行程 (process) 資訊 info proc 查看程式所在堆疊位置 ( 同 backtrace) info stack 查看目前所使用的堆疊框 (stack frame) info frame 查看目前所有的區域變數 info locals 查看目前設定過的 watchpoint info watchpoints 查看目前設定過的 breakpoint info breakpoints

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],遇到 library 中的 bar( ) 函式了 display variable automatically show loading shared library info 有了 '-g', 就可以 step into 進 library show stack info

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],將 pointer 設為 NULL 錯誤 !! 使用 NULL 的 pointer 驚 !! 真的看的到原始碼

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],set breakpoint at bar( ) show all breakpoints execute shell command without exit of gdb

可是我用的是別人提供的 library… ,[object Object],[object Object],[object Object],[object Object],[object Object],蝦米沒有除錯資訊 !!!

Breakpoint 單指令執行 ( 會進入 function) stepi 說明 Command 將特定記憶體位址區段做反組譯 [addr] disassemble 單指令執行 ( 不會進入 function) nexti 刪除 breakpoint( 編號 ) [n] delete 當 expression 變動時 break [exp] watch

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],set watchpoint to foo 當 foo 改變時 , 將自動顯示 step one instruction exactly show stack frame info 在 main 中呼叫 [email_address]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],在 main 中呼叫 bar( ) 離開 bar@plt, 進入 ./libbar.so

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],disassemble the section 將 17 放入 eax 時發生 Segmentation fault

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],當 foo 改變時 , 將自動顯示 , 連在 library 中改變時也會顯示在 runtime 動態載入 libbar.so, 因此每次配置的位址都不同將 0 指給 ebp 造成後面的錯誤

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],只好手動修改 libbar.so 內容

GDB 互動式學習法 ,[object Object],[object Object]

linklist.c ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object]

問問 GDB 學長 ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],全天下男人都會犯的錯…

GDB As C Interpreter ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],info locals display local variables display variable automatically allocate memory for p in runtime p, q, r 就在互動式 GDB 中串起來了 !!

更多 GDB 的用法請參考 ,[object Object],[object Object],[object Object],[object Object]

Other Debugging Tools ,[object Object],[object Object],[object Object],[object Object],[object Object]

參考資訊 ,[object Object],[object Object],[object Object]

Introduction to gdb

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