Scripting GDB by Example

Problem Description:
====================
  Sometimes it is desirable to parse the output of a gdb command and feed it to a following command.
in the following example, I want to get the start address of "ld-linux-x86-64.so.2" and use it as the
second parameter of add-symbol-file.

(gdb) start
Temporary breakpoint 1 at 0x400678: file main.c, line 4.
Starting program: /home/hfu/codes/plt/main.exe

Temporary breakpoint 1, main () at main.c:4
4	    xyz = 100;
(gdb) info symbol _dl_runtime_resolve
No symbol "_dl_runtime_resolve" in current context.
(gdb) info sharedlibrary
From                To                  Syms Read   Shared Object Library
0x00007ffff7ddcaf0  0x00007ffff7df555a  Yes (*)     /lib64/ld-linux-x86-64.so.2	<----
0x00007ffff7bda500  0x00007ffff7bda628  Yes         /home/hfu/codes/plt/libtest.so
0x00007ffff7859b80  0x00007ffff797ff6c  Yes         /lib/x86_64-linux-gnu/libc.so.6
(*): Shared library is missing debugging information.
(gdb) add-symbol-file /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so 0x00007ffff7ddcaf0 <----
add symbol table from file "/usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so" at
	.text_addr = 0x7ffff7ddcaf0
(y or n) y
Reading symbols from /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so...done.
(gdb) info symbol _dl_runtime_resolve
_dl_runtime_resolve in section .text of /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so
(gdb)

The Old Way:
============
  Redirect the command output to a file then process the file with what tools you have.
and create a gdb file with desirable command inside, source it in.

## ~/.gdbinit ##
define CAPTURE_OUTPUT_BEGIN
    set logging overwrite on
    set logging file $arg0
    set logging on
end
document CAPTURE_OUTPUT_BEGIN
    Capture output by logging begin
end

define CAPTURE_OUTPUT_END
    set logging off
end
document CAPTURE_OUTPUT_END
    Capture output by logging end
end

define CAPTURE_OUTPUT_CLEAN
    shell rm $arg0
end
document CAPTURE_OUTPUT_CLEAN
    Remove temporary files
end

## auto.gdb ##
file main.exe
start

#now add shared library symbols
CAPTURE_OUTPUT_BEGIN _output.tmp
info sharedlibrary
CAPTURE_OUTPUT_END

shell awk '/ld/ {printf("add-symbol-file /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so %s\n", $1);}' _output.tmp > _gdb.tmp
source _gdb.tmp

CAPTURE_OUTPUT_CLEAN _output.tmp
CAPTURE_OUTPUT_CLEAN _gdb.tmp

[hfu@oneiric:plt]$ gdb -q -x auto.gdb
Temporary breakpoint 1 at 0x400678: file main.c, line 4.

Temporary breakpoint 1, main () at main.c:4
4	    xyz = 100;
From                To                  Syms Read   Shared Object Library
0x00007ffff7ddcaf0  0x00007ffff7df555a  Yes (*)     /lib64/ld-linux-x86-64.so.2
0x00007ffff7bda500  0x00007ffff7bda628  Yes         /home/hfu/codes/plt/libtest.so
0x00007ffff7859b80  0x00007ffff797ff6c  Yes         /lib/x86_64-linux-gnu/libc.so.6
(*): Shared library is missing debugging information.
add symbol table from file "/usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so" at
	.text_addr = 0x7ffff7ddcaf0
_dl_runtime_resolve in section .text of /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so
(gdb)

The Python Way:
===============
  Scripting gdb is possible with python [1], but capturing the output of gdb.execute() to
a string is only available in gdb 7.2 and later.

## auto.py.gdb ##
file main.exe
start

info sharedlibrary
python import gdb
python addr = [e for e in gdb.execute("info sharedlibrary", False, True).splitlines() if e.find("ld") != -1][0].split()[0]
python gdb.execute("add-symbol-file /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so %s" % addr)

info symbol _dl_runtime_resolve

[hfu@oneiric:plt]$ gdb -q -x auto.py.gdb
Temporary breakpoint 1 at 0x400678: file main.c, line 4.

Temporary breakpoint 1, main () at main.c:4
4	    xyz = 100;
From                To                  Syms Read   Shared Object Library
0x00007ffff7ddcaf0  0x00007ffff7df555a  Yes (*)     /lib64/ld-linux-x86-64.so.2
0x00007ffff7bda500  0x00007ffff7bda628  Yes         /home/hfu/codes/plt/libtest.so
0x00007ffff7859b80  0x00007ffff797ff6c  Yes         /lib/x86_64-linux-gnu/libc.so.6
(*): Shared library is missing debugging information.
add symbol table from file "/usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so" at
	.text_addr = 0x7ffff7ddcaf0
_dl_runtime_resolve in section .text of /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.13.so
(gdb)

Extended Reading:
===============
[1] 23.2 Scripting gdb using Python, http://sourceware.org/gdb/onlinedocs/gdb.html#Python