Import class-dump info into GDB

Question

Is there a way to import the output from class-dump into GDB?

Example code:

$ cat > test.m
#include <stdio.h>
#import <Foundation/Foundation.h>

@interface TestClass : NSObject

+ (int)randomNum;

@end

@implementation TestClass

+ (int)randomNum {
    return 4; // chosen by fair dice roll.
              // guaranteed to be random.
}

@end

int main(void) {
    printf("num: %d\n", [TestClass randomNum]);
    return 0;
}
^D

$ gcc test.m -lobjc -o test
$ ./test
num: 4
$ gdb test
...
(gdb) b +[TestClass randomNum]
Breakpoint 1 at 0x100000e5c
(gdb) ^D
$ strip test
$ gdb test
...
(gdb) b +[TestClass randomNum]
Function "+[TestClass randomNum]" not defined.
(gdb) ^D

$ class-dump -A test
...
@interface TestClass : NSObject
{
}

+ (int)randomNum;   // IMP=0x0000000100000e50

@end

I know I can now use b *0x0000000100000e50 in gdb, but is there a way of modifying GDB's symbol table to make it accept b +[TestClass randomNum]?

Edit: It would be preferably if it would work with GDB v6 and not only GDB v7, as GDB v6 is the latest version with Apple's patches.

Answer 1

It’s possible to load a symbol file in gdb with the add-symbol-file command. The hardest part is to produce this symbol file.

With the help of libMachObjC (which is part of class-dump), it’s very easy to dump all addresses and their corresponding Objective-C methods. I have written a small tool, objc-symbols which does exactly this.

Let’s use Calendar.app as an example. If you try to list the symbols with the nm tool, you will notice that the Calendar app has been stripped:

$ nm -U /Applications/Calendar.app/Contents/MacOS/Calendar 
0000000100000000 T __mh_execute_header
0000000005614542 - 00 0000   OPT radr://5614542

But with objc-symbols you can easily retrieve the addresses of all the missing Objective-C methods:

$ objc-symbols /Applications/Calendar.app
00000001000c774c +[CALCanvasAttributedText textWithPosition:size:text:]
00000001000c8936 -[CALCanvasAttributedText createTextureIfNeeded]
00000001000c8886 -[CALCanvasAttributedText bounds]
00000001000c883b -[CALCanvasAttributedText updateBezierRepresentation]
...
00000001000309eb -[CALApplication applicationDidFinishLaunching:]
...

Then, with SymTabCreator you can create a symbol file, which is just actually an empty dylib with all the symbols.

Using objc-symbols and SymTabCreator together is straightforward:

$ objc-symbols /Applications/Calendar.app | SymTabCreator -o Calendar.stabs

You can check that Calendar.stabs contains all the symbols:

$ nm Calendar.stabs 
000000010014a58b T +[APLCALSource printingCachedTextSize]
000000010013e7c5 T +[APLColorSource alternateGenerator]
000000010013e780 T +[APLColorSource defaultColorSource]
000000010013e7bd T +[APLColorSource defaultGenerator]
000000010011eb12 T +[APLConstraint constraintOfClass:withProperties:]
...
00000001000309eb T -[CALApplication applicationDidFinishLaunching:]
...

Now let’s see what happens in gdb:

$ gdb --silent /Applications/Calendar.app
Reading symbols for shared libraries ................................. done

Without the symbol file:

(gdb) b -[CALApplication applicationDidFinishLaunching:]
Function "-[CALApplication applicationDidFinishLaunching:]" not defined.
Make breakpoint pending on future shared library load? (y or [n]) n

And after loading the symbol file:

(gdb) add-symbol-file Calendar.stabs 
add symbol table from file "Calendar.stabs"? (y or n) y
Reading symbols from /Users/0xced/Calendar.stabs...done.
(gdb) b -[CALApplication applicationDidFinishLaunching:]
Breakpoint 1 at 0x1000309f2

You will notice that the breakpoint address does not exactly match the symbol address (0x1000309f2 vs 0x1000309eb, 7 bytes of difference), this is because gdb automatically recognizes the function prologue and sets the breakpoint just after.

---

GDB script

You can use this GDB script to automate this, given that the stripped executable is the current target.

Add the script from below to your .gdbinit, target the stripped executable and run the command objc_symbols in gdb:

$ gdb test
...
(gdb) b +[TestClass randomNum]
Function "+[TestClass randomNum]" not defined.
(gdb) objc_symbols
(gdb) b +[TestClass randomNum]
Breakpoint 1 at 0x100000ee1
(gdb) ^D

---

define objc_symbols
    shell rm -f /tmp/gdb-objc_symbols

    set logging redirect on
    set logging file /tmp/gdb-objc_symbols
    set logging on

    info target

    set logging off

    shell target="$(head -1 /tmp/gdb-objc_symbols | head -1 | awk -F '"' '{ print $2 }')"; objc-symbols "$target" | SymTabCreator -o /tmp/gdb-symtab

    set logging on
    add-symbol-file /tmp/gdb-symtab
    set logging off
end