Expected behavior of gdb.lookup_symbol()

Gulshan Singh gsingh2011@gmail.com
Wed Oct 12 03:31:46 GMT 2022


Hi,

I've noticed that the `gdb.lookup_symbol()` function in the Python API
returns None for certain types of symbols. For example:

$ gdb -q -nx -ex 'catch load libc' -ex 'run' /bin/ls
Reading symbols from /bin/ls...
(No debugging symbols found in /bin/ls)
Catchpoint 1 (load)
Starting program: /usr/bin/ls
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Catchpoint 1
  Inferior loaded /lib/x86_64-linux-gnu/libselinux.so.1
    /lib/x86_64-linux-gnu/libc.so.6
    /lib/x86_64-linux-gnu/libpcre2-8.so.0
__GI__dl_debug_state () at ./elf/dl-debug.c:116
116     ./elf/dl-debug.c: No such file or directory.
(gdb) p __libc_malloc
$1 = {void *(size_t)} 0x7ffff7df3120 <__GI___libc_malloc>
(gdb) pi gdb.lookup_symbol('__libc_malloc')
(None, False)
(gdb) pi gdb.lookup_symbol('__GI___libc_malloc')
(<gdb.Symbol object at 0x7f896853e760>, False)

Here I was expecting both calls to `gdb.lookup_symbol()` to return a
`gdb.Symbol` object. I have debugging symbols installed. I determined the
name of the file containing the debug symbols like this:

(gdb) pi gdb.lookup_symbol('__GI___libc_malloc')[0].symtab.objfile
<gdb.Objfile
filename=/usr/lib/debug/.build-id/69/389d485a9793dbe873f0ea2c93e02efaa9aa3d.debug>

And I looked for those symbols in that file like this:

$ readelf -a
/usr/lib/debug/.build-id/69/389d485a9793dbe873f0ea2c93e02efaa9aa3d.debug |
grep __libc_malloc
  5933: 00000000000a5120   828 FUNC    LOCAL  DEFAULT   15
__GI___libc_malloc
  9639: 00000000000a5120   828 FUNC    GLOBAL DEFAULT   15 __libc_malloc

It seems that the only difference is one is a local binding and one is a
global binding. Is this expected behavior? If so, why? And is there another
way to get the address of a symbol like this, or should I just resort to
calling `info address` with `gdb.execute()`?

My GDB version is GNU gdb (Ubuntu 12.0.90-0ubuntu1) 12.0.90, and I'm on
Ubuntu 22.04.1 LTS.


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