gdb-python : Parsing structure's each field and print them with proper value, if exists

Question

I am writing a python script to automate debugging core dump from gdb. i am trying to print data structure which includes kernel data structures and lists(e.g. struct list_head). For example the structure is something like this:

struct my_struct {
  struct my_hardware_context ahw;
  struct net_device *netdev;
  struct pci_dev *pdev;
  struct list_head mac_list;
  ....
  ....
};

i am using following API tp print this structure:

gdb.execute('p (*(struct my_struct *)dev_base->priv)')

so i am able to print the content of 'struct my_struct' , struct my_hardware_context ahw, but not the content of pointers and list ( e.g. struct net_device *netdev, struct pci_dev *pdev, struct list_head mac_list) automatically (only address is printed). So how to print the content of *netdev, *pdev and mac_list using gdb-python script?

EDITED : to make my question more clear

I am writing a python script to automate debugging core dump from gdb. i am trying to print data structure which includes kernel data structures and lists(e.g. struct list_head). For example the structure is something like this:

struct my_struct {
   struct my_hardware_context ahw;
   struct net_device *netdev;
   struct pci_dev *pdev;
   struct list_head mac_list;
   ....
   ....
};

i am using following API to print this structure: (it can be assumed that i have right core dump and added proper symbols.

main_struct = gdb.execute('p (*(struct my_struct *)dev_base->priv)')

print main_struct

Now it will print the values of all members of struct my_struct but upto one level , meaning it will print the whole content of struct my_hardware_context ahw because it is an instance but it will not print the content of struct net_device *netdev, struct pci_dev *pdev, struct list_head mac_list etc. so now manually i need to do it like below:

netdev = gdb.parse_and_eval('*(*(struct my_struct *)dev_base->next->priv).netdev')

print netdev

pdev = gdb.parse_and_eval('*(*(struct my_struct *)dev_base->next->priv).pdev')

print pdev

so i want to automate these steps. Is there any gdb-python API or way by which it can iterate the struct my_struct and print the pointers, arrays and lists values also automatically?

Thanks.

Answer 1

struct net_device, struct pci_dev from Linux are meant to be used by kernel and not userspace code. They're not even exported in the sanitized kernel headers you get with make headers_install for use with libc.

GDB can't print struct net_device, struct pci_dev because it doesn't have debug info describing the definition of those structures. Your userspace struct my_struct is declared to have opaque pointers to those structures. I don't think you should be doing that in the first place.

Edit After Core Dump Clarification

The trick is loading debug info from both the kernel and your driver module into GDB:

• Grab a kernel with debuginfo (CONFIG_DEBUG_INFO). e.g. for Centos, get the matching kernel-debuginfo package from http://debuginfo.centos.org/6/x86_64/.
• Get the .text, .data and .bss load addresses of your driver module by inspecting /sys/module/MY-DRIVER/sections/{.text,.data,.bss} from a system running your driver under normal operation.

Assuming the kernel with debug info is located at /usr/lib/debug/lib/modules/3.9.4-200.fc18.x86_64/vmlinux, run:

$ gdb /usr/lib/debug/lib/modules/3.9.4-200.fc18.x86_64/vmlinux vmcore
(gdb) add-symbol-file MY-DRIVER.ko TEXT-ADDR -s .data DATA-ADDR -s .bss BSS-ADDR

while replacing TEXT-ADDR, DATA-ADDR and BSS-ADDR with the address from the files under /sys/module/MY-DRIVER/sections/. (I think just lying and using an address of 0 would probably work in this case)

Verify that ptype struct net_device, ptype struct pci_dev, ptype my_struct work. Then after obtaining the address of a struct *my_struct the way you did before you should be able print its contents.

Traversing a Struct While Following Pointers

print-struct-follow-pointers.py

import gdb

def is_container(v):
    c = v.type.code
    return (c == gdb.TYPE_CODE_STRUCT or c == gdb.TYPE_CODE_UNION)

def is_pointer(v):
    return (v.type.code == gdb.TYPE_CODE_PTR)

def print_struct_follow_pointers(s, level_limit = 3, level = 0):
    indent = ' ' * level

    if not is_container(s):
        gdb.write('%s\n' % (s,))
        return

    if level >= level_limit:
        gdb.write('%s { ... },\n' % (s.type,))
        return

    gdb.write('%s {\n' % (s.type,))
    for k in s.type.keys():
        v = s[k]
        if is_pointer(v):
            gdb.write('%s %s: %s' % (indent, k, v))
            try:
                v1 = v.dereference()
                v1.fetch_lazy()
            except gdb.error:
                gdb.write(',\n')
                continue
            else:
                gdb.write(' -> ')
            print_struct_follow_pointers(v1, level_limit, level + 1)
        elif is_container(v):
            gdb.write('%s %s: ' % (indent, k))
            print_struct_follow_pointers(v, level_limit, level + 1)
        else:
            gdb.write('%s %s: %s,\n' % (indent, k, v))
    gdb.write('%s},\n' % (indent,))

class PrintStructFollowPointers(gdb.Command):
    '''
    print-struct-follow-pointers [/LEVEL_LIMIT] STRUCT-VALUE
    '''
    def __init__(self): 
        super(PrintStructFollowPointers, self).__init__(
            'print-struct-follow-pointers',
            gdb.COMMAND_DATA, gdb.COMPLETE_SYMBOL, False)

    def invoke(self, arg, from_tty):
        s = arg.find('/')
        if s == -1:
            (expr, limit) = (arg, 3)
        else:
            if arg[:s].strip():
                (expr, limit) = (arg, 3)
            else:
                i = s + 1
                for (i, c) in enumerate(arg[s+1:], s + 1):
                    if not c.isdigit():
                        break
                end = i
                digits = arg[s+1:end]
                try:
                    limit = int(digits)
                except ValueError:
                    raise gdb.GdbError(PrintStructFollowPointers.__doc__)
                (expr, limit) = (arg[end:], limit)
        try:
            v = gdb.parse_and_eval(expr)
        except gdb.error, e:
            raise gdb.GdbError(e.message)

        print_struct_follow_pointers(v, limit)

PrintStructFollowPointers()

Sample Session

(gdb) source print-struct-follow-pointers.py
(gdb) print-struct-follow-pointers *p

You can limit the levels of embedded structures printed:

(gdb) print-struct-follow-pointers/4 *p