gdb python中的container_of宏

Question

我正在尝试访问内核链表，结构是

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

};

使用 gdb，我可以通过以下方式打印：

(gdb)p *(qlcnic_wait_event_t *)(((struct my_struct *)dev_base->next->priv).wait_list)

输出是：

$17 = {
list = {
  next = 0x410026a14ff0,
  prev = 0x410026a14ff0
},
comp_id = 0x0,
trigger = 0x0,
active = 0x0,
rsp_word = 0x0 <buses_init at vmkdrivers/src_9/vmklinux_9/linux/drivers/base/bus.c:1061>

}

要迭代列表，我需要转到“下一步”wait_list并使用“container_of”，获取地址的基址。所以我使用 container_of 宏，代码是

#!/usr/bin/env python
import gdb

long_type = None

def get_type(type_name):
        t = gdb.lookup_type(type_name)
        if t == None:
         raise gdb.GdbError("cannot resolve type '%s'" % type_name)
        return t

def get_long_type():
        global long_type
        if long_type == None:
          long_type = get_type("long")
        return long_type

def offset_of(typeobj, field):
        element = gdb.Value(0).cast(typeobj)
        return int(str(element[field].address), 16)

def container_of(ptr, typeobj, member):
        return (ptr.cast(get_long_type()) - offset_of(typeobj, member)).cast(typeobj)


class ContainerOf(gdb.Function):
        __doc__ = "Return pointer to containing data structure.\n" \
         "\n" \
         "$container_of(PTR, \"TYPE\", \"ELEMENT\"): Given PTR, return a pointer to the\n" \
         "data structure of the type TYPE in which PTR is the address of ELEMENT.\n" \
     "Note that TYPE and ELEMENT have to be quoted as strings."

        def __init__(self):
        super(ContainerOf, self).__init__("container_of")

        def invoke(self, ptr, typename, elementname):
         return container_of(ptr,
        gdb.lookup_type(typename.string()).pointer(),
        elementname.string())

ContainerOf()
ptr = gdb.parse_and_eval('(qlcnic_wait_event_t *)(((struct my_struct *)dev_base->next->priv).wait_list)').address
print '%s'%(ptr)
c = container_of(ptr,"qlcnic_wait_event_t","list")

做完之后 (gdb) source container_of.py

输出是：

wait_list = {
 list = {
   next = 0x410026a14ff0,
   prev = 0x410026a14ff0
 },
 comp_id = 0x0,
 trigger = 0x0,
 active = 0x0,
 rsp_word = 0x0 <buses_init at /src_9/linux_9/drivers/base/bus.c:1061>
}
ptr = 0x410026a14ff0
Traceback (most recent call last):
  File "container_of.py", line 64, in ?
    next = container_of(ptr,"struct qlcnic_wait_event_s","list")
  File "container_of.py", line 23, in container_of
    return (ptr.cast(get_long_type()) - offset_of(typeobj, member)).cast(typeobj)
  File "container_of.py", line 19, in offset_of
    element = gdb.Value(0).cast(typeobj)
RuntimeError: Argument must be a type.

为什么它不起作用？如何实现这个 container_of ？

Answer 1

您的代码的问题在于 cast() 此时需要 gdb.Type，而不是字符串。调用 gdb.lookup_type() 将修复该部分。

关于 offsetof/container_of：到目前为止，最方便的方法是使用 gdb 的宏工具。这只是因为访问函数或命令更容易。

(gdb) macro define offsetof(_type, _memb) \
      ((long)(&((_type *)0)->_memb))
(gdb) macro define container_of(_ptr, _type, _memb) \
      ((_type *)((void *)(_ptr) - offsetof(_type, _memb)))

这可能会进入您的 .gdbinit。

(gdb) print offsetof(struct foo, bar)
...

要在 Python 中使用它，您可以以与开始时相同的方式重新实现它，使用 gdb.Type/Field 和强制转换操作，或者再次依赖宏定义：

(gdb) python print gdb.parse_and_eval("offsetof(struct foo, bar)")
...