526互联

crash —— 查看数据结构内部成员的偏移量和地址

发布时间 2023-09-24 10:54:27作者: 摩斯电码

whatis

如果提前知道数据类型的定义，可以直接用struct、union等，否则可以直接用whatis。

crash> whatis -o page
struct page {
   [0] unsigned long flags;
       union {
           struct {
               union {
   [8]             struct list_head lru;
                   struct {
   [8]                 void *__filler;
  [16]                 unsigned int mlock_count;
                   };
   [8]             struct list_head buddy_list;
   [8]             struct list_head pcp_list;
               };
  [24]         struct address_space *mapping;
               union {
  [32]             unsigned long index;
  [32]             unsigned long share;
               };
  [40]         unsigned long private;
           };
           struct {
   [8]         unsigned long pp_magic;
  [16]         struct page_pool *pp;
  [24]         unsigned long _pp_mapping_pad;
  [32]         unsigned long dma_addr;
               union {
  [40]             unsigned long dma_addr_upper;
  [40]             atomic_long_t pp_frag_count;
               };
           };
...
  [52] atomic_t _refcount;
  [56] unsigned long memcg_data;
}
SIZE: 64

struct

上面显示page是struct类型，那么也可以直接用struct，struct的输出格式更丰富，这里struct也可以用*代替。

crash> *page -xo
struct page {
   [0x0] unsigned long flags;
         union {
             struct {
                 union {
   [0x8]             struct list_head lru;
                     struct {
   [0x8]                 void *__filler;
  [0x10]                 unsigned int mlock_count;
                     };
   [0x8]             struct list_head buddy_list;
   [0x8]             struct list_head pcp_list;
                 };
  [0x18]         struct address_space *mapping;
                 union {
  [0x20]             unsigned long index;
  [0x20]             unsigned long share;
                 };
  [0x28]         unsigned long private;
             };
...
  [0x34] atomic_t _refcount;
  [0x38] unsigned long memcg_data;
}
SIZE: 0x40

上面显示的是结构体成员的的偏移，如果我们已经知道某个结构体变量的地址，那么可以可以用下面的方法获取其中每个成员的地址：

crash> *page -ox ffffea0000000440
struct page {
  [ffffea0000000440] unsigned long flags;
         union {
             struct {
                 union {
  [ffffea0000000448]             struct list_head lru;
                     struct {
  [ffffea0000000448]                 void *__filler;
  [ffffea0000000450]                 unsigned int mlock_count;
                     };
  [ffffea0000000448]             struct list_head buddy_list;
  [ffffea0000000448]             struct list_head pcp_list;
                 };
  [ffffea0000000458]         struct address_space *mapping;
                 union {
  [ffffea0000000460]             unsigned long index;
  [ffffea0000000460]             unsigned long share;
                 };
  [ffffea0000000468]         unsigned long private;
             };
...
  [ffffea0000000474] atomic_t _refcount;
  [ffffea0000000478] unsigned long memcg_data;
}
SIZE: 0x40

完。