linux capability.h如何为34个元素使用32位掩码？

Question

/usr/include/linux/capability.h中的文件#defines 34个可能的功能。 它是这样：

#define CAP_CHOWN   0 

#define CAP_DAC_OVERRIDE  1 

..... 

#define CAP_MAC_ADMIN  33 

#define CAP_LAST_CAP   CAP_MAC_ADMIN 

每个进程又正是如此定义

typedef struct __user_cap_data_struct { 

     __u32 effective; 
     __u32 permitted; 
     __u32 inheritable; 
} * cap_user_data_t; 

能力我很困惑 - 这个过程可以具有有效功能的32位，但能力在能力所限定的总量.h是34.如何在32位掩码中编码34个位置？

Answer 1

因为您还没有阅读全部手册。

的capget手动启动说服你不使用它：

These two functions are the raw kernel interface for getting and set‐ 
ting thread capabilities. Not only are these system calls specific to 
Linux, but the kernel API is likely to change and use of these func‐ 
tions (in particular the format of the cap_user_*_t types) is subject 
to extension with each kernel revision, but old programs will keep 
working. 

The portable interfaces are cap_set_proc(3) and cap_get_proc(3); if 
possible you should use those interfaces in applications. If you wish 
to use the Linux extensions in applications, you should use the easier- 
to-use interfaces capsetp(3) and capgetp(3). 

当前细节

Now that you have been warned, some current kernel details. The struc‐ 
tures are defined as follows. 

#define _LINUX_CAPABILITY_VERSION_1 0x19980330 
#define _LINUX_CAPABILITY_U32S_1  1 

#define _LINUX_CAPABILITY_VERSION_2 0x20071026 
#define _LINUX_CAPABILITY_U32S_2  2 

[...] 
effective, permitted, inheritable are bitmasks of the capabilities 
defined in capability(7). Note the CAP_* values are bit indexes and 
need to be bit-shifted before ORing into the bit fields. 
[...] 
Kernels prior to 2.6.25 prefer 32-bit capabilities with version 
_LINUX_CAPABILITY_VERSION_1, and kernels 2.6.25+ prefer 64-bit capabil‐ 
ities with version _LINUX_CAPABILITY_VERSION_2. Note, 64-bit capabili‐ 
ties use datap[0] and datap[1], whereas 32-bit capabilities only use 
datap[0]. 

其中datap较早的指针__user_cap_data_struct定义。所以你只是代表一个64bit的值，其中两个__u32在两个__user_cap_data_struct的数组中。

这个，单独告诉我永远不要使用这个API，所以我没有阅读手册的其余部分。

Answer 2

他们不是位掩码，他们只是常数。例如。 CAP_MAC_ADMIN设置多个位。在二进制中，33是什么，10001？