检查SUID属性以获取内核模块中的打开文件

Question

I'm writing a kernel module which needs to check the attribute suid of a given file descriptor. I checked through the various kernel structure, but I couldn't find any clue on the topic.

I expected somehow to find a field containing a bitwise Unix-stile permission, but I couldn't find any.

My goal is to write a function which just states if the file referenced by a given file descriptor has its suid attribute set or not.

int issuid(file *f){
 ...
}

My best bet would be looking into the inode structure, where I did search (wihout success), but considering the inode as the in-memory representation of a file on the physical disk, and considering some filesystem do not have Unix-style file permissions, I'm not still sure I'm looking in the right direction.

Answer 1

I was already in the right direction, but somehow, I didn't realize it.

The inode structure has a field:

umode_t                     i_mode;

Which represents the file permissions.

In the file include/uapi/linux/stat.h you can see the file permissions starting from its very beginning.

#define S_IFMT  00170000
#define S_IFSOCK 0140000
#define S_IFLNK  0120000
#define S_IFREG  0100000
#define S_IFBLK  0060000
#define S_IFDIR  0040000
#define S_IFCHR  0020000
#define S_IFIFO  0010000
#define S_ISUID  0004000
#define S_ISGID  0002000
#define S_ISVTX  0001000

#define S_ISLNK(m)  (((m) & S_IFMT) == S_IFLNK)
#define S_ISREG(m)  (((m) & S_IFMT) == S_IFREG)
#define S_ISDIR(m)  (((m) & S_IFMT) == S_IFDIR)
#define S_ISCHR(m)  (((m) & S_IFMT) == S_IFCHR)
#define S_ISBLK(m)  (((m) & S_IFMT) == S_IFBLK)
#define S_ISFIFO(m) (((m) & S_IFMT) == S_IFIFO)
#define S_ISSOCK(m) (((m) & S_IFMT) == S_IFSOCK)

#define S_IRWXU 00700
#define S_IRUSR 00400
#define S_IWUSR 00200
#define S_IXUSR 00100

#define S_IRWXG 00070
#define S_IRGRP 00040
#define S_IWGRP 00020
#define S_IXGRP 00010

#define S_IRWXO 00007
#define S_IROTH 00004
#define S_IWOTH 00002
#define S_IXOTH 00001

I hope this answer can be useful to someone like me who can't see the obvious.