在Linux内核源代码情景分析-从路径名到目标节点，一文中path_walk代码中，err = permission(inode, MAY_EXEC)当前进程是否可以访问这个节点，代码如下：
int permission(struct inode * inode,int mask)在ext2_read_inode中，i_op可以设置为ext2_file_inode_operations，ext2_dir_inode_operations，ext2_fast_symlink_inode_operations，page_symlink_inode_operations，均没有permission指针。所以执行vfs_permission，代码如下：
{
if (inode->i_op && inode->i_op->permission) {
int retval;
lock_kernel();
retval = inode->i_op->permission(inode, mask);
unlock_kernel();
return retval;
}
return vfs_permission(inode, mask);
}
int vfs_permission(struct inode * inode,int mask){int mode = inode->i_mode;if ((mask & S_IWOTH) && IS_RDONLY(inode) && (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))return -EROFS; //如果要求的写文件，并且是只读系统，而且是常规文件，目录，或者链接时，返回-EROFS，表示不能访问if ((mask & S_IWOTH) && IS_IMMUTABLE(inode)) //Nobody gets write access to an immutable filereturn -EACCES; if (current->fsuid == inode->i_uid)//如果当前进程的fsuid和inode结构的i_uid相等，那么应该比较mode中S_IRUSR、S_IWUSR、S_IXUSR位mode >>= 6;else if (in_group_p(inode->i_gid))//如果当前进程的fsgid和inode结构的i_gid相等，那么应该比较mode中S_IRGRP、S_IWGRP、S_IXGRP位mode >>= 3;if (((mode & mask & S_IRWXO) == mask) || capable(CAP_DAC_OVERRIDE))//mask相当于要求，mode是现有的当前用户可以访问的权限，如果相等就返回0。假如不相等，如果当前进程得到了授权，允许其CAP_DAC_OVERRIDE，即可以凌驾于文件系统的访问权限控制机制DAC之上。return 0;/* read and search access */if ((mask == S_IROTH) || (S_ISDIR(inode->i_mode) && !(mask & ~(S_IROTH | S_IXOTH))))if (capable(CAP_DAC_READ_SEARCH))return 0;return -EACCES;}
其中mask为：
#define MAY_EXEC 1#define MAY_WRITE 2#define MAY_READ 4inode->i_mode为：
用于对三种不同用户的访问权限：
#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
参考这张图就好理解了。
还有三个标志位，是一个状态占一个位：
#define S_ISUID 0004000#define S_ISGID 0002000#define S_ISVTX 0001000现在16位只剩下4位了，表示文件类型，要为每种文件类型都分配一个标志位就不够了，所以表示文件的类型的这4位是编码的。
#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
capable，代码如下：
static inline int capable(int cap){#if 1 /* ok now */if (cap_raised(current->cap_effective, cap))#elseif (cap_is_fs_cap(cap) ? current->fsuid == 0 : current->euid == 0)#endif{current->flags |= PF_SUPERPRIV;return 1;}return 0;}
#define cap_raised(c, flag) (cap_t(c) & CAP_TO_MASK(flag))
#define cap_t(x) (x)
#define CAP_TO_MASK(x) (1 << (x))
current->cap_effective是在sys_execve->do_execve->prepare_binprm初始设置的。
int prepare_binprm(struct linux_binprm *bprm){int mode;struct inode * inode = bprm->file->f_dentry->d_inode;mode = inode->i_mode;/* Huh? We had already checked for MAY_EXEC, WTF do we check this? */if (!(mode & 0111))/* with at least _one_ execute bit set */return -EACCES;if (bprm->file->f_op == NULL)return -EACCES;bprm->e_uid = current->euid;bprm->e_gid = current->egid;if(!IS_NOSUID(inode)) {/* Set-uid? */if (mode & S_ISUID)bprm->e_uid = inode->i_uid;/* Set-gid? *//* * If setgid is set but no group execute bit then this * is a candidate for mandatory locking, not a setgid * executable. */if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))bprm->e_gid = inode->i_gid;}/* We don't have VFS support for capabilities yet */cap_clear(bprm->cap_inheritable);cap_clear(bprm->cap_permitted);cap_clear(bprm->cap_effective);/* To support inheritance of root-permissions and suid-root * executables under compatibility mode, we raise all three * capability sets for the file. * * If only the real uid is 0, we only raise the inheritable * and permitted sets of the executable file. */if (!issecure(SECURE_NOROOT)) {if (bprm->e_uid == 0 || current->uid == 0) {cap_set_full(bprm->cap_inheritable);cap_set_full(bprm->cap_permitted);}if (bprm->e_uid == 0) cap_set_full(bprm->cap_effective);//这里设置的}memset(bprm->buf,0,BINPRM_BUF_SIZE);return kernel_read(bprm->file,0,bprm->buf,BINPRM_BUF_SIZE);}