According to the POSIX standard，symbolic links can always be read, except that the value of the file mode bits returned in the st_mode field of the stat structure is unspecified.
Linux implemented like this: stat() always returns st_mode 0777 for almost symbolic link files, but on FreeBSD returns its own real permission.
Some linux programs do obtain the symbol link file permissions in proc/sys or sys folders and check whether it is equal to 777, so I think maybe linux_stat return 0777 better at this time.
BTW.
BTW, I found two exceptions on Linux, I don’t know if there are any other exceptions.
1. lr-x------ /proc/xxx/fd/yyy
If file is open for reading, mode is S_IRUSR | S_IXUSR.
If file is open for writing, mode is S_IWUSR | S_IXUSR.
linux code 5.7-rc1:
static void tid_fd_update_inode(struct task_struct *task, struct inode *inode,```
static void tid_fd_update_inode(struct task_struct *task, struct inode *inode, fmode_t f_mode)
{
task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
if (S_ISLNK(inode->i_mode)) {
unsigned i_mode = S_IFLNK;
if (f_mode & FMODE_READ)
i_mode |= S_IRUSR | S_IXUSR;
if (f_mode & FMODE_WRITE)
i_mode |= S_IWUSR | S_IXUSR;
inode->i_mode = i_mode;
}
security_task_to_inode(task, inode);
}
```
In this case, we let fdescfs handle it.
2. lr-------- /proc/aaa/map_files/bbb-ccc
If file is open for reading, mode is S_IRUSR.
If file is open for writing, mode is S_IWUSR.
linux code 5.7-rc1:
```
static struct dentry *
proc_map_files_instantiate(struct dentry *dentry, struct task_struct *task, const void *ptr)
struct task_struct *task, const void *ptr)
{
fmode_t mode = (fmode_t)(unsigned long)ptr;
struct proc_inode *ei;
struct inode *inode;
inode = proc_pid_make_inode(dentry->d_sb, task, S_IFLNK |
((mode & FMODE_READ ) ? S_IRUSR : 0) |
((mode & FMODE_WRITE) ? S_IWUSR : 0));
if (!inode)
return ERR_PTR(-ENOENT);
ei = PROC_I(inode);
ei->op.proc_get_link = map_files_get_link;
inode->i_op = &proc_map_files_link_inode_operations;
inode->i_size = 64;
d_set_d_op(dentry, &tid_map_files_dentry_operations);
return d_splice_alias(inode, dentry);
}
```
In this case, we don't have such files and just skip.