On Sun, Aug 27, 2023 at 3:41 PM Florian Weimer <fweimer@xxxxxxxxxx> wrote: > To what degree is this dependent on the file system? Does the VFS layer > restrict these error codes for anything else? I don't think so. Maybe > strictly speaking, the added wording is still accurate, but the > conclusion that ENXIO means socket would be incorrect draw, I think. -ENXIO is returned by no_open() in fs/inode.c, which is automatically set as the ->open() function (by inode_init_always()) for every inode that isn't a symlink and doesn't set its ->i_fop to something else. As far as I am aware, every filesystem uses this fallback only for files of socket type; all non-socket inodes (except for dummy inodes) set either ->i_fop or ->i_op->read_link(). Note that this doesn't apply only to Unix domain sockets, but to all sockets, if one attempts to open() them via their links in /proc/pid/fd. Open sockets in another process can only be duplicated (AFAIK) using pidfd_getfd() or SCM_RIGHTS. Of course, some filesystems also return -ENXIO for their own error conditions, e.g., opening a pipe for writing with O_NONBLOCK when it has no readers. Matthew House
