On Do, 22.08.24 17:06, Florian Weimer (fweimer@xxxxxxxxxx) wrote: > > You don't really have to live with such a limitation. In systemd we > > have code that works around this limitation via O_PATH. i.e. when > > connect()ing you first open the socket inode with O_PATH, and then you > > fire the connect() specifying /proc/self/fd/<fd> as path. That always > > fits into the 108ch limit. > > Do you think this is useful more generally? What about opening files > with names longer than PATH_MAX? Internally in systemd, we have a function chase() which basically does something like canonicalize_file_name(), but built around O_PATH, and ultimately returning an O_PATH fd. It resolves paths in userspace, taking alternative roots into account, being careful with symlinks and so on. As a side effect this means we do not have any limits on paths lengths anywhere. I mean, things like this I sense are really fundamental if you want to safely use the POSIX file system API, without risking being tricked into opening/creating files one shouldn't use by the various tricks available. But I have no illusions there: I think it's really tough to wrap this into a public libc-level API. > > bind()ing to an overly long unix socket path is also doable, but > > harder (since you cannot O_PATH on an inode that doesn't exist > > yet). The way I'd do it is via chdir() to the dir of the target path > > and binding to a relative path then. But chdir() is of course icky, > > since it's a global property of a process, hence will affect all > > threads. Hence, maybe do this in a short-lived forked off process. > > I would have expected that it's possible to use a directory descriptor > under /proc/self/fd as the base, but that doesn't seem to work for some > reason. Well, the path gets propagated into what /proc/net/unix shows, hence I don't think going via /proc/self/fd/ is desriable there. I think the best option is to use a relative bind then, via chdir() as I mentioned. It will then just show a relative path, which is not perfect, but at least communicates clearly what's going on, and still is descriptive.
