Get path from file descriptor when path is longer than PATH_MAX

Question

I receive filesystem events from fanotify. Sometimes I want to get an absolute path to a file that's being accessed.

Usually, it's not a problem - fanotify_event_metadata contains a file descriptor fd, so I can call readlink on /proc/self/fd/<fd> and get my path.

However, if a path exceeds PATH_MAX readlink can no longer be used - it fails with ENAMETOOLONG. I'm wondering if there's a way to get a file path in this case.

Obviously, I can fstat the descriptor I get from a fanotify and traverse the entire filesystem looking for files with identical device ID and inode number. But this approach is not feasible for me performance-wise (even if I optimize it to ignore paths shorter than PATH_MAX).

I've tried getting a parent directory by reopening fd with O_PATH and calling openat(fd, "..", ...). Obviously, that failed because fd doesn't refer to a directory. I've also tried examining contents of a buffer after a failed readlink call (hoping it contains partial path). That didn't work either.

So far I've managed to get long paths for files inside the working directory of a process that opened them (fanotify events contain a pid of a target process, so I can read /proc/<pid>/cwd and get the path to the root from there). But that is a partial solution.

Is there a way to get an absolute path from a file descriptor without traversing the whole filesystem? Preferably the one that will work with kernel 2.6.32/glibc 2.11.

Update: For the curious. I've figured out why calling readlink("/proc/self/fd/<fd>", ... with a buffer large enough to store the entire path doesn't work.

Look at the implementation of do_proc_readlink. Notice that it doesn't use provided buffer directly. Instead, it allocates a single page and uses it as a temporary buffer when it calls d_path. In other words, no matter how large is buffer, d_path will always be limited to a size of a page. Which is 4096 bytes on amd64. Same as PATH_MAX! The -ENAMETOOLONG itself is returned by prepend when it runs out of mentioned page.

Answer 1

readlink can be used with a link target that's longer than PATH_MAX. There are two restrictions: the name of the link itself must be shorter than PATH_MAX (check, "/proc/self/fd/<fd>" is about 20 characters) and the provided output buffer must be large enough. You might want to call lstat first to figure out how big the output buffer should be, or just call readlink repeatedly with growing buffers.