Re: [musl] getdents64 lost direntries with SMB/NFS and buffer size < unknown threshold

Florian Weimer
2019-11-20T19:57:44Z

* Rich Felker:

> An issue was reported today on the Alpine Linux tracker at
> https://gitlab.alpinelinux.org/alpine/aports/issues/10960 regarding
> readdir results from SMB/NFS shares with musl libc.
>
> After a good deal of analysis, we determined the root cause to be that
> the second and subsequent calls to getdents64 are dropping/skipping
> direntries (that have not yet been deleted) when some entries were
> deleted following the previous call. The issue appears to happen only
> when the buffer size passed to getdents64 is below some threshold
> greater than 2k (the size musl uses) but less than 32k (the size glibc
> uses, with which we were unable to reproduce the issue).

From the Gitlab issue:

while ((dp = readdir(dir)) != NULL) {
unlink(dp->d_name);
++file_cnt;
}

I'm not sure that this is valid code to delete the contents of a
directory. It's true that POSIX says this:

| If a file is removed from or added to the directory after the most
| recent call to opendir() or rewinddir(), whether a subsequent call
| to readdir() returns an entry for that file is unspecified.

But many file systems simply provide not the necessary on-disk data
structures which are need to ensure stable iteration in the face of
modification of the directory. There are hacks, of course, such as
compacting the on-disk directory only on file creation, which solves
the file removal case.

For deleting an entire directory, that is not really a problem because
you can stick another loop around this while loop which re-reads the
directory after rewinddir. Eventually, it will become empty.