[PATCH 4/4] resolve_ref_unsafe(): close race condition reading loose refs

We read loose references in two steps.  The code is roughly:

    lstat()
    if error ENOENT:
        loose ref is missing; look for corresponding packed ref
    else if S_ISLNK:
        readlink()
        if error:
            report failure
    else if S_ISDIR:
        report failure
    else
        open()
        if error:
            report failure
        read()

The problem is that the first filesystem call, to lstat(), is not atomic with the second filesystem call, to readlink() or open(). Therefore it is possible for another process to change the file between our two calls, for example:

* If the other process deletes the file, our second call will fail
  with ENOENT, which we *should* interpret as "loose ref is missing;
  look for corresponding packed ref".  This can arise if the other
  process is pack-refs; it might have just written a new packed-refs
  file containing the old contents of the reference then deleted the
  loose ref.

* If the other process changes a symlink into a plain file, our call
  to readlink() will fail with EINVAL, which we *should* respond to by
  trying to open() and read() the file.

The old code treats the reference as missing in both of these cases, which is incorrect.

So instead, wrap the above code in a loop. If the result of readline()/open() is a failure that is inconsistent with the result of the previous lstat(), then just loop again starting with a fresh call to lstat().

One race is still possible and undetected: another process could change the file from a regular file into a symlink between the call to lstat and the call to open(). The open() call would silently follow the symlink and not know that something is wrong. I don't see a way to detect this situation without the use of the O_NOFOLLOW option, which is not portable and is not used elsewhere in our code base.

However, we don't use symlinks anymore, so this situation is unlikely. And it doesn't appear that treating a symlink as a regular file would have grave consequences; after all, this is exactly how the code handles non-relative symlinks.

Note that this solves only the part of the race within resolve_ref_unsafe. In the situation described above, we may still be depending on a cached view of the packed-refs file; that race will be dealt with in a future patch.

This problem was reported and diagnosed by Jeff King <peff@peff.net>, and this solution is derived from his patch.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
Please note that if there is some bizarre filesystem somewhere for
which, for a single, static file

    lstat() reports S_ISLNK and readlink() fails with ENOENT or EINVAL

or

    lstat() reports neither S_ISLNK nor S_ISDIR and open() fails with ENOENT

then the inner loop would never terminate. I can't imagine this happening, but if somebody is worried about it the solution is simple: limit the inner loop to 3 iterations or so.

Another obvious way to solve the problem would be to skip the lstat() call altogether, and to rely on errno to distinguish the cases:

    readlink()
    if error ENOENT:
        loose ref is missing; look for corresponding packed ref
    else if error EINVAL:
        must not be a symlink; fall through to open()
    else if other error:
        report failure
    else:
        handle as symlink

    open()
    if error ENOENT:
        file must have been deleted since readlink(); look for packed ref
    else if other error:
        report failure
    else:
        read() and handle file contents

There is still a gap between readlink() and open(), during which the file could have been changed from a regular file into a symlink, so the remaining race mentioned above would still be undetected. I don't see a strong reason to prefer the looping code in this patch vs. the no-lstat() alternative so I took the variant that was closer to the old code.

 refs.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)