makeStoreWritable: Ask forgiveness, not permission

It is surprisingly impossible to check if a mountpoint is a bind mount
on Linux, and in my previous commit I forgot to check if /nix/store was
even a mountpoint at all. statvfs.f_flag is not populated with MS_BIND
(and even if it were, my check was wrong in the previous commit).

Luckily, the semantics of mount with MS_REMOUNT | MS_BIND make both
checks unnecessary: if /nix/store is not a mountpoint, then mount will
fail with EINVAL, and if /nix/store is not a bind-mount, then it will
not be made writable. Thus, if /nix/store is not a mountpoint, we fail
immediately (since we don't know how to make it writable), and if
/nix/store IS a mountpoint but not a bind-mount, we fail at first write
(see below for why we can't check and fail immediately).

Note that, due to what is IMO buggy behavior in Linux, calling mount
with MS_REMOUNT | MS_BIND on a non-bind readonly mount makes the
mountpoint appear writable in two places: In the sixth (but not the
10th!) column of mountinfo, and in the f_flags member of struct statfs.
All other syscalls behave as if the mount point were still readonly (at
least for Linux 3.9-rc1, but I don't think this has changed recently or
is expected to soon). My preferred semantics would be for MS_REMOUNT |
MS_BIND to fail on a non-bind mount, as it doesn't make sense to remount
a non bind-mount as a bind mount.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 4086e553c0a9..687ad7e47f19 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -437,12 +437,12 @@ void LocalStore::makeStoreWritable()
 {
 #if HAVE_UNSHARE && HAVE_STATVFS && HAVE_SYS_MOUNT_H && defined(MS_BIND) && defined(MS_REMOUNT)
     if (getuid() != 0) return;
-    /* Check if /nix/store is a read-only bind mount. */
+    /* Check if /nix/store is on a read-only mount. */
     struct statvfs stat;
     if (statvfs(settings.nixStore.c_str(), &stat) !=0)
         throw SysError("Getting info of nix store mountpoint");
 
-    if (stat.f_flag & (ST_RDONLY | MS_BIND)) {
+    if (stat.f_flag & ST_RDONLY) {
         if (unshare(CLONE_NEWNS) == -1)
             throw SysError("setting up a private mount namespace");