From cc63db1dd5c37aead3e3d2e20e2d2f548cc24830 Mon Sep 17 00:00:00 2001 From: Shea Levy Date: Thu, 7 Mar 2013 22:53:49 -0500 Subject: [PATCH] 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. --- src/libstore/local-store.cc | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc index 4086e553c..687ad7e47 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");