Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
>
> Since March 2009 the kernel has treated the state that if no
> MS_..ATIME flags are passed then the kernel defaults to relatime.
>
> Defaulting to relatime instead of the existing atime state during a
> remount is silly, and causes problems in practice for people who don't
> specify any MS_...ATIME flags and to get the default filesystem atime
> setting. Those users may encounter a permission error because the
> default atime setting does not work.
>
> A default that does not work and causes permission problems is
> ridiculous, so preserve the existing value to have a default
> atime setting that is always guaranteed to work.
>
> Using the default atime setting in this way is particularly
> interesting for applications built to run in restricted userspace
> environments without /proc mounted, as the existing atime mount
> options of a filesystem can not be read from /proc/mounts.
>
> In practice this fixes user space that uses the default atime
> setting on remount that are broken by the permission checks
> keeping less privileged users from changing more privileged users
> atime settings.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Acked-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
> ---
> fs/namespace.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index dd9c93b5a9d5..7886176232c1 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2473,6 +2473,14 @@ long do_mount(const char *dev_name, const char *dir_name,
> if (flags & MS_RDONLY)
> mnt_flags |= MNT_READONLY;
>
> + /* The default atime for remount is preservation */
> + if ((flags & MS_REMOUNT) &&
> + ((flags & (MS_NOATIME | MS_NODIRATIME | MS_RELATIME |
> + MS_STRICTATIME)) == 0)) {
> + mnt_flags &= ~MNT_ATIME_MASK;
> + mnt_flags |= path.mnt->mnt_flags & MNT_ATIME_MASK;
> + }
> +
> flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
> MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
> MS_STRICTATIME);
> --
> 1.9.1
>
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