"mount -o remount" does not need to validate the target directory for the same reasons that "umount" doesn't. It just needs to find the mountpoint and verify it is a mountpount.
So change do_mount() to use user_path_mountpoint_at() in the MS_REMOUNT case. This means that mount(.., MS_REMOUNT|MS_RO, ..) on a network attached filesystem will only access the network if there is data to be written out. This reduces the chance of a hang when the network is down. Systemd-shutdown currently calls mount(NULL, path, NULL, MS_REMOUNT|MS_RDONLY, ...); umount2(path, 0); which is not unreasonable for local filesystems which may still be in use, but causes a hang for NFS filesystems which have not been unmounted. Note that this change does not affect /usr/bin/mount -o remount,ro ... as that currently calls "lstat()" on the mount point. Signed-off-by: NeilBrown <ne...@suse.com> --- fs/namespace.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index f8893dc6a989..31ded3a1cdff 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -2718,7 +2718,10 @@ long do_mount(const char *dev_name, const char __user *dir_name, ((char *)data_page)[PAGE_SIZE - 1] = 0; /* ... and get the mountpoint */ - retval = user_path(dir_name, &path); + if (flags & MS_REMOUNT) + retval = user_path_mountpoint_at(AT_FDCWD, dir_name, 0, &path); + else + retval = user_path(dir_name, &path); if (retval) return retval; -- 2.12.2
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