linux-kernel - Re: [PATCH 3/3] autofs - fix AT_NO

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Date:   Thu, 23 Nov 2017 11:04:06 +0800
From:   Ian Kent <raven@...maw.net>
To:     NeilBrown <neilb@...e.com>, Al Viro <viro@...IV.linux.org.uk>
Cc:     Colin Walters <walters@...hat.com>, Ondrej Holy <oholy@...hat.com>,
        autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 3/3] autofs - fix AT_NO_AUTOMOUNT not being honored

On 23/11/17 10:46, Ian Kent wrote:
> On 23/11/17 10:21, NeilBrown wrote:
>> On Thu, Nov 23 2017, Ian Kent wrote:
>>
>>>
>>> Hey Neil, I'm looking at this again because RH QE have complained about
>>> a regression test failing with a kernel that has this change.
>>>
>>> Maybe I'm just dumb but I though a "find <base directory> <options>"
>>> would, well, just look at the contents below <base directory> but an
>>> strace shows that it reads and calls fstatat() on "every entry in the
>>> mount table" regardless of the path.
>>
>> weird ... I can only get find to look at the mount table if given the
>> -fstyp option, and even then it doesn't fstatat anything that isn't in
>> the tree it is searching.
> 
> It's probably the -xautofs (exclude autofs fs'es) that was used in
> the test that requires reading the mount table to get info about
> excluding autofs mounts but the fstatat() on all the entries,
> regardless of path, that was a surprise to me.
> 
> find did use AT_SYMLINK_NOFOLLOW which historically behaved like
> AT_NO_AUTOMOUNT.

Check this simple reproducer out:

[root@...el-mahobay-02 ~]# uname -r
4.14.0-0.rc8.2.el7a.x86_64
[root@...el-mahobay-02 ~]# cat /etc/exports
/exportdir-bz1482065 *(rw,no_root_squash)
[root@...el-mahobay-02 ~]# cat /etc/auto.master
/- /etc/auto.direct
[root@...el-mahobay-02 ~]# cat /etc/auto.direct 
/mnt/autofs/test localhost:/exportdir-bz1482065
[root@...el-mahobay-02 ~]# ls /mnt/autofs/
bz1482065  test
[root@...el-mahobay-02 ~]# ls /exportdir-bz1482065
bz1482065
[root@...el-mahobay-02 ~]# find /mnt/autofs -xautofs  -name '*bz1482065*'
[root@...el-mahobay-02 ~]# find /mnt/autofs -xautofs  -name '*bz1482065*'
/mnt/autofs/bz1482065
/mnt/autofs/test/bz1482065

The first find mounts the direct mount /mnt/autofs/test/bz1482065 which causes the
second to list /mnt/autofs/bz1482065 (not sure yet where this comes from, it must
be a plain directory within /mnt/autofs) and the now mounted bind mount (ie. the
localhost: host name in the map entry) /mnt/autofs/test/bz1482065.

The strace is:
execve("/usr/bin/find", ["find", "/mnt/autofs", "-xautofs", "-name", "*bz1482065*"], [/* 45 vars */]) = 0

snip ...

newfstatat(AT_FDCWD, "/mnt/autofs", {st_mode=S_IFDIR|0755, st_size=35, ...}, AT_SYMLINK_NOFOLLOW) = 0
open("/etc/mtab", O_RDONLY|O_CLOEXEC)   = 4
futex(0x7efec9ba55b0, FUTEX_WAKE_PRIVATE, 2147483647) = 0
fstat(4, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7efeca2ea000
read(4, "sysfs /sys sysfs rw,seclabel,nos"..., 1024) = 1024
read(4, "oup/net_cls,net_prio cgroup rw,n"..., 1024) = 1024
read(4, "ges hugetlbfs rw,seclabel,relati"..., 1024) = 594
read(4, "", 1024)                       = 0
close(4)                                = 0
munmap(0x7efeca2ea000, 4096)            = 0
newfstatat(AT_FDCWD, "/sys", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/proc", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/dev", {st_mode=S_IFDIR|0755, st_size=3280, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/kernel/security", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/dev/shm", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=40, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/dev/pts", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/run", {st_mode=S_IFDIR|0755, st_size=1020, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup", {st_mode=S_IFDIR|0755, st_size=340, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/systemd", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/pstore", {st_mode=S_IFDIR|0750, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/blkio", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/cpu,cpuacct", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/pids", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/net_cls,net_prio", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/devices", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/freezer", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/perf_event", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/cpuset", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/memory", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/cgroup/hugetlb", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/kernel/config", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/", {st_mode=S_IFDIR|0555, st_size=4096, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/fs/selinux", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/proc/sys/fs/binfmt_misc", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/sys/kernel/debug", {st_mode=S_IFDIR|0700, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/dev/mqueue", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=40, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/dev/hugepages", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/proc/fs/nfsd", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/boot", {st_mode=S_IFDIR|0555, st_size=4096, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/home", {st_mode=S_IFDIR|0755, st_size=18, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/var/lib/nfs/rpc_pipefs", {st_mode=S_IFDIR|0555, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/run/user/0", {st_mode=S_IFDIR|0700, st_size=40, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/proc/sys/fs/binfmt_misc", {st_mode=S_IFDIR|0755, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(AT_FDCWD, "/mnt/autofs/test", {st_mode=S_IFDIR|0755, st_size=23, ...}, AT_SYMLINK_NOFOLLOW) = 0
fchdir(3)                               = 0
close(3)                                = 0
close(1)                                = 0
close(2)                                = 0
exit_group(0)                           = ?