BPF Archive on lore.kernel.org
 help / color / Atom feed
From: Carlos Antonio Neira Bustos <cneirabustos@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"ebiederm@xmission.com" <ebiederm@xmission.com>,
	"brouer@redhat.com" <brouer@redhat.com>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf-next v10 2/4] bpf: new helper to obtain namespace data from current task New bpf helper bpf_get_current_pidns_info.
Date: Mon, 9 Sep 2019 14:45:22 -0300
Message-ID: <20190909174522.GA17882@frodo.byteswizards.com> (raw)
In-Reply-To: <7d196a64-cf36-c2d5-7328-154aaeb929eb@fb.com>

Thanks a lot, Al Viro and Yonghong for taking the time to review this patch and
provide technical insights needed on this one.
But how do we move this forward? 
Al Viro's review is clear that this will not work and we should strip the name 
resolution code (thanks for your detailed analysis).
As there is currently only one instance of the nsfs device on the system,  
I think we could leave out the retrieval of the pidns device number and address it
when the situation changes.
What do you think?


On Sat, Sep 07, 2019 at 06:34:39AM +0000, Yonghong Song wrote:
> 
> 
> On 9/6/19 5:10 PM, Al Viro wrote:
> > On Fri, Sep 06, 2019 at 11:21:14PM +0000, Yonghong Song wrote:
> > 
> >> -bash-4.4$ readlink /proc/self/ns/pid
> >> pid:[4026531836]
> >> -bash-4.4$ stat /proc/self/ns/pid
> >>     File: ‘/proc/self/ns/pid’ -> ‘pid:[4026531836]’
> >>     Size: 0               Blocks: 0          IO Block: 1024   symbolic link
> >> Device: 4h/4d   Inode: 344795989   Links: 1
> >> Access: (0777/lrwxrwxrwx)  Uid: (128203/     yhs)   Gid: (  100/   users)
> >> Context: user_u:base_r:base_t
> >> Access: 2019-09-06 16:06:09.431616380 -0700
> >> Modify: 2019-09-06 16:06:09.431616380 -0700
> >> Change: 2019-09-06 16:06:09.431616380 -0700
> >>    Birth: -
> >> -bash-4.4$
> >>
> >> Based on a discussion with Eric Biederman back in 2019 Linux
> >> Plumbers, Eric suggested that to uniquely identify a
> >> namespace, device id (major/minor) number should also
> >> be included. Although today's kernel implementation
> >> has the same device for all namespace pseudo files,
> >> but from uapi perspective, device id should be included.
> >>
> >> That is the reason why we try to get device id which holds
> >> pid namespace pseudo file.
> >>
> >> Do you have a better suggestion on how to get
> >> the device id for 'current' pid namespace? Or from design, we
> >> really should not care about device id at all?
> > 
> > What the hell is "device id for pid namespace"?  This is the
> > first time I've heard about that mystery object, so it's
> > hard to tell where it could be found.
> > 
> > I can tell you what device numbers are involved in the areas
> > you seem to be looking in.
> > 
> > 1) there's whatever device number that gets assigned to
> > (this) procfs instance.  That, ironically, _is_ per-pidns, but
> > that of the procfs instance, not that of your process (and
> > those can be different).  That's what you get in ->st_dev
> > when doing lstat() of anything in /proc (assuming that
> > procfs is mounted there, in the first place).  NOTE:
> > that's lstat(2), not stat(2).  stat(1) uses lstat(2),
> > unless given -L (in which case it's stat(2) time).  The
> > difference:
> > 
> > root@kvm1:~# stat /proc/self/ns/pid
> >    File: /proc/self/ns/pid -> pid:[4026531836]
> >    Size: 0               Blocks: 0          IO Block: 1024   symbolic link
> > Device: 4h/4d   Inode: 17396       Links: 1
> > Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
> > Access: 2019-09-06 19:43:11.871312319 -0400
> > Modify: 2019-09-06 19:43:11.871312319 -0400
> > Change: 2019-09-06 19:43:11.871312319 -0400
> >   Birth: -
> > root@kvm1:~# stat -L /proc/self/ns/pid
> >    File: /proc/self/ns/pid
> >    Size: 0               Blocks: 0          IO Block: 4096   regular empty file
> > Device: 3h/3d   Inode: 4026531836  Links: 1
> > Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
> > Access: 2019-09-06 19:43:15.955313293 -0400
> > Modify: 2019-09-06 19:43:15.955313293 -0400
> > Change: 2019-09-06 19:43:15.955313293 -0400
> >   Birth: -
> > 
> > The former is lstat, the latter - stat.
> > 
> > 2) device number of the filesystem where the symlink target lives.
> > In this case, it's nsfs and there's only one instance on the entire
> > system.  _That_ would be obtained by looking at st_dev in stat(2) on
> > /proc/self/ns/pid (0:3 above).
> > 
> > 3) device number *OF* the symlink.  That would be st_rdev in lstat(2).
> > There's none - it's a symlink, not a character or block device.  It's
> > always zero and always will be zero.
> > 
> > 4) the same for the target; st_rdev in stat(2) results and again,
> > there's no such beast - it's neither character nor block device.
> > 
> > Your code is looking at (3).  Please, reread any textbook on Unix
> > in the section that would cover stat(2) and discussion of the
> > difference between st_dev and st_rdev.
> > 
> > I have no idea what Eric had been talking about - it's hard to
> > reconstruct by what you said so far.  Making nsfs per-userns,
> > perhaps?  But that makes no sense whatsoever, not that userns
> > ever had...  Cheap shots aside, I really can't guess what that's
> > about.  Sorry.
> 
> Thanks for the detailed information. The device number we want
> is nsfs. Indeed, currently, there is only one instance
> on the entire system. But not exactly sure what is the possibility
> to have more than one nsfs device in the future. Maybe per-userns
> or any other criteria?
> 
> > 
> > In any case, pathname resolution is *NOT* for the situations where
> > you can't block.  Even if it's procfs (and from the same pidns as
> > the process) mounted there, there is no promise that the target
> > of /proc/self has already been looked up and not evicted from
> > memory since then.  And in case of cache miss pathwalk will
> > have to call ->lookup(), which requires locking the directory
> > (rw_sem, shared).  You can't do that in such context.
> > 
> > And that doesn't even go into the possibility that process has
> > something very different mounted on /proc.
> > 
> > Again, I don't know what it is that you want to get to, but
> > I would strongly recommend finding a way to get to that data
> > that would not involve going anywhere near pathname resolution.
> > 
> > How would you expect the userland to work with that value,
> > whatever it might be?  If it's just a 32bit field that will
> > never be read, you might as well store there the same value
> > you store now (0, that is) in much cheaper and safer way ;-)
> 
> Suppose inside pid namespace, user can pass the device number,
> say n1, (`stat -L /proc/self/ns/pid`) to bpf program (through map
> or JIT). At runtime, bpf program will try to get device number,
> say n2, for the 'current' process. If n1 is not the same as
> n2, that means they are not in the same namespace. 'current'
> is in the same pid namespace as the user iff
> n1 == n2 and also pidns id is the same for 'current' and
> the one with `lsns -t pid`.
> 
> Are you aware of any way to get the pidns device number
> for 'current' without going through the pathname
> lookup?
> 

  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-06 15:09 [PATCH bpf-next v10 0/4] BPF: New helper to obtain namespace data from current task Carlos Neira
2019-09-06 15:09 ` [PATCH bpf-next v10 1/4] fs/namei.c: make available filename_lookup() for bpf helpers Carlos Neira
2019-09-06 15:09 ` [PATCH bpf-next v10 2/4] bpf: new helper to obtain namespace data from current task New bpf helper bpf_get_current_pidns_info Carlos Neira
2019-09-06 15:24   ` Al Viro
2019-09-06 15:46     ` Al Viro
2019-09-06 16:00       ` Al Viro
2019-09-06 23:21         ` Yonghong Song
2019-09-07  0:10           ` Al Viro
2019-09-07  6:34             ` Yonghong Song
2019-09-09 17:45               ` Carlos Antonio Neira Bustos [this message]
2019-09-10 22:35                 ` Yonghong Song
2019-09-10 23:15                   ` Al Viro
2019-09-11  8:16                     ` ebiederm
2019-09-12  5:49                       ` Yonghong Song
     [not found]                         ` <CACiB22j9M2gmccnh7XqqFp8g7qKFuiOrSAVJiA2tQHLB0pmoSQ@mail.gmail.com>
2019-09-13  2:56                           ` Yonghong Song
2019-09-13 11:58                             ` Carlos Antonio Neira Bustos
2019-09-13 16:59                         ` ebiederm
2019-09-13 17:28                           ` Yonghong Song
2019-09-11  4:32                   ` Carlos Antonio Neira Bustos
2019-09-11  8:17                     ` ebiederm
2019-09-10 22:46   ` Yonghong Song
2019-09-11  4:33     ` Carlos Antonio Neira Bustos
2019-09-06 15:09 ` [PATCH bpf-next v10 3/4] tools: Added bpf_get_current_pidns_info helper Carlos Neira
2019-09-06 15:09 ` [PATCH bpf-next v10 4/4] tools/testing/selftests/bpf: Add self-tests for helper bpf_get_pidns_info Carlos Neira
2019-09-10 22:55   ` Yonghong Song

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190909174522.GA17882@frodo.byteswizards.com \
    --to=cneirabustos@gmail.com \
    --cc=bpf@vger.kernel.org \
    --cc=brouer@redhat.com \
    --cc=ebiederm@xmission.com \
    --cc=netdev@vger.kernel.org \
    --cc=viro@zeniv.linux.org.uk \
    --cc=yhs@fb.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

BPF Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/bpf/0 bpf/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 bpf bpf/ https://lore.kernel.org/bpf \
		bpf@vger.kernel.org bpf@archiver.kernel.org
	public-inbox-index bpf

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.bpf


AGPL code for this site: git clone https://public-inbox.org/ public-inbox