From: Mimi Zohar <zohar@linux.ibm.com>
To: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Cc: jmorris@namei.org, serge@hallyn.com,
linux-integrity@vger.kernel.org,
Matthew Garrett <mjg59@google.com>
Subject: Re: Fwd: a8d5875ce5 ("Default enable RCU list lockdep debugging with .."): WARNING: suspicious RCU usage
Date: Tue, 28 Apr 2020 08:59:01 -0400 [thread overview]
Message-ID: <1588078741.5195.6.camel@linux.ibm.com> (raw)
In-Reply-To: <20200428112349.GA19116@madhuparna-HP-Notebook>
On Tue, 2020-04-28 at 16:53 +0530, Madhuparna Bhowmik wrote:
> On Mon, Apr 27, 2020 at 08:58:26PM -0400, Mimi Zohar wrote:
> > [Cc'ing Matthew Garrett)
> >
> > Hi Madhuparna,
> >
> > On Sat, 2020-04-25 at 16:33 +0530, Madhuparna Bhowmik wrote:
> > > Hi,
> > >
> > > This is regarding the warning reported by kernel test bot regarding
> > > suspicious RCU usage.
> > > Using a simple git grep, I can only see the following usage of RCU:
> > >
> > > evm_crypto.c: list_for_each_entry_rcu(xattr, &evm_config_xattrnames,
> > > list) {
> > > evm_main.c: list_for_each_entry_rcu(xattr, &evm_config_xattrnames,
> > > list) {
> > > evm_main.c: list_for_each_entry_rcu(xattr, &evm_config_xattrnames,
> > > list) {
> > > evm_secfs.c: list_add_tail_rcu(&xattr->list, &evm_config_xattrnames);
> > >
> > > So, the evm_config_xattrnames list is traversed using
> > > list_for_each_entry_rcu() but without the protection of rcu_read_lock()?
> > > If these are not really RCU read-side CS, and other locks are held then
> > > there is no need to use list_for_each_entry_rcu().
> > > And maybe we can completely remove the usage of rcu primitives here.
> > > Or if there is a bug and rcu_read_lock() should be held, please let me know
> > > and I can try fixing this.
> >
> > Thank you for forwarding this report. The list of EVM xattrs is
> > protected by the xattr_list_mutex, which is used when reading or
> > appending to the EVM list itself. Entries in the list can not be
> > removed.
> >
> Hi Mimi,
>
> Thank you for your reply.
> So, if the list is protected by xattr_list mutex and it is used during
> both reading and writing to the list, can we remove the usage of RCU
> here?
I should have said the mutex is used when cat'ing the securityfs file
(security/integrity/evm/evm_xattrs) and when adding to the list, but
not in the above cases when walking the list.
> Since the read side critical section is already protected by the
> xattr_list mutex, we do not need list_for_each_entry_rcu() to read the
> list. Also, we can just simply add to the list using list_add_tail(),
> RCU primitives are not really required here.
>
> Please let me know is this is fine, and I can send a patch removing the
> usage of RCU here.
Matthew, please correct me if I'm wrong, the reason it is safe, is not
because there is a mutex, but because entries are never removed from
the list.
Mimi
> > The examples, above, are all readers, which walk the EVM xattr list in
> > order to calculate or verify a file's security.evm xattr.
next prev parent reply other threads:[~2020-04-28 12:59 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <5ea3a0e3.ruR9Zw9VIGN+NGIb%lkp@intel.com>
[not found] ` <CAD=jOEYd-pAQMo3hukx6AhXN7CbH8yGLVLHe2=92wCq-HWS++Q@mail.gmail.com>
2020-04-28 0:58 ` Fwd: a8d5875ce5 ("Default enable RCU list lockdep debugging with .."): WARNING: suspicious RCU usage Mimi Zohar
2020-04-28 11:23 ` Madhuparna Bhowmik
2020-04-28 12:59 ` Mimi Zohar [this message]
2020-04-29 10:04 ` Madhuparna Bhowmik
2020-04-29 22:40 ` Paul E. McKenney
2020-04-30 13:07 ` Madhuparna Bhowmik
2020-04-30 13:32 ` Mimi Zohar
2020-04-30 15:08 ` Madhuparna Bhowmik
Reply instructions:
You may reply publicly 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=1588078741.5195.6.camel@linux.ibm.com \
--to=zohar@linux.ibm.com \
--cc=jmorris@namei.org \
--cc=linux-integrity@vger.kernel.org \
--cc=madhuparnabhowmik10@gmail.com \
--cc=mjg59@google.com \
--cc=serge@hallyn.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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).