Message-ID: <20230124221915.GW2948950@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 24 Jan 2023 14:19:15 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc:     Alan Stern <stern@...land.harvard.edu>, parri.andrea@...il.com,
        will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com, dlustig@...dia.com,
        joel@...lfernandes.org, urezki@...il.com, quic_neeraju@...cinc.com,
        frederic@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tools/memory-model Flag suspicious use of srcu cookies

On Tue, Jan 24, 2023 at 08:36:53PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/24/2023 8:15 PM, Paul E. McKenney wrote:
> > On Tue, Jan 24, 2023 at 12:19:24PM -0500, Alan Stern wrote:
> > > On Tue, Jan 24, 2023 at 03:39:51PM +0100, Jonas Oberhauser wrote:
> > > > The herd model of LKMM deviates from actual implementations in the
> > > > range of cookies that might be returned by srcu_lock() and similar
> > > > functions.  As a consequence, code that relies on srcu_lock()
> > > > returning specific values might pass on the herd model but fail in
> > > > the real world.
> > > > 
> > > > This patch flags any code that looks at the value of a cookie
> > > > without passing it on to an srcu_unlock().  This indicates that the
> > > > cookie value might be being used in ways that can lead herd to
> > > > produce incorrect results, as in the following (contrived) case:
> > > > 
> > > > P0(struct srcu_struct *ss)
> > > > {
> > > > 	int r = srcu_read_lock(ss);
> > > > 	if (r==0)
> > > > 		srcu_read_unlock(ss, r);
> > > > }
> > > > 
> > > > Without this patch, the code passes herd7 without any warnings.
> > > > 
> > > > With this patch, this code is flagged with illegal-srcu-cookie-ctrl,
> > > > indicating that a cookie is used to compute a control condition.
> > > > Such scenarios potentially lead to other branches of the code that
> > > > are possible in real usage not being evaluated by herd7.  In this
> > > > example, this affects the branch where r!=0, which would lead to
> > > > an unmatched read side critical section and thus to hangs of
> > > > synchronize_srcu() calls.
> > > > 
> > > > Besides use of cookies in control conditions, the patch also flags
> > > > use in address computation and any time a cookie is inspected but
> > > > not later passed to srcu_read_unlock().
> > > > 
> > > > Signed-off-by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
> > > > ---
> > > Acked-by: Alan Stern <stern@...land.harvard.edu>
> > Thank you both!
> > 
> > I wordsmithed the commit log as follows, but then realized that this
> > depends on Alan's earlier patch.
> 
> Yeah, I don't know if I did this correctly. I based it on the
> lkmm-srcu.2023.01.20a branch.
> Let me know if I should have done this differently.

You got it right.

> Looking through your changes to learn for future submissions:
> 
> > [...]
> > 
> > This patch flags any code that looks at the value of a cookie
> > without passing it on to an srcu_unlock().
> 
> You missed this one : )

No I didn't!  I missed *two*!  ;-)

Though that comment could be Srcu-unlock, I suppose.  But making it
srcu_read_unlock() seems more straightforward.  Update below!

							Thanx, Paul

------------------------------------------------------------------------

From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: paulmck@...nel.org
Cc: stern@...land.harvard.edu,
	parri.andrea@...il.com,
	will@...nel.org,
	peterz@...radead.org,
	boqun.feng@...il.com,
	npiggin@...il.com,
	dhowells@...hat.com,
	j.alglave@....ac.uk,
	luc.maranget@...ia.fr,
	akiyks@...il.com,
	dlustig@...dia.com,
	joel@...lfernandes.org,
	urezki@...il.com,
	quic_neeraju@...cinc.com,
	frederic@...nel.org,
	linux-kernel@...r.kernel.org,
	Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Subject: [PATCH] tools/memory-model Flag suspicious use of srcu cookies
Date: Tue, 24 Jan 2023 15:39:51 +0100

The herd model of LKMM deviates from actual implementations in the
range of cookies that might be returned by srcu_read_lock() and
similar functions.  As a consequence, code that makes special use of
values returned by srcu_read_lock() might pass in herd but fail in the
real world.

This patch flags any code that looks at the value of a cookie without
passing it on to an srcu_read_unlock().  This indicates that the cookie
value might be being used in ways that can lead herd to produce incorrect
results, as in the following (contrived) case:

P0(struct srcu_struct *ss)
{
	int r = srcu_read_lock(ss);
	if (r==0)
		srcu_read_unlock(ss, r);
}

Without this patch, the above code passes herd7 without any warnings.
However, real-world use of this code could result in SRCU grace-period
hangs.

With this patch, this code is flagged with illegal-srcu-cookie-ctrl,
indicating that a cookie is used to compute a control condition.
Real-world use of such code can result in executions that herd7 does
not evaluate.  In this example, when srcu_read_lock() returns a non-zero
value, there is no matching srcu_read_unlock(), which (as noted above)
would result in calls to synchronize_srcu() never returning.

Besides use of cookies in control conditions, the patch also flags use
in address computation and any time a cookie is inspected but not later
passed to srcu_read_unlock().

Signed-off-by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Acked-by: Alan Stern <stern@...land.harvard.edu>
---
 tools/memory-model/linux-kernel.bell | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/memory-model/linux-kernel.bell b/tools/memory-model/linux-kernel.bell
index 6e702cda15e1..db5993acc241 100644
--- a/tools/memory-model/linux-kernel.bell
+++ b/tools/memory-model/linux-kernel.bell
@@ -58,7 +58,8 @@ flag ~empty Rcu-unlock \ range(rcu-rscs) as unbalanced-rcu-unlock
 
 (* Compute matching pairs of nested Srcu-lock and Srcu-unlock *)
 let carry-srcu-data = (data ; [~ Srcu-unlock] ; rf)*
-let srcu-rscs = ([Srcu-lock] ; carry-srcu-data ; data ; [Srcu-unlock]) & loc
+let pass-cookie = carry-srcu-data ; data
+let srcu-rscs = ([Srcu-lock] ; pass-cookie ; [Srcu-unlock]) & loc
 
 (* Validate nesting *)
 flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-lock
@@ -71,6 +72,15 @@ flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep
 (* Validate SRCU dynamic match *)
 flag ~empty different-values(srcu-rscs) as bad-srcu-value-match
 
+(*
+ * Check that srcu cookies are only used for passing to srcu_read_unlock()
+ * Note: this check is only approximate
+ *)
+flag ~empty [Srcu-lock] ; pass-cookie ; rf ;
+	[~ domain(pass-cookie ; [Srcu-unlock])] as suspicious-srcu-cookie-use
+flag ~empty [Srcu-lock] ; carry-srcu-data ; ctrl as illegal-srcu-cookie-ctrl
+flag ~empty [Srcu-lock] ; carry-srcu-data ; addr as illegal-srcu-cookie-addr
+
 (* Compute marked and plain memory accesses *)
 let Marked = (~M) | IW | Once | Release | Acquire | domain(rmw) | range(rmw) |
 		LKR | LKW | UL | LF | RL | RU | Srcu-lock | Srcu-unlock
-- 
2.17.1

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