linux-kernel - [PATCH] acct: fix the acct->needcheck check in check_free

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Message-ID: <20171213133940.GA6554@redhat.com>
Date:   Wed, 13 Dec 2017 14:39:40 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        TSUKADA Koutaro <tsukada@...ade.co.jp>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] acct: fix the acct->needcheck check in check_free_space()

As Tsukada explains, the time_is_before_jiffies(acct->needcheck) check
is very wrong, we need time_is_after_jiffies() to make sys_acct() work.

Ignoring the overflows, the code should "goto out" if needcheck > jiffies,
while currently it checks "needcheck < jiffies" and thus in the likely
case check_free_space() does nothing until jiffies overflow.

In particular this means that sys_acct() is simply broken, acct_on() sets
acct->needcheck = jiffies and expects that check_free_space() should set
acct->active = 1 after the free-space check, but this won't happen if
jiffies increments in between.

This was broken by commit 32dc73086015 ("get rid of timer in kern/acct.c")
in 2011, then another (correct) commit 795a2f22a8ea ("acct() should honour
the limits from the very beginning") made the problem more visible.

Fixes: 32dc73086015 ("get rid of timer in kern/acct.c")
Cc: stable@...r.kernel.org
Reported-by: TSUKADA Koutaro <tsukada@...ade.co.jp>
Sugested-by: TSUKADA Koutaro <tsukada@...ade.co.jp>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/acct.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/acct.c b/kernel/acct.c
index 5b12843..bf90c09 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -101,7 +101,7 @@ static int check_free_space(struct bsd_acct_struct *acct)
 {
 	struct kstatfs sbuf;

-	if (time_is_before_jiffies(acct->needcheck))
+	if (time_is_after_jiffies(acct->needcheck))
 		goto out;

 	/* May block */
-- 
2.5.0