[PATCH] acct: fix the acct->needcheck check in check_free_space()

[Oleg Nesterov <oleg@redhat.com>]

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@vger.kernel.org
Reported-by: TSUKADA Koutaro <tsukada@ascade.co.jp>
Sugested-by: TSUKADA Koutaro <tsukada@ascade.co.jp>
Signed-off-by: Oleg Nesterov <oleg@redhat.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