[PATCH 3.16 169/254] kernel/acct.c: fix the acct->needcheck check in check_free_space()

[Ben Hutchings]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Oleg Nesterov <oleg@xxxxxxxxxx>

commit 4d9570158b6260f449e317a5f9ed030c2504a615 upstream.

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.

Link: http://lkml.kernel.org/r/20171213133940.GA6554@xxxxxxxxxx
Fixes: 32dc73086015 ("get rid of timer in kern/acct.c")
Reported-by: TSUKADA Koutaro <tsukada@xxxxxxxxxxxx>
Suggested-by: TSUKADA Koutaro <tsukada@xxxxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/acct.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -107,7 +107,7 @@ static int check_free_space(struct bsd_a
 
 	spin_lock(&acct_lock);
 	res = acct->active;
-	if (!file || time_is_before_jiffies(acct->needcheck))
+	if (!file || time_is_after_jiffies(acct->needcheck))
 		goto out;
 	spin_unlock(&acct_lock);