3.16.55-rc1 review patch. If anyone has any objections, please let me know.
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From: Oleg Nesterov oleg@redhat.com
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@redhat.com Fixes: 32dc73086015 ("get rid of timer in kern/acct.c") Reported-by: TSUKADA Koutaro tsukada@ascade.co.jp Suggested-by: TSUKADA Koutaro tsukada@ascade.co.jp Signed-off-by: Oleg Nesterov oleg@redhat.com Cc: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- 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);