[RESEND][PATCH 12/15] tty: audit: Simplify first-use allocation

From: Peter Hurley
Date: Sun Jan 10 2016 - 00:01:33 EST


The first-use tty audit buffer allocation is a potential race
amongst multiple attempts at 'first-use'; only one 'winner' is
acceptable.

The successful buffer assignment occurs if tty_audit_buf == NULL
(which will also be the return from cmpxchg()); otherwise, another
racer 'won' and this buffer allocation is freed.

Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/tty_audit.c | 22 +++++++---------------
1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/drivers/tty/tty_audit.c b/drivers/tty/tty_audit.c
index 71ba8ba..6e33e41 100644
--- a/drivers/tty/tty_audit.c
+++ b/drivers/tty/tty_audit.c
@@ -181,30 +181,22 @@ int tty_audit_push(void)
*/
static struct tty_audit_buf *tty_audit_buf_get(void)
{
- struct tty_audit_buf *buf, *buf2;
- unsigned long flags;
+ struct tty_audit_buf *buf;

buf = current->signal->tty_audit_buf;
if (buf)
return buf;

- buf2 = tty_audit_buf_alloc();
- if (buf2 == NULL) {
+ buf = tty_audit_buf_alloc();
+ if (buf == NULL) {
audit_log_lost("out of memory in TTY auditing");
return NULL;
}

- spin_lock_irqsave(&current->sighand->siglock, flags);
- buf = current->signal->tty_audit_buf;
- if (!buf) {
- current->signal->tty_audit_buf = buf2;
- buf = buf2;
- buf2 = NULL;
- }
- spin_unlock_irqrestore(&current->sighand->siglock, flags);
- if (buf2)
- tty_audit_buf_free(buf2);
- return buf;
+ /* Race to use this buffer, free it if another wins */
+ if (cmpxchg(&current->signal->tty_audit_buf, NULL, buf) != NULL)
+ tty_audit_buf_free(buf);
+ return current->signal->tty_audit_buf;
}

/**
--
2.7.0