linux-kernel - [PATCH] genirq: __setup_irq(): fix type of literal 1 in shift

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Message-Id: <20171030213548.16831-1-linux@rasmusvillemoes.dk>
Date:   Mon, 30 Oct 2017 22:35:47 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] genirq: __setup_irq(): fix type of literal 1 in shift

If we ever get a value >= 31 from ffz(), we'd be invoking UB; in the >
31 case, probably assigning the same thread_mask to to multiple
irqactions (at least on x86_64, where the shift count is implicitly
truncated to 5 bits).

In practice, I think the bug is mostly harmless, since when we first
get ffz == 31, the RHS is - ignoring UB - (int)(0x80000000), and
sign-extension means that the 32nd irqaction just ends up eating the
top 33 bits of thread_mask, so all subsequent __setup_irq calls would
just end up hitting the -EBUSY branch. (AFAICT, no code seems to rely
on ->thread_mask being a single bit; it's all whole-sale |= and &=).

However, a sufficiently aggressive optimizer may use the UB of 1<<31
to decide that doesn't happen, and hence elide the sign-extension
code, so that subsequent calls can indeed get ffz > 31. In any case,
this is the right thing to do.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 kernel/irq/manage.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 4bff6a10ae8e..a1a448e1760d 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -1305,7 +1305,7 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 		 * thread_mask assigned. See the loop above which or's
 		 * all existing action->thread_mask bits.
 		 */
-		new->thread_mask = 1 << ffz(thread_mask);
+		new->thread_mask = 1UL << ffz(thread_mask);

 	} else if (new->handler == irq_default_primary_handler &&
 		   !(desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)) {
-- 
2.11.0