linux-kernel - Re: [PATCH] usb: gadget: dummy_hcd: execute hrtimer callback in softirq context

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Message-ID: <CAG_fn=Uafd4y9eetxBKWWROpSdDYWTOpjhOsCU4ZVf2Z1LvvVA@mail.gmail.com>
Date: Fri, 9 Aug 2024 17:22:00 +0200
From: Alexander Potapenko <glider@...gle.com>
To: andrey.konovalov@...ux.dev
Cc: Alan Stern <stern@...land.harvard.edu>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Marcello Sylvester Bauer <sylv@...v.io>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Aleksandr Nogikh <nogikh@...gle.com>, Marco Elver <elver@...gle.com>, kasan-dev@...glegroups.com, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-usb@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+2388cdaeb6b10f0c13ac@...kaller.appspotmail.com, 
	syzbot+17ca2339e34a1d863aad@...kaller.appspotmail.com, stable@...r.kernel.org, 
	Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] usb: gadget: dummy_hcd: execute hrtimer callback in
 softirq context

On Mon, Jul 29, 2024 at 4:23 AM <andrey.konovalov@...ux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@...il.com>
>
> Commit a7f3813e589f ("usb: gadget: dummy_hcd: Switch to hrtimer transfer
> scheduler") switched dummy_hcd to use hrtimer and made the timer's
> callback be executed in the hardirq context.
>
> With that change, __usb_hcd_giveback_urb now gets executed in the hardirq
> context, which causes problems for KCOV and KMSAN.
>
> One problem is that KCOV now is unable to collect coverage from
> the USB code that gets executed from the dummy_hcd's timer callback,
> as KCOV cannot collect coverage in the hardirq context.
>
> Another problem is that the dummy_hcd hrtimer might get triggered in the
> middle of a softirq with KCOV remote coverage collection enabled, and that
> causes a WARNING in KCOV, as reported by syzbot. (I sent a separate patch
> to shut down this WARNING, but that doesn't fix the other two issues.)
>
> Finally, KMSAN appears to ignore tracking memory copying operations
> that happen in the hardirq context, which causes false positive
> kernel-infoleaks, as reported by syzbot.

Hi Andrey,

FWIW this problem is tracked as
https://github.com/google/kmsan/issues/92, I'll try to revisit it in
September.