From: Rafael Aquini <aquini@redhat.com>
To: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Audra Mitchell <audra@redhat.com>,
linux-kernel@vger.kernel.org, tj@kernel.org,
jiangshanlai@gmail.com, hirokazu.yamauchi.hk@hitachi.com,
ddouwsma@redhat.com, loberman@redhat.com, raquini@redhat.com
Subject: Re: [PATCH v2] workqueue.c: Increase workqueue name length
Date: Wed, 10 Jan 2024 17:31:49 -0500 [thread overview]
Message-ID: <ZZ8a1RvwpDZvwfg9@optiplex-fbsd> (raw)
In-Reply-To: <f5ded466-cbe1-4a46-b042-1c65839c9e02@prevas.dk>
On Wed, Jan 10, 2024 at 11:06:22PM +0100, Rasmus Villemoes wrote:
> On 10/01/2024 22.52, Rafael Aquini wrote:
> > On Wed, Jan 10, 2024 at 09:47:56PM +0100, Rasmus Villemoes wrote:
> >> On 10/01/2024 21.29, Audra Mitchell wrote:
> >>
> >>> @@ -4663,9 +4663,10 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
> >>> unsigned int flags,
> >>> int max_active, ...)
> >>> {
> >>> - va_list args;
> >>> + va_list args, args_copy;
> >>> struct workqueue_struct *wq;
> >>> struct pool_workqueue *pwq;
> >>> + int len;
> >>>
> >>> /*
> >>> * Unbound && max_active == 1 used to imply ordered, which is no longer
> >>> @@ -4692,6 +4693,13 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
> >>> }
> >>>
> >>> va_start(args, max_active);
> >>> + va_copy(args_copy, args);
> >>> + len = vsnprintf(NULL, 0, fmt, args_copy);
> >>> + WARN(len > WQ_NAME_LEN,
> >>> + "workqueue: wq->name too long (%d). Truncated to WQ_NAME_LEN (%d)\n",
> >>> + len, WQ_NAME_LEN);
> >>> +
> >>> + va_end(args_copy);
> >>> vsnprintf(wq->name, sizeof(wq->name), fmt, args);
> >>
> >> Eh, why not just _not_ throw away the return value from the existing
> >> vsnprintf() and do "len >= sizeof(wq->name)" to know if truncation
> >> happened? There's really no need need to do vsnprintf() twice. (And yes,
> >> you want >=, not >).
> >>
> >
> > The extra vsnprintf call is required because the return of the existing
> > vsnprintf() is going to be already capped by sizeof(wq->name).
>
> No, it is not. vsnprintf() returns the length of the would-be-created
> string if the buffer was big enough. That is independent of whether one
> does a dummy NULL,0 call or just calls it with a real, but possibly too
> small, buffer.
>
> This is true for userspace (as required by posix) as well as the kernel
> implementation of vsnprintf(). What makes you think otherwise?
>
this snippet from PRINTF(3) man page
RETURN VALUE
Upon successful return, these functions return the number of characters
printed (excluding the null byte used to end output to strings).
> The kernel _also_ happens to have a non-standardized function called
> vscnprintf (note the c) which returns the possibly-truncated result. But
> that's irrelevant here.
>
> >> Oh, and definitely not WARN, pr_warn() or pr_warn_once() please.
> >>
> >
> > Then you lose the ability to figure out what was trying to create the
> > wq with the inflated name. Also, the _once variants don't seem to do
> > good here, because alloc_workqueue() can be called by different
> > drivers.
>
> I assume that whatever creates the wq will do so on every boot, and the
> name is most likely some fixed thing. So you're essentially setting up
> some configurations to do a WARN on every single boot, not to mention
> that for some machines that implies a panic... It really is not
> something that warrants a WARN.
>
> As for figuring out what caused that too-long name, well, I'd hope that
> the 31 meaningful bytes that did get produced would provide a
> sufficiently good hint.
>
> Rasmus
>
next prev parent reply other threads:[~2024-01-10 22:31 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-15 19:39 [PATCH] workqueue.c: Change workqueue to accept variable length name Audra Mitchell
2023-12-21 21:39 ` Tejun Heo
[not found] ` <CA+bDH-v6T5vvyOwsphseHwgihdGQta7TZ9tOtt-Fnij92kvU6A@mail.gmail.com>
2023-12-22 17:53 ` Tejun Heo
2024-01-09 13:29 ` Audra Mitchell
2024-01-10 20:29 ` [PATCH v2] workqueue.c: Increase workqueue name length Audra Mitchell
2024-01-10 20:47 ` Rasmus Villemoes
2024-01-10 21:52 ` Rafael Aquini
2024-01-10 22:06 ` Rasmus Villemoes
2024-01-10 22:31 ` Rafael Aquini [this message]
2024-01-10 22:45 ` Rafael Aquini
2024-01-10 22:58 ` Rasmus Villemoes
2024-01-10 22:52 ` Rasmus Villemoes
2024-01-10 23:08 ` Rafael Aquini
2024-01-15 17:08 ` [PATCH v3] " Audra Mitchell
2024-01-16 18:31 ` Tejun Heo
2024-01-17 14:40 ` Audra Mitchell
2024-01-17 17:09 ` Tejun Heo
2024-01-19 20:30 ` Audra Mitchell
2024-01-19 23:44 ` Tejun Heo
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