netdev - Re: [PATCH RFC v2] mac80211: debugfs option to force TX status frames

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Date:   Mon, 11 Mar 2019 14:52:40 +0000
From:   Jeremy Sowden <jeremy@...zel.net>
To:     Kalle Valo <kvalo@...eaurora.org>
Cc:     ga58taw@...um.de, Julius Niedworok <julius.n@....net>,
        linux-wireless@...r.kernel.org, david@...hat.com, nc@....in.tum.de,
        Johannes Berg <johannes@...solutions.net>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2] mac80211: debugfs option to force TX status frames

On 2019-03-11, at 16:03:38 +0200, Kalle Valo wrote:
> ga58taw@...um.de writes:
> > On Thu, Mar 07, 2019 at 05:42:04PM +0200, Kalle Valo wrote:
> > > > +   len = scnprintf(buf, sizeof(buf), "%d\n",
> > > >                    (int)local->force_tx_status);
> > >
> > > I wonder about the cast, is it guaranteed that a bool is always of
> > > the same size as an int?
> >
> > Why is this a problem? If a bool is smaller than an int, the
> > compiler emits code that will prepend the value of force_tx_status
> > with zeros.
>
> Let's say that a bool is a byte and int is four bytes. If you use "%d"
> I would guess that in that case scnprintf() writes 4 bytes, meaning
> that 3 bytes will be overwriting either padding or some other field in
> the struct.

It's the value that matters, not the type.  It will only be too big for
the buffer if the result of casting local->force_tx_status to int is
less than -9 or greeater than 99.

  scnprintf(buf, size(buf), "%lld\n", (long long)local->force_tx_status)

would also be fine if the value were in range.  Note also that scnprintf
will not overrun the buffer: it will truncate the string.

> But I'm no compiler expert so I'm not going to argue about this
> anymore.  I just wanted to point out that that the cast looks
> dangerous and I would not do it.

As it happens, arguments to variadic functions are subject to the
"default argument promotions," so if local->force_tx_status is in fact a
bool (I can't find the definition), it would be promoted to int and the
cast is superfluous.

J.

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