linux-kernel - RE: Build fail in drivers/gpu/drm/i915/display/intel

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Date:   Fri, 10 Nov 2023 09:00:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        "imre.deak@...el.com" <imre.deak@...el.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: Build fail in drivers/gpu/drm/i915/display/intel_tc.c

From: Linus Torvalds
> Sent: 10 November 2023 00:52
> 
> On Thu, 9 Nov 2023 at 15:34, Imre Deak <imre.deak@...el.com> wrote:
> >
> > The compiler warn should be fixed/suppressed by:
> > https://lore.kernel.org/all/20231026125636.5080-1-nirmoy.das@intel.com
> 
> Ugh, so now it's a dynamic allocation, wasting memory, and a pointer
> to it, using as much memory as the array did in the first place.
> 
> All because of a pointless warning that was a false positive - and was
> always harmless anyway, since snprintf() is safe (ie it was only a
> "might be truncated").

That entire warning for snprintf() is a false positive.
The ones that are likely to overflow unexpectedly are the ones
with a "%s" format for a 'char *' pointer where there is no
implied length.

The same check for printf() using the implied buffer length
probably does make sense.

I don't even think there is a way of avoiding it on a case by case
basis - apart from passing both the buffer address and length
to an inline asm that the compiler has to assume might change
the values, but that tends to generate an extra 'mov' instruction
for no good reason at all.

> 
> Please don't do this. Either do that ((tc_port & 7) + 1) suggestion of
> David's, or just do '%c' and make the expression be
> 
>   '1' + tc_port
> 
> which should be fine since I915_MAX_PORTS is 8 or whatever.

If I'd though for 2 seconds that is what I'd have done.
But I wanted to get something through the compiler.

> I do wonder why those ports are printed out as '1-8', when the 'enum
> port' is PORT_A..I.

They look like TC_PORT_[1..6] to me - the enum values are 0..5
which is why there is a 'random' '+ 1'.

	David

> 
> So it would actually have made more sense to print them out as %c with
> the expression being
> 
>    'A'+tc_port
> 
> but I guess by now people might depend on the 1..8 naming?
> 
>              Linus

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