linux-hardening - RE: [PATCH net-next v3 1/7] overflow: add DEFINE

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Date:   Fri, 18 Aug 2023 10:49:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Przemek Kitszel' <przemyslaw.kitszel@...el.com>,
        'Kees Cook' <keescook@...omium.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jacob Keller <jacob.e.keller@...el.com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        Steven Zou <steven.zou@...el.com>
Subject: RE: [PATCH net-next v3 1/7] overflow: add DEFINE_FLEX() for on-stack
 allocs

From: Przemek Kitszel
> Sent: Friday, August 18, 2023 11:28 AM
...
> >>> I'm not sure you should be forcing the memset() either.
> >>
> >> This already got discussed: better to fail safe.
> >
> > Perhaps call it DEFINE_FLEX_Z() to make this clear and
> > give the option for a non-zeroing version later.
> > Not everyone wants the expense of zeroing everything.
> 
> per Kees, zeroing should be removed by compiler when not needed:
> https://lore.kernel.org/intel-wired-lan/202308101128.C4F0FA235@keescook/

Expect in the most trivial cases the compiler is pretty much never
going to remove the zeroing of the data[] part.

I'm also not at all sure what happens if there is a function
call between the initialisation and any assignments.

With a bit of effort you should be able to pass the '= {}'
through into an inner #define.
Possibly with the alternative of a caller-provider
 '= { .obj = call_supplied_initialiser }'
The 'not _Z' form would pass an empty argument.

	David

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