linux-kernel - Re: [PATCH] Drivers: hv: vmbus: Split memcpy of flex-array

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Message-ID: <202209232119.E32C14857@keescook>
Date:   Fri, 23 Sep 2022 21:22:55 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "K. Y. Srinivasan" <kys@...rosoft.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        linux-hyperv@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Drivers: hv: vmbus: Split memcpy of flex-array

On Fri, Sep 23, 2022 at 10:42:38PM -0500, Gustavo A. R. Silva wrote:
> On Fri, Sep 23, 2022 at 08:07:41PM -0700, Kees Cook wrote:
> > To work around a misbehavior of the compiler's ability to see into
> > composite flexible array structs (as detailed in the coming memcpy()
> > hardening series[1]), split the memcpy() of the header and the payload
> > so no false positive run-time overflow warning will be generated. As it
> > turns out, this appears to actually reduce the text size:

Er, actually, I can't read/math. ;) It _does_ grow the text size. (That's
2_3_ not 22 at the start of the text size...) On examination, it appears
to unroll the already inlined memcpy further.

> > 
> > $ size drivers/hv/vmbus_drv.o.before drivers/hv/vmbus_drv.o
> >    text    data     bss     dec     hex filename
> >   22968    5239     232   28439    6f17 drivers/hv/vmbus_drv.o.before
> >   23032    5239     232   28503    6f57 drivers/hv/vmbus_drv.o
       ^

-Kees

-- 
Kees Cook