linux-kernel - Re: [PATCH] fs/select: avoid clang stack usage warning

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Message-ID: <202210071614.4297AB65D8@keescook>
Date:   Fri, 7 Oct 2022 16:17:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Christoph Hellwig <hch@....de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Paul Kirth <paulkirth@...gle.com>
Subject: Re: [PATCH] fs/select: avoid clang stack usage warning

On Fri, Oct 07, 2022 at 11:42:51PM +0200, Arnd Bergmann wrote:
> On Fri, Oct 7, 2022, at 9:04 PM, Nick Desaulniers wrote:
> > On Fri, Oct 7, 2022 at 1:28 AM Arnd Bergmann <arnd@...db.de> wrote:
> >> - The behavior of -ftrivial-auto-var-init= is a bit odd here: with =zero or
> >>   =pattern, the stack usage is just below the limit (1020), without the
> >>   option it is up to 1044. It looks like your .config picks =zero, which
> >>   was dropped in the latest clang version, so it falls back to not
> >
> > Huh? What do you mean by "was dropped?"
> >
> > The config I sent has:
> > CONFIG_CC_HAS_AUTO_VAR_INIT_PATTERN=y
> > CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_BARE=y
> > CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO=y
> > # CONFIG_INIT_STACK_NONE is not set
> > CONFIG_INIT_STACK_ALL_ZERO=y
> 
> When I use this config on my kernel tree (currently on top of
> next-20220929 for unrelated work) and build with clang-16,
> CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO is disabled, so it falls
> back from CONFIG_INIT_STACK_NONE instead of the unavailable
> CONFIG_INIT_STACK_ALL_ZERO.

I think you have a very recent Clang but are building a tree that
doesn't have commit 607e57c6c62c ("hardening: Remove Clang's enable flag
for -ftrivial-auto-var-init=zero").

-- 
Kees Cook