linux-kernel - Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wj8fkosQ7=bps5K+DDazBXk=ypfn49A0sEq+7-nZnyfXA@mail.gmail.com>
Date:   Mon, 28 Feb 2022 12:03:34 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christian König <christian.koenig@....com>
Cc:     Jakob Koschel <jakobkoschel@...il.com>,
        alsa-devel@...a-project.org, linux-aspeed@...ts.ozlabs.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-iio@...r.kernel.org, nouveau@...ts.freedesktop.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        samba-technical@...ts.samba.org,
        linux1394-devel@...ts.sourceforge.net, drbd-dev@...ts.linbit.com,
        linux-arch <linux-arch@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-staging@...ts.linux.dev, "Bos, H.J." <h.j.bos@...nl>,
        Jason Gunthorpe <jgg@...pe.ca>,
        intel-wired-lan@...ts.osuosl.org,
        kgdb-bugreport@...ts.sourceforge.net,
        bcm-kernel-feedback-list@...adcom.com,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergman <arnd@...db.de>,
        Linux PM <linux-pm@...r.kernel.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        v9fs-developer@...ts.sourceforge.net,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-sgx@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>, linux-usb@...r.kernel.org,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux F2FS Dev Mailing List 
        <linux-f2fs-devel@...ts.sourceforge.net>,
        tipc-discussion@...ts.sourceforge.net,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        dma <dmaengine@...r.kernel.org>,
        linux-mediatek@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

On Mon, Feb 28, 2022 at 11:56 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I do wish we could actually poison the 'pos' value after the loop
> somehow - but clearly the "might be uninitialized" I was hoping for
> isn't the way to do it.

Side note: we do need *some* way to do it.

Because if we make that change, and only set it to another pointer
when not the head, then we very much change the semantics of
"list_for_each_head()". The "list was empty" case would now exit with
'pos' not set at all (or set to NULL if we add that). Or it would be
set to the last entry.

And regardless, that means that all the

        if (pos == head)

kinds of checks after the loop would be fundamentally broken.

Darn. I really hoped for (and naively expected) that we could actually
have the compiler warn about the use-after-loop case. That whole
"compiler will now complain about bad use" was integral to my clever
plan to use the C99 feature of declaring the iterator inside the loop.

But my "clever plan" was apparently some ACME-level Wile E. Coyote sh*t.

Darn.

                   Linus