linux-kernel - Re: [PATCH] x86/uaccess: fix code generation in put

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-=wg9L3EZk=cBjt5R3LkE8Y6swwOZ8sxhpQYcJO3Fj1wLbQ@mail.gmail.com>
Date:   Fri, 23 Oct 2020 14:11:19 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Anvin <hpa@...or.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/uaccess: fix code generation in put_user()

On Fri, Oct 23, 2020 at 2:00 PM <hpa@...or.com> wrote:
>
> There is no same reason to mess around with hacks when we are talking about dx:ax, though.

Sure there is.

"A" doesn't actually mean %edx:%eax like you seem to think.

It actually means %eax OR %edx, and then if given a 64-bit value, it
will use the combination (with %edx being the high bits).

So using "A" unconditionally doesn't work - it gives random behavior
for 32-bit (or smaller) types.

Or you'd have to cast the value to always be 64-bit, and have the
extra code generation.

IOW, an unconditional "A" is wrong.

And the alternative is to just duplicate things, and go back to the
explicit size testing, but honestly, I really think that's much worse
than relying on a documented feature of "register asm()" that gcc
_documents_ is for this kind of inline asm use.

So the "don't do pointless conditional duplication" is certainly a
very sane reason for the code.

            Linus