linux-kernel - Re: [PATCH 13/43] kmsan: add KMSAN runtime core

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Message-ID: <CAG_fn=WvjRCNH+F65QuuCnrmLcicz1zu0s-uu8DrmUtr0tcZ7Q@mail.gmail.com>
Date:   Mon, 21 Mar 2022 14:17:44 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/43] kmsan: add KMSAN runtime core

> > +       KMSAN_WARN_ON(!src_slots || !dst_slots);
> > +       KMSAN_WARN_ON((src_slots < 1) || (dst_slots < 1));
>
> The above 2 checks look equivalent.
Right, I'll drop the first one.

> > +       KMSAN_WARN_ON((src_slots - dst_slots > 1) ||
> > +                     (dst_slots - src_slots < -1));
> > +       backwards = dst > src;
> > +       i = backwards ? min(src_slots, dst_slots) - 1 : 0;
> > +       iter = backwards ? -1 : 1;
> > +
> > +       align_shadow_src =
> > +               (u32 *)ALIGN_DOWN((u64)shadow_src, KMSAN_ORIGIN_SIZE);
> > +       for (step = 0; step < min(src_slots, dst_slots); step++, i += iter) {
> > +               KMSAN_WARN_ON(i < 0);
> > +               shadow = align_shadow_src[i];
> > +               if (i == 0) {
> > +                       /*
> > +                        * If |src| isn't aligned on KMSAN_ORIGIN_SIZE, don't
> > +                        * look at the first |src % KMSAN_ORIGIN_SIZE| bytes
> > +                        * of the first shadow slot.
> > +                        */
> > +                       skip_bits = ((u64)src % KMSAN_ORIGIN_SIZE) * 8;
> > +                       shadow = (shadow << skip_bits) >> skip_bits;
>
> Is this correct?...
> For the first slot we want to ignore some of the first (low) bits. To
> ignore low bits we need to shift right and then left, no?

Yes, you are right, I forgot about the endianness. Will try to add
some tests for this case.

> > +               }
> > +               if (i == src_slots - 1) {
> > +                       /*
> > +                        * If |src + n| isn't aligned on
> > +                        * KMSAN_ORIGIN_SIZE, don't look at the last
> > +                        * |(src + n) % KMSAN_ORIGIN_SIZE| bytes of the
> > +                        * last shadow slot.
> > +                        */
> > +                       skip_bits = (((u64)src + n) % KMSAN_ORIGIN_SIZE) * 8;
> > +                       shadow = (shadow >> skip_bits) << skip_bits;
>
> Same here.
Done
>


> This can be a bit shorted and w/o the temp var as:
>
> new_origin = kmsan_internal_chain_origin(old_origin);
> /*
> * kmsan_internal_chain_origin() may return
> * NULL, but we don't want to lose the previous
> * origin value.
> */
> if (!new_origin)
>    new_origin = old_origin;

Done.

>
>
> > +               }
> > +               if (shadow)
> > +                       origin_dst[i] = new_origin;
>
> Are we sure that origin_dst is aligned here?
Yes, kmsan_get_metadata(..., KMSAN_META_ORIGIN) always returns aligned pointers.



--
Alexander Potapenko
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