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From:   Alexander Potapenko <glider@google.com>
Date:   Mon, 21 Mar 2022 14:17:44 +0100
Message-ID: <CAG_fn=WvjRCNH+F65QuuCnrmLcicz1zu0s-uu8DrmUtr0tcZ7Q@mail.gmail.com>
Subject: Re: [PATCH 13/43] kmsan: add KMSAN runtime core
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Christoph Hellwig <hch@lst.de>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ilya Leoshkevich <iii@linux.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Pekka Enberg <penberg@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Vegard Nossum <vegard.nossum@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Linux Memory Management List <linux-mm@kvack.org>,
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> > +       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 =3D dst > src;
> > +       i =3D backwards ? min(src_slots, dst_slots) - 1 : 0;
> > +       iter =3D backwards ? -1 : 1;
> > +
> > +       align_shadow_src =3D
> > +               (u32 *)ALIGN_DOWN((u64)shadow_src, KMSAN_ORIGIN_SIZE);
> > +       for (step =3D 0; step < min(src_slots, dst_slots); step++, i +=
=3D iter) {
> > +               KMSAN_WARN_ON(i < 0);
> > +               shadow =3D align_shadow_src[i];
> > +               if (i =3D=3D 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 =3D ((u64)src % KMSAN_ORIGIN_SIZE) * =
8;
> > +                       shadow =3D (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 =3D=3D 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 =3D (((u64)src + n) % KMSAN_ORIGIN_SI=
ZE) * 8;
> > +                       shadow =3D (shadow >> skip_bits) << skip_bits;
>
> Same here.
Done
>


> This can be a bit shorted and w/o the temp var as:
>
> new_origin =3D 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 =3D old_origin;

Done.

>
>
> > +               }
> > +               if (shadow)
> > +                       origin_dst[i] =3D new_origin;
>
> Are we sure that origin_dst is aligned here?
Yes, kmsan_get_metadata(..., KMSAN_META_ORIGIN) always returns aligned poin=
ters.


--
Alexander Potapenko
Software Engineer

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