linux-kernel - Re: [PATCH] kcov: fix struct layout for kcov_remote

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Date:   Fri, 6 Dec 2019 14:31:53 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH] kcov: fix struct layout for kcov_remote_arg

On Fri, Dec 6, 2019 at 2:14 PM Marco Elver <elver@...gle.com> wrote:
>
> On Fri, 6 Dec 2019 at 14:05, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> >
> > Make the layout of kcov_remote_arg the same for 32-bit and 64-bit code.
> > This makes it more convenient to write userspace apps that can be compiled
> > into 32-bit or 64-bit binaries and still work with the same 64-bit kernel.
> > Also use proper __u32 types in uapi headers instead of unsigned ints.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> > ---
> >
> > Hi Andrew,
> >
> > We've noticed failures on 32 bit syzbot instances when the kcov patches
> > got merged into mainline. The reason is that the layout of kcov_remote_arg
> > depends on the alignment rules, which are different for 32/64 bit code.
> > We can deal with this issue in syzkaller [1], but I think it would be
> > cleander to get this fixed in the kernel.
> >
> > I hope this patch is acceptable, since the change has just been merged and
> > is not included into a release kernel version. The patch breaks the newly
> > introduced kcov API for 32 bit apps.
> >
> > Sorry for not testing it with 32 bit code earlier.
> >
> > Thanks!
> >
> > [1] https://github.com/google/syzkaller/commit/ba97c611a36b7729d489ebca5f97183c2ba7a90a
> >
> >  Documentation/dev-tools/kcov.rst |  7 ++++---
> >  include/uapi/linux/kcov.h        | 11 ++++++-----
> >  2 files changed, 10 insertions(+), 8 deletions(-)
> >
> > diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> > index 36890b026e77..744df2bae1ed 100644
> > --- a/Documentation/dev-tools/kcov.rst
> > +++ b/Documentation/dev-tools/kcov.rst
> > @@ -251,9 +251,10 @@ selectively from different subsystems.
> >  .. code-block:: c
> >
> >      struct kcov_remote_arg {
> > -       unsigned        trace_mode;
> > -       unsigned        area_size;
> > -       unsigned        num_handles;
> > +       uint32_t        trace_mode;
> > +       uint32_t        area_size;
> > +       uint32_t        num_handles;
> > +       uint32_t        reserved;
> >         uint64_t        common_handle;
> >         uint64_t        handles[0];
> >      };
> > diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> > index 409d3ad1e6e2..53267f9f1665 100644
> > --- a/include/uapi/linux/kcov.h
> > +++ b/include/uapi/linux/kcov.h
> > @@ -9,11 +9,12 @@
> >   * and the comment before kcov_remote_start() for usage details.
> >   */
> >  struct kcov_remote_arg {
> > -       unsigned int    trace_mode;     /* KCOV_TRACE_PC or KCOV_TRACE_CMP */
> > -       unsigned int    area_size;      /* Length of coverage buffer in words */
> > -       unsigned int    num_handles;    /* Size of handles array */
> > -       __u64           common_handle;
> > -       __u64           handles[0];
> > +       __u32   trace_mode;     /* KCOV_TRACE_PC or KCOV_TRACE_CMP */
> > +       __u32   area_size;      /* Length of coverage buffer in words */
> > +       __u32   num_handles;    /* Size of handles array */
> > +       __u32   reserved;
>
> The kernel provides __aligned_u64 for this purpose (32/64 bit uapi
> compatibility).

Oh yes, this is much better, sent v2, thank you!

> Is adding an explicit 'reserved' better here? If so,
> it'd be good to document.