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SubjectRe: [PATCH] kcov: fix struct layout for kcov_remote_arg
On Fri, Dec 6, 2019 at 2:14 PM Marco Elver <elver@google.com> wrote:
>
> On Fri, 6 Dec 2019 at 14:05, Andrey Konovalov <andreyknvl@google.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@google.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.

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