[PATCH] kcov: fix struct layout for kcov_remote_arg

[PATCH] kcov: fix struct layout for kcov_remote_arg

[Andrey Konovalov]

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;
+	__u64	common_handle;
+	__u64	handles[0];
 };
 
 #define KCOV_REMOTE_MAX_HANDLES		0x100
-- 
2.24.0.393.g34dc348eaf-goog

Re: [PATCH] kcov: fix struct layout for kcov_remote_arg

[Marco Elver]

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

Thanks,
-- Marco

Re: [PATCH] kcov: fix struct layout for kcov_remote_arg

[Andrey Konovalov]

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.