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

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Message-Id: <46cee2a09d53921057cd4b87d1ed3796a2ab15bb.1575637022.git.andreyknvl@google.com>
Date:   Fri,  6 Dec 2019 14:04:55 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kernel@...r.kernel.org
Cc:     Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: [PATCH] kcov: fix struct layout for kcov_remote_arg

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