[PATCH] userfaultfd: update the uffd_msg structure to be the same on 32/64bit

[Andrea Arcangeli]

Avoiding to using packed allowed the code to be nicer and it avoided
the reserved1/2/3 but the structure must be the same for 32bit and
64bit archs so x86 applications built with the 32bit ABI can run on
the 64bit kernel without requiring translation of the data read
through the read syscall.

$ gcc -m64 p.c && ./a.out
32
0
16
8
8
16
24
$ gcc -m32 p.c && ./a.out
32
0
16
8
8
16
24

int main()
{
	printf("%lu\n", sizeof(struct uffd_msg));
	printf("%lu\n", (unsigned long) &((struct uffd_msg *) 0)->event);
	printf("%lu\n", (unsigned long) &((struct uffd_msg *) 0)->arg.pagefault.address);
	printf("%lu\n", (unsigned long) &((struct uffd_msg *) 0)->arg.pagefault.flags);
	printf("%lu\n", (unsigned long) &((struct uffd_msg *) 0)->arg.reserved.reserved1);
	printf("%lu\n", (unsigned long) &((struct uffd_msg *) 0)->arg.reserved.reserved2);
	printf("%lu\n", (unsigned long) &((struct uffd_msg *) 0)->arg.reserved.reserved3);
}

Reported-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
 include/uapi/linux/userfaultfd.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index c8a543f..00d28e2 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -59,9 +59,13 @@
 struct uffd_msg {
 	__u8	event;
 
+	__u8	reserved1;
+	__u16	reserved2;
+	__u32	reserved3;
+
 	union {
 		struct {
-			__u32	flags;
+			__u64	flags;
 			__u64	address;
 		} pagefault;
 
@@ -72,7 +76,7 @@ struct uffd_msg {
 			__u64	reserved3;
 		} reserved;
 	} arg;
-};
+} __attribute__((packed));
 
 /*
  * Start at 0x12 and not at 0 to be more strict against bugs.


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