icmp: support rfc 4884

Add setsockopt SOL_IP/IP_RECVERR_4884 to return the offset to an
extension struct if present.

ICMP messages may include an extension structure after the original
datagram. RFC 4884 standardized this behavior. It stores the offset
in words to the extension header in u8 icmphdr.un.reserved[1].

The field is valid only for ICMP types destination unreachable, time
exceeded and parameter problem, if length is at least 128 bytes and
entire packet does not exceed 576 bytes.

Return the offset to the start of the extension struct when reading an
ICMP error from the error queue, if it matches the above constraints.

Do not return the raw u8 field. Return the offset from the start of
the user buffer, in bytes. The kernel does not return the network and
transport headers, so subtract those.

Also validate the headers. Return the offset regardless of validation,
as an invalid extension must still not be misinterpreted as part of
the original datagram. Note that !invalid does not imply valid. If
the extension version does not match, no validation can take place,
for instance.

For backward compatibility, make this optional, set by setsockopt
SOL_IP/IP_RECVERR_RFC4884. For API example and feature test, see
github.com/wdebruij/kerneltools/blob/master/tests/recv_icmp_v2.c

For forward compatibility, reserve only setsockopt value 1, leaving
other bits for additional icmp extensions.

Changes
  v1->v2:
  - convert word offset to byte offset from start of user buffer
    - return in ee_data as u8 may be insufficient
  - define extension struct and object header structs
  - return len only if constraints met
  - if returning len, also validate

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 22 insertions, 0 deletions

diff --git a/include/uapi/linux/icmp.h b/include/uapi/linux/icmp.h
index 5589eeb791ca..fb169a50895e 100644
--- a/include/uapi/linux/icmp.h
+++ b/include/uapi/linux/icmp.h
@@ -19,6 +19,7 @@
 #define _UAPI_LINUX_ICMP_H
 
 #include <linux/types.h>
+#include <asm/byteorder.h>
 
 #define ICMP_ECHOREPLY		0	/* Echo Reply			*/
 #define ICMP_DEST_UNREACH	3	/* Destination Unreachable	*/
@@ -95,5 +96,26 @@ struct icmp_filter {
 	__u32		data;
 };
 
+/* RFC 4884 extension struct: one per message */
+struct icmp_ext_hdr {
+#if defined(__LITTLE_ENDIAN_BITFIELD)
+	__u8		reserved1:4,
+			version:4;
+#elif defined(__BIG_ENDIAN_BITFIELD)
+	__u8		version:4,
+			reserved1:4;
+#else
+#error	"Please fix <asm/byteorder.h>"
+#endif
+	__u8		reserved2;
+	__sum16		checksum;
+};
+
+/* RFC 4884 extension object header: one for each object */
+struct icmp_extobj_hdr {
+	__be16		length;
+	__u8		class_num;
+	__u8		class_type;
+};
 
 #endif /* _UAPI_LINUX_ICMP_H */