linux-user: fix cmsg conversion in case of multiple headers

Currently, __target_cmsg_nxthdr compares a pointer derived from
target_cmsg against the msg_control field of target_msgh (through
subtraction).  This failed for me when emulating i386 code under x86_64,
because pointers in the host address space and pointers in the guest
address space were not the same.  This patch passes the initial value of
target_cmsg into __target_cmsg_nxthdr.

I found and fixed two more related bugs:
- __target_cmsg_nxthdr now returns the new cmsg pointer instead of the
  old one.
- tgt_space (in host_to_target_cmsg) doesn't count "sizeof (struct
  target_cmsghdr)" twice anymore.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

1 files changed, 9 insertions, 5 deletions

diff --git a/linux-user/syscall_defs.h b/linux-user/syscall_defs.h
index cdc8db421c..7ca33a6f62 100644
--- a/linux-user/syscall_defs.h
+++ b/linux-user/syscall_defs.h
@@ -235,7 +235,8 @@ struct target_cmsghdr {
 };
 
 #define TARGET_CMSG_DATA(cmsg) ((unsigned char *) ((struct target_cmsghdr *) (cmsg) + 1))
-#define TARGET_CMSG_NXTHDR(mhdr, cmsg) __target_cmsg_nxthdr (mhdr, cmsg)
+#define TARGET_CMSG_NXTHDR(mhdr, cmsg, cmsg_start) \
+                               __target_cmsg_nxthdr(mhdr, cmsg, cmsg_start)
 #define TARGET_CMSG_ALIGN(len) (((len) + sizeof (abi_long) - 1) \
                                & (size_t) ~(sizeof (abi_long) - 1))
 #define TARGET_CMSG_SPACE(len) (TARGET_CMSG_ALIGN (len) \
@@ -243,17 +244,20 @@ struct target_cmsghdr {
 #define TARGET_CMSG_LEN(len)   (TARGET_CMSG_ALIGN (sizeof (struct target_cmsghdr)) + (len))
 
 static __inline__ struct target_cmsghdr *
-__target_cmsg_nxthdr (struct target_msghdr *__mhdr, struct target_cmsghdr *__cmsg)
+__target_cmsg_nxthdr(struct target_msghdr *__mhdr,
+                     struct target_cmsghdr *__cmsg,
+                     struct target_cmsghdr *__cmsg_start)
 {
   struct target_cmsghdr *__ptr;
 
   __ptr = (struct target_cmsghdr *)((unsigned char *) __cmsg
                                     + TARGET_CMSG_ALIGN (tswapal(__cmsg->cmsg_len)));
-  if ((unsigned long)((char *)(__ptr+1) - (char *)(size_t)tswapal(__mhdr->msg_control))
-      > tswapal(__mhdr->msg_controllen))
+  if ((unsigned long)((char *)(__ptr+1) - (char *)__cmsg_start)
+      > tswapal(__mhdr->msg_controllen)) {
     /* No more entries.  */
     return (struct target_cmsghdr *)0;
-  return __cmsg;
+  }
+  return __ptr;
 }
 
 struct target_mmsghdr {