[Qemu-devel] [PATCH] linux-user: fix cmsg conversion in case of multiple headers

["Jonathan Neuschäfer" <j.neuschaefer@gmx.net>]

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.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

--
Changes since v1:
- Follow Peter Maydell's advice on how to fix the first bug
- The "two more related bugs"
---
 linux-user/syscall.c      | 14 +++++++++-----
 linux-user/syscall_defs.h |  9 +++++----
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index f62c698..12a6cd2 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -1181,7 +1181,7 @@ static inline abi_long target_to_host_cmsg(struct msghdr *msgh,
     struct cmsghdr *cmsg = CMSG_FIRSTHDR(msgh);
     abi_long msg_controllen;
     abi_ulong target_cmsg_addr;
-    struct target_cmsghdr *target_cmsg;
+    struct target_cmsghdr *target_cmsg, *target_cmsg_start;
     socklen_t space = 0;
     
     msg_controllen = tswapal(target_msgh->msg_controllen);
@@ -1189,6 +1189,7 @@ static inline abi_long target_to_host_cmsg(struct msghdr *msgh,
         goto the_end;
     target_cmsg_addr = tswapal(target_msgh->msg_control);
     target_cmsg = lock_user(VERIFY_READ, target_cmsg_addr, msg_controllen, 1);
+    target_cmsg_start = target_cmsg;
     if (!target_cmsg)
         return -TARGET_EFAULT;
 
@@ -1247,7 +1248,8 @@ static inline abi_long target_to_host_cmsg(struct msghdr *msgh,
         }
 
         cmsg = CMSG_NXTHDR(msgh, cmsg);
-        target_cmsg = TARGET_CMSG_NXTHDR(target_msgh, target_cmsg);
+        target_cmsg = TARGET_CMSG_NXTHDR(target_msgh, target_cmsg,
+                                         target_cmsg_start);
     }
     unlock_user(target_cmsg, target_cmsg_addr, 0);
  the_end:
@@ -1261,7 +1263,7 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
     struct cmsghdr *cmsg = CMSG_FIRSTHDR(msgh);
     abi_long msg_controllen;
     abi_ulong target_cmsg_addr;
-    struct target_cmsghdr *target_cmsg;
+    struct target_cmsghdr *target_cmsg, *target_cmsg_start;
     socklen_t space = 0;
 
     msg_controllen = tswapal(target_msgh->msg_controllen);
@@ -1269,6 +1271,7 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
         goto the_end;
     target_cmsg_addr = tswapal(target_msgh->msg_control);
     target_cmsg = lock_user(VERIFY_WRITE, target_cmsg_addr, msg_controllen, 0);
+    target_cmsg_start = target_cmsg;
     if (!target_cmsg)
         return -TARGET_EFAULT;
 
@@ -1382,14 +1385,15 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
         }
 
         target_cmsg->cmsg_len = tswapal(tgt_len);
-        tgt_space = TARGET_CMSG_SPACE(tgt_len);
+        tgt_space = TARGET_CMSG_SPACE(len);
         if (msg_controllen < tgt_space) {
             tgt_space = msg_controllen;
         }
         msg_controllen -= tgt_space;
         space += tgt_space;
         cmsg = CMSG_NXTHDR(msgh, cmsg);
-        target_cmsg = TARGET_CMSG_NXTHDR(target_msgh, target_cmsg);
+        target_cmsg = TARGET_CMSG_NXTHDR(target_msgh, target_cmsg,
+                                         target_cmsg_start);
     }
     unlock_user(target_cmsg, target_cmsg_addr, space);
  the_end:
diff --git a/linux-user/syscall_defs.h b/linux-user/syscall_defs.h
index edd5f3c..8b0e354 100644
--- a/linux-user/syscall_defs.h
+++ b/linux-user/syscall_defs.h
@@ -234,7 +234,8 @@ 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) \
@@ -242,17 +243,17 @@ 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))
+  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 {
-- 
2.5.0