[RFC 4/5] osdep: add qemu_recvmsg() wrapper

[Stefan Hajnoczi <stefanha@xxxxxxxxxxxxxxxxxx>]

Usually we need to set O_CLOEXEC, which is platform-specific.  Add a
wrapper like qemu_open() but for qemu_recvmsg().

Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxxxxxxxxxx>
---
 block.c       |    5 +----
 osdep.c       |   46 ++++++++++++++++++++++++++++++++++++++++++++++
 qemu-common.h |    2 ++
 3 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/block.c b/block.c
index d3bf443..e66d64f 100644
--- a/block.c
+++ b/block.c
@@ -176,10 +176,7 @@ int file_open(const char *filename, int flags, mode_t mode)
     msg.msg_control = &msg_control;
     msg.msg_controllen = sizeof(msg_control);
 
-    do {
-        ret = recvmsg(remote_file_fd, &msg, 0);
-    } while (ret == -1 && errno == EINTR);
-    if (ret != sizeof(OpenResponse)) {
+    if (qemu_recvmsg(remote_file_fd, &msg, 0) != sizeof(OpenResponse)) {
         errno = EPIPE;
         return -1;
     }
diff --git a/osdep.c b/osdep.c
index 3e6bada..834e78f 100644
--- a/osdep.c
+++ b/osdep.c
@@ -103,6 +103,52 @@ int qemu_open(const char *name, int flags, ...)
 }
 
 /*
+ * Receive a message from a socket
+ *
+ * This behaves like recvmsg(2) except that EINTR is handled internally and
+ * never returned.  Passed file descriptors will have O_CLOEXEC set.
+ */
+ssize_t qemu_recvmsg(int fd, struct msghdr *msg, int flags)
+{
+    ssize_t ret;
+
+#ifdef MSG_CMSG_CLOEXEC
+    /* Receive file descriptors with O_CLOEXEC */
+    flags |= MSG_CMSG_CLOEXEC;
+#endif
+
+    do {
+        ret = recvmsg(fd, msg, flags);
+    } while (ret == -1 && errno == EINTR);
+    if (ret < 0) {
+        return ret;
+    }
+
+#ifndef MSG_CMSG_CLOEXEC
+    /* As a fallback, set O_CLOEXEC in a way that is not thread-safe */
+    {
+        struct cmsghdr *cmsg;
+        for (cmsg = CMSG_FIRSTHDR(msg); cmsg; cmsg = CMSG_NXTHDR(msg, cmsg)) {
+            int new_fd;
+
+            if (cmsg->cmsg_len != CMSG_LEN(sizeof(int)) ||
+                cmsg->cmsg_level != SOL_SOCKET ||
+                cmsg->cmsg_type != SCM_RIGHTS) {
+                continue;
+            }
+
+            new_fd = *((int *)CMSG_DATA(cmsg));
+            if (new_fd >= 0) {
+                qemu_set_cloexec(new_fd);
+            }
+        }
+    }
+#endif
+
+    return ret;
+}
+
+/*
  * A variant of write(2) which handles partial write.
  *
  * Return the number of bytes transferred.
diff --git a/qemu-common.h b/qemu-common.h
index 50f659a..e3a6c4d 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -184,6 +184,8 @@ const char *path(const char *pathname);
 void *qemu_oom_check(void *ptr);
 
 int qemu_open(const char *name, int flags, ...);
+struct msghdr;
+ssize_t qemu_recvmsg(int fd, struct msghdr *msg, int flags);
 ssize_t qemu_write_full(int fd, const void *buf, size_t count)
     QEMU_WARN_UNUSED_RESULT;
 ssize_t qemu_send_full(int fd, const void *buf, size_t count, int flags)
-- 
1.7.10

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