[PATCH v4 4/6] util: introduce qemu_open and qemu_create with error reporting

["Daniel P. Berrangé" <berrange@redhat.com>]

qemu_open_old() works like open(): set errno and return -1 on failure.
It has even more failure modes, though.  Reporting the error clearly
to users is basically impossible for many of them.

Our standard cure for "errno is too coarse" is the Error object.
Introduce two new helper methods:

  int qemu_open(const char *name, int flags, Error **errp);
  int qemu_create(const char *name, int flags, mode_t mode, Error **errp);

Note that with this design we no longer require or even accept the
O_CREAT flag. Avoiding overloading the two distinct operations
means we can avoid variable arguments which would prevent 'errp' from
being the last argument. It also gives us a guarantee that the 'mode' is
given when creating files, avoiding a latent security bug.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 include/qemu/osdep.h |  6 ++++++
 util/osdep.c         | 21 +++++++++++++++++----
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 18333e9006..13a821845b 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -497,7 +497,13 @@ int qemu_madvise(void *addr, size_t len, int advice);
 int qemu_mprotect_rwx(void *addr, size_t size);
 int qemu_mprotect_none(void *addr, size_t size);
 
+/*
+ * Don't introduce new usage of this function, prefer the following
+ * qemu_open/qemu_create that take an "Error **errp"
+ */
 int qemu_open_old(const char *name, int flags, ...);
+int qemu_open(const char *name, int flags, Error **errp);
+int qemu_create(const char *name, int flags, mode_t mode, Error **errp);
 int qemu_close(int fd);
 int qemu_unlink(const char *name);
 #ifndef _WIN32
diff --git a/util/osdep.c b/util/osdep.c
index 9c7118d3cb..a4956fbf6b 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -344,10 +344,7 @@ qemu_open_internal(const char *name, int flags, mode_t mode, Error **errp)
 #endif /* ! O_CLOEXEC */
 
     if (ret == -1) {
-        const char *action = "open";
-        if (flags & O_CREAT) {
-            action = "create";
-        }
+        const char *action = flags & O_CREAT ? "create" : "open";
         error_setg_errno(errp, errno, "Could not %s '%s' flags 0x%x",
                          action, name, flags);
     }
@@ -357,6 +354,22 @@ qemu_open_internal(const char *name, int flags, mode_t mode, Error **errp)
 }
 
 
+int qemu_open(const char *name, int flags, Error **errp)
+{
+    assert(!(flags & O_CREAT));
+
+    return qemu_open_internal(name, flags, 0, errp);
+}
+
+
+int qemu_create(const char *name, int flags, mode_t mode, Error **errp)
+{
+    assert(!(flags & O_CREAT));
+
+    return qemu_open_internal(name, flags | O_CREAT, mode, errp);
+}
+
+
 int qemu_open_old(const char *name, int flags, ...)
 {
     va_list ap;
-- 
2.26.2