[Qemu-devel] [PATCH v4 1/3] memfd: split qemu_memfd_alloc()

[Marc-André Lureau]

Add a function to only create a memfd, without mmap. The function is
used in the following memory backend.

Signed-off-by: Marc-André Lureau <address@hidden>
---
 include/qemu/memfd.h |  2 ++
 util/memfd.c         | 42 +++++++++++++++++++++++++-----------------
 2 files changed, 27 insertions(+), 17 deletions(-)

diff --git a/include/qemu/memfd.h b/include/qemu/memfd.h
index 745a8c501e..30c1ab1d91 100644
--- a/include/qemu/memfd.h
+++ b/include/qemu/memfd.h
@@ -16,6 +16,8 @@
 #define F_SEAL_WRITE    0x0008  /* prevent writes */
 #endif
 
+int qemu_memfd_create(const char *name, size_t size, unsigned int seals,
+                      bool must_seal);
 void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
                        int *fd);
 void qemu_memfd_free(void *ptr, size_t size, int fd);
diff --git a/util/memfd.c b/util/memfd.c
index 4571d1aba8..e6476df083 100644
--- a/util/memfd.c
+++ b/util/memfd.c
@@ -55,45 +55,53 @@ static int memfd_create(const char *name, unsigned int 
flags)
 #define MFD_ALLOW_SEALING 0x0002U
 #endif
 
-/*
- * This is a best-effort helper for shared memory allocation, with
- * optional sealing. The helper will do his best to allocate using
- * memfd with sealing, but may fallback on other methods without
- * sealing.
- */
-void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
-                       int *fd)
+int qemu_memfd_create(const char *name, size_t size, unsigned int seals,
+    bool must_seal)
 {
-    void *ptr;
     int mfd = -1;
 
-    *fd = -1;
-
 #ifdef CONFIG_LINUX
     if (seals) {
         mfd = memfd_create(name, MFD_ALLOW_SEALING | MFD_CLOEXEC);
     }
 
-    if (mfd == -1) {
+    if (mfd == -1 && !must_seal) {
         /* some systems have memfd without sealing */
         mfd = memfd_create(name, MFD_CLOEXEC);
         seals = 0;
     }
-#endif
 
-    if (mfd != -1) {
+    if (mfd >= 0) {
         if (ftruncate(mfd, size) == -1) {
             perror("ftruncate");
             close(mfd);
-            return NULL;
+            return -1;
         }
 
         if (seals && fcntl(mfd, F_ADD_SEALS, seals) == -1) {
             perror("fcntl");
             close(mfd);
-            return NULL;
+            return -1;
         }
-    } else {
+    }
+#endif
+
+    return mfd;
+}
+
+/*
+ * This is a best-effort helper for shared memory allocation, with
+ * optional sealing. The helper will do his best to allocate using
+ * memfd with sealing, but may fallback on other methods without
+ * sealing.
+ */
+void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
+                       int *fd)
+{
+    void *ptr;
+    int mfd = qemu_memfd_create(name, size, seals, false);
+
+    if (mfd == -1) {
         const char *tmpdir = g_get_tmp_dir();
         gchar *fname;
 
-- 
2.13.1.395.gf7b71de06