[PATCH v1 1/3] softmmu/physmem: fallback to opening guest RAM file as readonly in a MAP_PRIVATE mapping

[David Hildenbrand]

From: Thiner Logoer <logoerthin...@163.com>

Users may specify
* "-mem-path" or
* "-object memory-backend-file,share=off,readonly=off"
and expect such COW (MAP_PRIVATE) mappings to work, even if the user
does not have write permissions to open the file.

For now, we would always fail in that case, always requiring file write
permissions. Let's detect when that failure happens and fallback to opening
the file readonly.

Warn the user, since there are other use cases where we want the file to
be mapped writable: ftruncate() and fallocate() will fail if the file
was not opened with write permissions.

Signed-off-by: Thiner Logoer <logoerthin...@163.com>
Co-developed-by: David Hildenbrand <da...@redhat.com>
Signed-off-by: David Hildenbrand <da...@redhat.com>
---
 softmmu/physmem.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 3df73542e1..d1ae694b20 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -1289,8 +1289,7 @@ static int64_t get_file_align(int fd)
 static int file_ram_open(const char *path,
                          const char *region_name,
                          bool readonly,
-                         bool *created,
-                         Error **errp)
+                         bool *created)
 {
     char *filename;
     char *sanitized_name;
@@ -1334,10 +1333,7 @@ static int file_ram_open(const char *path,
             g_free(filename);
         }
         if (errno != EEXIST && errno != EINTR) {
-            error_setg_errno(errp, errno,
-                             "can't open backing store %s for guest RAM",
-                             path);
-            return -1;
+            return -errno;
         }
         /*
          * Try again on EINTR and EEXIST.  The latter happens when
@@ -1946,9 +1942,23 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, 
MemoryRegion *mr,
     bool created;
     RAMBlock *block;
 
-    fd = file_ram_open(mem_path, memory_region_name(mr), readonly, &created,
-                       errp);
+    fd = file_ram_open(mem_path, memory_region_name(mr), readonly, &created);
+    if (fd == -EACCES && !(ram_flags & RAM_SHARED) && !readonly) {
+        /*
+         * We can have a writable MAP_PRIVATE mapping of a readonly file.
+         * However, some operations like ftruncate() or fallocate() might fail
+         * later, let's warn the user.
+         */
+        fd = file_ram_open(mem_path, memory_region_name(mr), true, &created);
+        if (fd >= 0) {
+            warn_report("backing store %s for guest RAM (MAP_PRIVATE) opened"
+                        " readonly because the file is not writable", 
mem_path);
+        }
+    }
     if (fd < 0) {
+        error_setg_errno(errp, -fd,
+                         "can't open backing store %s for guest RAM",
+                         mem_path);
         return NULL;
     }
 
-- 
2.41.0