softmmu/physmem: Never return directories from file_ram_open()

open() does not fail on directories when opening them readonly (O_RDONLY).

Currently, we succeed opening such directories and fail later during
mmap(), resulting in a misleading error message.

$ ./qemu-system-x86_64 \
    -object memory-backend-file,id=ram0,mem-path=tmp,readonly=true,size=1g
 qemu-system-x86_64: unable to map backing store for guest RAM: No such device

To identify directories and handle them accordingly in file_ram_open()
also when readonly=true was specified, detect if we just opened a directory
using fstat() instead. Then, fail file_ram_open() right away, similarly
to how we now fail if the file does not exist and we want to open the
file readonly.

With this change, we get a nicer error message:
 qemu-system-x86_64: can't open backing store tmp for guest RAM: Is a directory

Note that the only memory-backend-file will end up calling
memory_region_init_ram_from_file() -> qemu_ram_alloc_from_file() ->
file_ram_open().

Message-ID: <20230906120503.359863-8-david@redhat.com>
Reported-by: Thiner Logoer <logoerthiner1@163.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

softmmu/physmem.c

@@ -1299,6 +1299,25 @@ static int file_ram_open(const char *path,
     for (;;) {
         fd = open(path, readonly ? O_RDONLY : O_RDWR);
         if (fd >= 0) {
+            /*
+             * open(O_RDONLY) won't fail with EISDIR. Check manually if we
+             * opened a directory and fail similarly to how we fail ENOENT
+             * in readonly mode. Note that mkstemp() would imply O_RDWR.
+             */
+            if (readonly) {
+                struct stat file_stat;
+
+                if (fstat(fd, &file_stat)) {
+                    close(fd);
+                    if (errno == EINTR) {
+                        continue;
+                    }
+                    return -errno;
+                } else if (S_ISDIR(file_stat.st_mode)) {
+                    close(fd);
+                    return -EISDIR;
+                }
+            }
             /* @path names an existing file, use it */
             break;
         }