[PATCH v1 2/3] softmmu/physmem: fail creation of new files in file_ram_open() with readonly=true

[David Hildenbrand <david@redhat.com>]

Currently, if a file does not exist yet, file_ram_open() will create new
empty file and open it writable. However, it even does that when
readonly=true was specified. So instead of opening the file read-only,
we'll open it writable, implying that later fallocate() or ftruncate()
could succeed.

Specifying O_RDONLY instead would theoretically work, however,
ftruncate() will refuse to resize the new empty file and we'll get a
warning:
    ftruncate: Invalid argument

mmap() will succeed, but any later access to that memory would fail with
SIGBUS. Undesirable.

If someone intends to let QEMU open+mmap a file read-only, better
create+resize+fill that file ahead of time outside of QEMU context.

We'll now fail with:
./qemu-system-x86_64 \
    -object memory-backend-file,id=ram0,mem-path=tmp/ls,readonly=true,size=1g
qemu-system-x86_64: can't open backing store tmp/ls for guest RAM: No such file or directory

It's unlikely that this will harm existing users: especially R/O NVDIMMs
better expose some reasonable data that already exists. Everything else
would just hide user errors when accidentally specifying a non-existent
file.

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().

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 softmmu/physmem.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index d1ae694b20..9580567608 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -1304,6 +1304,13 @@ static int file_ram_open(const char *path,
             break;
         }
         if (errno == ENOENT) {
+            if (readonly) {
+                /*
+                 * O_RDONLY would later make ftruncate() fail, leading to
+                 * SIGBUS after mmap().
+                 */
+                return -ENOENT;
+            }
             /* @path names a file that doesn't exist, create it */
             fd = open(path, O_RDWR | O_CREAT | O_EXCL, 0644);
             if (fd >= 0) {
-- 
2.41.0