Re: [PATCH v3 06/11] softmmu/physmem: Fail creation of new files in file_ram_open() with readonly=true

[Peter Xu]

On Wed, Aug 23, 2023 at 05:34:06PM +0200, David Hildenbrand wrote:
> 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.
> 
> Specifying O_RDONLY instead to create a new readonly file would
> theoretically work, however, ftruncate() will refuse to resize the new
> empty file and we'll get a warning:
>     ftruncate: Invalid argument
> And later eventually more problems when actually mmap'ing that file and
> accessing it.
> 
> 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,readonly=true,size=1g
> qemu-system-x86_64: can't open backing store tmp for guest RAM: No such file 
> or directory
> 
> All use cases of readonly files (R/O NVDIMMs, VM templating) work on
> existing files, so silently creating new files might 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().
> 
> Move error reporting to the single caller.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu