Re: [PATCH] physmem: fix qemu_ram_alloc_from_fd size calculation

[Peter Xu <peterx@redhat.com>]

On Thu, Jan 02, 2025 at 12:34:50PM -0800, Steve Sistare wrote:
> qemu_ram_alloc_from_fd allocates space if file_size == 0.  If non-zero,
> it uses the existing space and verifies it is large enough, but the
> verification was broken when the offset parameter was introduced.  As
> a result, a file smaller than offset passes the verification and causes
> errors later.  Fix that, and update the error message to include offset.
> 
> Peter provides this concise reproducer:
> 
>   $ touch ramfile
>   $ truncate -s 64M ramfile
>   $ ./qemu-system-x86_64 -object memory-backend-file,mem-path=./ramfile,offset=128M,size=128M,id=mem1,prealloc=on
>   qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address
> 
> With the fix, the error message is:
>   qemu-system-x86_64: mem1 backing store size 0x4000000 is too small for 'size' option 0x8000000 plus 'offset' option 0x8000000
> 

Can have the cc tag here too to be super clear:

Cc: qemu-stable@nongnu.org

> Fixes: 4b870dc4d0c0 ("hostmem-file: add offset option")
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>

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

Please also feel free to attach this patch as the 1st patch when repost
cpr-transfer, it could make dependency easier no matter how this would
land.

Maybe it'll be still easier to go via a migration pull that collects cpr
transfer as a whole. May depend on whether there'll be a memory API pull
before that..

Thanks,

-- 
Peter Xu