[1/2] migration/compress: compress QEMUFile is not writable

We open a file with empty_ops for compress QEMUFile, which means this is
not writable.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
---
 migration/qemu-file.c | 16 +++-------------
 1 file changed, 3 insertions(+), 13 deletions(-)

* Wei Yang (richardw.yang@linux.intel.com) wrote:
> We open a file with empty_ops for compress QEMUFile, which means this is
> not writable.

That explanation sounds reasonable; but I'm confused by the history of
this;  the code was added by Liang Li in :

  b3be289 qemu-file: Fix qemu_put_compression_data flaw

  ( https://www.mail-archive.com/qemu-devel@nongnu.org/msg368974.html )

with almost exactly the opposite argument;  can we figure out why?

Dave

> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> ---
>  migration/qemu-file.c | 16 +++-------------
>  1 file changed, 3 insertions(+), 13 deletions(-)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index 26fb25ddc1..f3d99a96ec 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -744,11 +744,8 @@ static int qemu_compress_data(z_stream *stream, uint8_t *dest, size_t dest_len,
>  /* Compress size bytes of data start at p and store the compressed
>   * data to the buffer of f.
>   *
> - * When f is not writable, return -1 if f has no space to save the
> - * compressed data.
> - * When f is wirtable and it has no space to save the compressed data,
> - * do fflush first, if f still has no space to save the compressed
> - * data, return -1.
> + * Since the file is dummy file with empty_ops, return -1 if f has no space to
> + * save the compressed data.
>   */
>  ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
>                                    const uint8_t *p, size_t size)
> @@ -756,14 +753,7 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
>      ssize_t blen = IO_BUF_SIZE - f->buf_index - sizeof(int32_t);
>  
>      if (blen < compressBound(size)) {
> -        if (!qemu_file_is_writable(f)) {
> -            return -1;
> -        }
> -        qemu_fflush(f);
> -        blen = IO_BUF_SIZE - sizeof(int32_t);
> -        if (blen < compressBound(size)) {
> -            return -1;
> -        }
> +        return -1;
>      }
>  
>      blen = qemu_compress_data(stream, f->buf + f->buf_index + sizeof(int32_t),
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

On Thu, Nov 07, 2019 at 11:59:10AM +0000, Dr. David Alan Gilbert wrote:
>* Wei Yang (richardw.yang@linux.intel.com) wrote:
>> We open a file with empty_ops for compress QEMUFile, which means this is
>> not writable.
>
>That explanation sounds reasonable; but I'm confused by the history of
>this;  the code was added by Liang Li in :
>
>  b3be289 qemu-file: Fix qemu_put_compression_data flaw
>
>  ( https://www.mail-archive.com/qemu-devel@nongnu.org/msg368974.html )
>
>with almost exactly the opposite argument;  can we figure out why?
>

Hmm... sounds interesting.

Toke a look into the change log, which says

    Current qemu_put_compression_data can only work with no writable
    QEMUFile, and can't work with the writable QEMUFile. But it does
    not provide any measure to prevent users from using it with a
    writable QEMUFile.

    We should fix this flaw to make it works with writable QEMUFile.

While I don't see a chance to use writable QEMUFile. Do I miss something?

>Dave
>

"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> * Wei Yang (richardw.yang@linux.intel.com) wrote:
>> We open a file with empty_ops for compress QEMUFile, which means this is
>> not writable.
>
> That explanation sounds reasonable; but I'm confused by the history of
> this;  the code was added by Liang Li in :
>
>   b3be289 qemu-file: Fix qemu_put_compression_data flaw
>
>   ( https://www.mail-archive.com/qemu-devel@nongnu.org/msg368974.html )
>
> with almost exactly the opposite argument;  can we figure out why?

Comment of that patch is wrong, code agrees with this patch.
The patch that went in does:

-        return 0;
+        if (!qemu_file_is_writable(f)) {
+            return -1;
+        }
+        qemu_fflush(f);
+        blen = IO_BUF_SIZE - sizeof(int32_t);
+        if (blen < compressBound(size)) {
+            return -1;
+        }

i.e. it move from return 0 to return -1 if we are not able to write, or
if we are not able to get enough space.

>> @@ -756,14 +753,7 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
>>      ssize_t blen = IO_BUF_SIZE - f->buf_index - sizeof(int32_t);
>>  
>>      if (blen < compressBound(size)) {
>> -        if (!qemu_file_is_writable(f)) {
>> -            return -1;
>> -        }
>> -        qemu_fflush(f);
>> -        blen = IO_BUF_SIZE - sizeof(int32_t);
>> -        if (blen < compressBound(size)) {
>> -            return -1;
>> -        }
>> +        return -1;
>>      }

This moves from trying some things to just return -1.

And patch is ok.  compression file has a file with empty_os, so
qemu_fflush() will not help at all.

Reviewed-by: Juan Quintela <quintela@redhat.com>


Thanks, Juan.