Re: [Qemu-devel] [PATCH v2 resend 3/4] Block: readonly changes

[Kevin Wolf]

Am 04.02.2010 23:04, schrieb Naphtali Sprei:
> Open backing file for read-only
> During commit upgrade to read-write and back at end to read-only
> 
> Signed-off-by: Naphtali Sprei <address@hidden>
> ---
>  block.c     |   68 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++-------
>  block_int.h |    1 +
>  2 files changed, 61 insertions(+), 8 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 66564de..4a9df91 100644
> --- a/block.c
> +++ b/block.c
> @@ -451,7 +451,6 @@ int bdrv_open2(BlockDriverState *bs, const char 
> *filename, int flags,
>      if (flags & (BDRV_O_CACHE_WB|BDRV_O_NOCACHE))
>          bs->enable_write_cache = 1;
>  
> -    bs->read_only = (flags & BDRV_O_RDWR) == 0;
>      if (!(flags & BDRV_O_FILE)) {
>          open_flags = (flags & (BDRV_O_RDWR | 
> BDRV_O_CACHE_MASK|BDRV_O_NATIVE_AIO));
>          if (bs->is_temporary) { /* snapshot should be writeable */
> @@ -466,6 +465,7 @@ int bdrv_open2(BlockDriverState *bs, const char 
> *filename, int flags,
>          goto free_and_fail;
>      }
>  
> +    bs->keep_read_only = bs->read_only = !(open_flags & BDRV_O_RDWR);
>      if (drv->bdrv_getlength) {
>          bs->total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
>      }
> @@ -482,13 +482,28 @@ int bdrv_open2(BlockDriverState *bs, const char 
> *filename, int flags,
>                       filename, bs->backing_file);
>          if (bs->backing_format[0] != '\0')
>              back_drv = bdrv_find_format(bs->backing_format);
> +
> +        open_flags &= ~BDRV_O_RDWR; /* clear RW, then restore from orig */
> +        if (bs->is_temporary) {
> +            open_flags |= (flags & BDRV_O_RDWR);
> +        }
> +        
>          ret = bdrv_open2(bs->backing_hd, backing_filename, open_flags,
>                           back_drv);
> -        bs->backing_hd->read_only =  (open_flags & BDRV_O_RDWR) == 0;
> +        if (ret < 0) {
> +            open_flags &= ~BDRV_O_RDWR;  /* Fall-back to read-only for the 
> backing file */
> +            ret = bdrv_open2(bs->backing_hd, backing_filename, open_flags,
> +                             back_drv);
> +        }

Why is this needed? The only case in which a backing file is opened
read-write is during commit, right? For commit there is certainly no use
in opening it read-only instead.

This whole code looks like there are cases where a backing file is still
opened read-write by default, though the commit message says that no
such backing files exist. Am I missing something?

>          if (ret < 0) {
>              bdrv_close(bs);
>              return ret;
>          }
> +        if (!bs->is_temporary) {
> +            bs->backing_hd->keep_read_only = bs->keep_read_only; /* base 
> image inherits from "parent" and open read-only */

This looks like more than 80 characters on a line.

What would helps here and also would improve consistency in style is to
move the comments to the line before instead of sticking them at the end
of a code line. This is true even more if the comment actually applies
to a whole block and not only to the line in which it is written (you're
doing this in other places).

> +        } else {
> +            bs->backing_hd->keep_read_only = !(flags & BDRV_O_RDWR);
> +        }
>      }
>  
>      if (!bdrv_key_required(bs)) {
> @@ -564,19 +579,38 @@ int bdrv_commit(BlockDriverState *bs)
>  {
>      BlockDriver *drv = bs->drv;
>      int64_t i, total_sectors;
> -    int n, j;
> +    int n, j, ro, open_flags;
>      int ret = 0;
>      unsigned char sector[512];
> +    char filename[1024];
> +    BlockDriverState *bs_rw, *bs_ro;
>  
>      if (!drv)
>          return -ENOMEDIUM;
> +    
> +    if (!bs->backing_hd) {
> +     return -ENOTSUP;
> +    }
>  
> -    if (bs->read_only) {
> +    if (bs->backing_hd->keep_read_only) {
>       return -EACCES;
>      }
> +    
> +    ro = bs->backing_hd->read_only;
> +    strncpy(filename, bs->backing_hd->filename, sizeof(filename));
> +    open_flags =  bs->backing_hd->open_flags;
>  
> -    if (!bs->backing_hd) {
> -     return -ENOTSUP;
> +    if (ro) { /* re-open as RW */
> +        bdrv_close(bs->backing_hd);
> +        qemu_free(bs->backing_hd);

bdrv_delete is doing what you mean here. But actually, you don't need to
delete it, you can just reuse the old bs for re-opening the image.

> +
> +        bs_rw = bdrv_new("");
> +        ret = bdrv_open2(bs_rw, filename, open_flags | BDRV_O_RDWR, NULL);
> +        if (ret < 0) {
> +            bdrv_delete(bs_rw);
> +            return -EACCES;

Why don't you pass the right return value up? Apart from that, you
should re-open the backing file (read-only) or the VM will get into
trouble...

> +        }
> +        bs->backing_hd = bs_rw;

Eek... ;-) Well, it should work, as far as I know the block drivers.

>      }
>  
>      total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
> @@ -584,11 +618,13 @@ int bdrv_commit(BlockDriverState *bs)
>          if (drv->bdrv_is_allocated(bs, i, 65536, &n)) {
>              for(j = 0; j < n; j++) {
>                  if (bdrv_read(bs, i, sector, 1) != 0) {
> -                    return -EIO;
> +                    ret = -EIO;
> +                    goto ro_cleanup;
>                  }
>  
>                  if (bdrv_write(bs->backing_hd, i, sector, 1) != 0) {
> -                    return -EIO;
> +                    ret = -EIO;
> +                    goto ro_cleanup;
>                  }
>                  i++;
>           }
> @@ -608,6 +644,22 @@ int bdrv_commit(BlockDriverState *bs)
>       */
>      if (bs->backing_hd)
>          bdrv_flush(bs->backing_hd);
> +
> +ro_cleanup:
> +
> +    if (ro) { /* re-open as RO */
> +        bdrv_close(bs->backing_hd);
> +        qemu_free(bs->backing_hd);

Again, I think bdrv_delete is needed.

> +        bs_ro = bdrv_new("");
> +        ret = bdrv_open2(bs_ro, filename, open_flags & ~BDRV_O_RDWR, NULL);
> +        if (ret < 0) {
> +            bdrv_delete(bs_ro);
> +            return -EACCES;

Again, wrong return value.

> +        }
> +        bs->backing_hd = bs_ro;
> +        bs->backing_hd->keep_read_only = 0;
> +    }
> +
>      return ret;
>  }
>  
> diff --git a/block_int.h b/block_int.h
> index 9144d37..02fae5b 100644
> --- a/block_int.h
> +++ b/block_int.h
> @@ -130,6 +130,7 @@ struct BlockDriverState {
>      int64_t total_sectors; /* if we are reading a disk image, give its
>                                size in sectors */
>      int read_only; /* if true, the media is read only */
> +    int keep_read_only; /* if true, the media was requested to stay read 
> only */
>      int open_flags; /* flags used to open the file */
>      int removable; /* if true, the media can be removed */
>      int locked;    /* if true, the media cannot temporarily be ejected */

Kevin