Re: [Qemu-devel] [PATCH] qemu_file: use fwrite() correctly

[Paolo Bonzini]

Il 11/02/2014 22:56, Juan Quintela ha scritto:
> fwrite() returns the number of items written.  But when there is one
> error, it can return a short write.
>
> In the particular bug that I was tracking, I did a migration to a
> read-only filesystem.  And it was able to finish the migration
> correctly.  fwrite() never returned a negative error code, nor zero,
> always 4096. (migration writes chunks of about 14000 bytes).  And it
> was able to "complete" the migration with success (yes, reading the
> file was a bit more difficult).
>
> To add insult to injury, if your amount of memory was big enough (12GB
> on my case), it overwrote some important structure, and from them,
> malloc failed.  This check makes the problem go away.
>
> Signed-off-by: Juan Quintela <address@hidden>
> ---
>  qemu-file.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/qemu-file.c b/qemu-file.c
> index 9473b67..f074af1 100644
> --- a/qemu-file.c
> +++ b/qemu-file.c
> @@ -100,7 +100,14 @@ static int stdio_put_buffer(void *opaque, const uint8_t 
> *buf, int64_t pos,
>                              int size)
>  {
>      QEMUFileStdio *s = opaque;
> -    return fwrite(buf, 1, size, s->stdio_file);
> +    int res;
> +
> +    res = fwrite(buf, 1, size, s->stdio_file);
> +
> +    if (res != size) {
> +        return -EIO;   /* fake errno value */

Can you return -errno here?  No need for a fake value.

Paolo

> +    }
> +    return res;
>  }
>
>  static int stdio_get_buffer(void *opaque, uint8_t *buf, int64_t pos, int size)