Re: [Qemu-devel] [PATCH v2] qemu_file: use fwrite() correctly

[Markus Armbruster <armbru@redhat.com>]

Juan Quintela <quintela@redhat.com> writes:

> 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, the 1st
> time it returns 0, after that it returns 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).  On the 1st fwrite() for the read-only filesystem,
> it returns an errno of -EPIPE, that is exactly what has failed.
>
> 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 <quintela@redhat.com>
> ---
>
> v2: a.k.a Paolo was right
>
>     On the first call to fwrite() it returns 0, and errno is setup to
>     EPIPE, exactly what we wanted.
>
>     Once here, improve the commit message.
>
>  qemu-file.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/qemu-file.c b/qemu-file.c
> index 9473b67..e5ec798 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 -errno;
> +    }
> +    return res;
>  }
>
>  static int stdio_get_buffer(void *opaque, uint8_t *buf, int64_t pos, int size)

This is what got an R-by from Eric and me.  And this is what Peter
pulled from Juan:

  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 */
  +    }
  +    return res;
   }

Follow-up fix sent.