Re: [PATCH] bufio: fix the next_buf calculation

[Michael Chang <mchang@suse.com>]

On Tue, Apr 17, 2018 at 07:11:34PM +0200, Daniel Kiper wrote:
> CC-ing Vladimir.
> 
> On Mon, Apr 16, 2018 at 06:05:04PM +0800, Michael Chang wrote:
> > The next_buf is the offset to the next cached block rounded to the size of
> > bufio->block_size. However the calculation needs the block_size to be in power
> > of 2 is not always valid. As an example, files with smaller size than
> > block_size will have the block_size leveled to the size of file which can be
> > set arbitrary value.
> >
> > This patch fixes the next_buf calculation to accept any integers.
> >
> > Signed-off-by: Michael Chang <mchang@suse.com>
> > ---
> >  grub-core/io/bufio.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/grub-core/io/bufio.c b/grub-core/io/bufio.c
> > index 22438277d..d0b0f71b6 100644
> > --- a/grub-core/io/bufio.c
> > +++ b/grub-core/io/bufio.c
> > @@ -132,7 +132,7 @@ grub_bufio_read (grub_file_t file, char *buf, grub_size_t len)
> >      return res;
> >
> >    /* Need to read some more.  */
> > -  next_buf = (file->offset + res + len - 1) & ~((grub_off_t) bufio->block_size - 1);
> > +  next_buf = (grub_divmod64 (file->offset + res + len - 1, bufio->block_size, NULL)) * bufio->block_size;
> 
> Should not you fix this in grub_bufio_open()? I think that
> you should look for closest power of 2 in it.

Of course here we can round up or down the bufio->block_size to meet power of
2. The down side of round-down is inefficient for small files as it can't cache
entire file in one go. The round-up will have to allocate (slightly) larger
than needed buffer to hold small files which is not a big deal ...

If you insist, I can work a new patch to round up the block_size in
grub_bufio_open, just let me know.

Thanks,
Michael

> 
> Daniel