[PATCH] fs/squashfs: use do_div function for math

[Pali Rohár]

On Thursday 12 May 2022 12:19:01 Pali Rohár wrote:
> On Monday 09 May 2022 15:08:15 Miquel Raynal wrote:
> > Hello,
> > 
> > trini at konsulko.com wrote on Thu, 5 May 2022 09:15:00 -0400:
> > 
> > > On Thu, May 05, 2022 at 11:26:37AM +0200, Sean Nyekjaer wrote:
> > > 
> > > > From: Sean Nyekjaer <sean.nyekjaer.ext at siemensgamesa.com>
> > > > 
> > > > When compling for x86:
> > > > ld.bfd: fs/squashfs/sqfs.o: in function `sqfs_read':
> > > > u-boot/fs/squashfs/sqfs.c:1443: undefined reference to `__udivmoddi4'
> > > > ld.bfd: u-boot/fs/squashfs/sqfs.c:1521: undefined reference to `__udivmoddi4'
> > > > 
> > > > Signed-off-by: Sean Nyekjaer <sean.nyekjaer.ext at siemensgamesa.com>
> > > > ---
> > > >  fs/squashfs/sqfs.c | 5 +++--
> > > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/fs/squashfs/sqfs.c b/fs/squashfs/sqfs.c
> > > > index 5d9c52af80..6405db4ff3 100644
> > > > --- a/fs/squashfs/sqfs.c
> > > > +++ b/fs/squashfs/sqfs.c
> > > > @@ -8,6 +8,7 @@
> > > >   */
> > > >  
> > > >  #include <asm/unaligned.h>
> > > > +#include <div64.h>
> > > >  #include <errno.h>
> > > >  #include <fs.h>
> > > >  #include <linux/types.h>
> > > > @@ -1440,7 +1441,7 @@ int sqfs_read(const char *filename, void *buf, loff_t offset, loff_t len,
> > > >  	}
> > > >  
> > > >  	for (j = 0; j < datablk_count; j++) {
> > > > -		start = data_offset / ctxt.cur_dev->blksz;
> > > > +		start = do_div(data_offset, ctxt.cur_dev->blksz);
> > > >  		table_size = SQFS_BLOCK_SIZE(finfo.blk_sizes[j]);
> > > >  		table_offset = data_offset - (start * ctxt.cur_dev->blksz);
> > > >  		n_blks = DIV_ROUND_UP(table_size + table_offset,
> > > > @@ -1516,7 +1517,7 @@ int sqfs_read(const char *filename, void *buf, loff_t offset, loff_t len,
> > > >  		goto out;
> > > >  	}
> > > >  
> > > > -	start = frag_entry.start / ctxt.cur_dev->blksz;
> > > > +	start = do_div(frag_entry.start, ctxt.cur_dev->blksz);
> > > >  	table_size = SQFS_BLOCK_SIZE(frag_entry.size);
> > > >  	table_offset = frag_entry.start - (start * ctxt.cur_dev->blksz);
> > > >  	n_blks = DIV_ROUND_UP(table_size + table_offset, ctxt.cur_dev->blksz);  
> > > 
> > > Adding maintainers...
> > 
> > I guess that's a correct fix, so
> > 
> > Reviewed-by: Miquel Raynal <miquel.raynal at bootlin.com>
> > 
> > Thanks,
> > Miquèl
> 
> Hello! do_div() macro modifies its first argument. It is correct?
> Because previous code did not modify anything.

 * The semantics of do_div() are:
 *
 * u32 do_div(u64 *n, u32 base)
 * {
 *	u32 remainder = *n % base;
 *	*n = *n / base;
 *	return remainder;
 * }

And this patch replaced:

  start = data_offset / ctxt.cur_dev->blksz;

by:

  start = do_div(data_offset, ctxt.cur_dev->blksz);

which expands to:

  start = data_offset % ctxt.cur_dev->blksz;
  data_offset = data_offset / ctxt.cur_dev->blksz;

Which is wrong as it calculates something totally different.

So this patch is incorrect.