Re: [PATCH 2/3] fs/ext2/inode: Fix a type cast error for fsdax
From: Jan Kara
Date: Mon Jul 02 2018 - 03:55:54 EST
On Mon 02-07-18 14:23:42, Huaisheng Ye wrote:
> ---- On Mon, 02 Jul 2018 03:26:00 +0800 Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote ----
> > On Sun, Jul 01, 2018 at 02:18:47PM +0800, Huaisheng Ye wrote:
> > > From: Huaisheng Ye <yehs1@xxxxxxxxxx>
> > >
> > > The type of offset within struct iomap is loff_t, which represents
> > > file offset of mapping.
> > >
> > > In ext2_iomap_begin, iomap->offset shall be given a type cast as
> > > loff_t instead of u64.
> > Why is it an error? loff_t is uniformly typedefed to long long.
> > In which case the second variant is different from the first one
> > *and* does not step into nasal demon territory?
> Sorry for my inaccuracy.
> The type of iomap->offset is loff_t, is it better to cast first_block
> to loff_t, then do the left shift operation?
I think what Al meant is that in the kernel loff_t is 64-bit anyway and
furthermore it is signed which means most people have to look up C standard
to remember how right shift is (un)defined if it would overflow ;). So this
does not really make the code any more readable. Rather on contrary...
> > > - iomap->offset = (u64)first_block << blkbits;
> > > + iomap->offset = (loff_t)first_block << blkbits;
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR