Re: [PATCH v2] fscrypt: fix left shift underflow when inode->i_blkbits > PAGE_SHIFT

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 04, 2025 at 10:10:06AM -0800, Eric Biggers wrote:
> On Tue, Nov 04, 2025 at 03:12:53AM -0800, Christoph Hellwig wrote:
> > On Mon, Nov 03, 2025 at 08:48:29AM -0800, Eric Biggers wrote:
> > > >  	*inode_ret = inode;
> > > > -	*lblk_num_ret = ((u64)folio->index << (PAGE_SHIFT - inode->i_blkbits)) +
> > > > +	*lblk_num_ret = (((u64)folio->index << PAGE_SHIFT) >> inode->i_blkbits) +
> > 
> > This should be using folio_pos() instead of open coding the arithmetics.
> 
> Well, folio_pos() doesn't work with sizes greater than S64_MAX, and it
> uses multiplication rather than a shift.

What do you mean with "sizes greater than S64_MAX"?  folio_pos works
on a folio and is the MM designated helper to get the file offset
from a folio, where a file offset is a loff_t, aka s64.

And as answered to the previous mail, the compiler turns that
multiplication into a shift.

> Anyway, the trivial version avoids having to consider any of this...

I see it the other way around - folio_pos is the defined way to get
the index into the inode (block device inode here) in the abstract
way.