From: Christoph Hellwig <hch@infradead.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>,
Yongpeng Yang <yangyongpeng.storage@gmail.com>,
Jaegeuk Kim <jaegeuk@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
linux-fscrypt@vger.kernel.org,
Yongpeng Yang <yangyongpeng@xiaomi.com>,
linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
Luis Chamberlain <mcgrof@kernel.org>
Subject: Re: [PATCH v2] fscrypt: fix left shift underflow when inode->i_blkbits > PAGE_SHIFT
Date: Wed, 5 Nov 2025 05:40:23 -0800 [thread overview]
Message-ID: <aQtTxx8ufd00_sJf@infradead.org> (raw)
In-Reply-To: <20251104181006.GC1780@sol>
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.
prev parent reply other threads:[~2025-11-05 13:40 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-30 7:29 [PATCH v2] fscrypt: fix left shift underflow when inode->i_blkbits > PAGE_SHIFT Yongpeng Yang
2025-11-03 16:48 ` Eric Biggers
2025-11-04 11:12 ` Christoph Hellwig
2025-11-04 15:05 ` Yongpeng Yang
2025-11-05 13:38 ` Christoph Hellwig
2025-11-04 18:10 ` Eric Biggers
2025-11-05 13:40 ` Christoph Hellwig [this message]
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