linux-kernel - Re: [PATCH 1/7] 9P: Cast to loff

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Message-ID: <20201007184730.GW20115@casper.infradead.org>
Date:   Wed, 7 Oct 2020 19:47:30 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, ericvh@...il.com, lucho@...kov.net,
        viro@...iv.linux.org.uk, jlayton@...nel.org, idryomov@...il.com,
        mark@...heh.com, jlbec@...lplan.org, joseph.qi@...ux.alibaba.com,
        v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        ceph-devel@...r.kernel.org, ocfs2-devel@....oracle.com,
        linux-btrfs@...r.kernel.org, clm@...com, josef@...icpanda.com,
        dsterba@...e.com, stable@...r.kernel.org
Subject: Re: [PATCH 1/7] 9P: Cast to loff_t before multiplying

On Wed, Oct 07, 2020 at 06:48:49AM +0100, Christoph Hellwig wrote:
> > -		.range_start = vma->vm_pgoff * PAGE_SIZE,
> > +		.range_start = (loff_t)vma->vm_pgoff * PAGE_SIZE,
> 
> Given the may places where this issue shows up I think we really need
> a vma_offset or similar helper for it.  Much better than chasing missing
> casts everywhere.

Good point.  I think these patches need to go in to fix the bugs in
the various stable releases, but we should definitely have a helper
for the future.  Also, several of these patches are for non-VMA
pgoff_t.

vma_offset() is a bit weird for me -- vmas have all kinds of offsets.
vma_file_offset() would work or vma_fpos().  I tend to prefer the shorter
function name ;-)

A quick grep shows we probably want a vmf_fpos() too:

arch/powerpc/platforms/cell/spufs/file.c:       unsigned long area, offset = vmf->pgoff << PAGE_SHIFT;
arch/x86/entry/vdso/vma.c:      sym_offset = (long)(vmf->pgoff << PAGE_SHIFT) +
drivers/gpu/drm/gma500/framebuffer.c:   address = vmf->address - (vmf->pgoff << PAGE_SHIFT);
drivers/scsi/cxlflash/ocxl_hw.c:        offset = vmf->pgoff << PAGE_SHIFT;

I'm sure a lot of this will never run on a 32-bit kernel or with a 4GB
file, but it's not good to have bad code around for people to copy from.