linux-kernel - Re: [RFC PATCH 11/20] hfs: Explicitly set hsb->nls_disk when hsb->nls

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Message-ID: <YRFnz6kn1UbSCN/S@casper.infradead.org>
Date:   Mon, 9 Aug 2021 18:37:19 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Viacheslav Dubeyko <slava@...eyko.com>
Cc:     Pali Rohár <pali@...nel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-ntfs-dev@...ts.sourceforge.net, linux-cifs@...r.kernel.org,
        jfs-discussion@...ts.sourceforge.net,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.cz>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Salah Triki <salah.triki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Kleikamp <shaggy@...nel.org>,
        Anton Altaparmakov <anton@...era.com>,
        Pavel Machek <pavel@....cz>,
        Marek Behún <marek.behun@....cz>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC PATCH 11/20] hfs: Explicitly set hsb->nls_disk when
 hsb->nls_io is set

On Mon, Aug 09, 2021 at 10:31:55AM -0700, Viacheslav Dubeyko wrote:
> > On Aug 8, 2021, at 9:24 AM, Pali Rohár <pali@...nel.org> wrote:
> > 
> > It does not make any sense to set hsb->nls_io (NLS iocharset used between
> > VFS and hfs driver) when hsb->nls_disk (NLS codepage used between hfs
> > driver and disk) is not set.
> > 
> > Reverse engineering driver code shown what is doing in this special case:
> > 
> >    When codepage was not defined but iocharset was then
> >    hfs driver copied 8bit character from disk directly to
> >    16bit unicode wchar_t type. Which means it did conversion
> >    from Latin1 (ISO-8859-1) to Unicode because first 256
> >    Unicode code points matches 8bit ISO-8859-1 codepage table.
> >    So when iocharset was specified and codepage not, then
> >    codepage used implicit value "iso8859-1".
> > 
> > So when hsb->nls_disk is not set and hsb->nls_io is then explicitly set
> > hsb->nls_disk to "iso8859-1".
> > 
> > Such setup is obviously incompatible with Mac OS systems as they do not
> > support iso8859-1 encoding for hfs. So print warning into dmesg about this
> > fact.
> > 
> > After this change hsb->nls_disk is always set, so remove code paths for
> > case when hsb->nls_disk was not set as they are not needed anymore.
> 
> 
> Sounds reasonable. But it will be great to know that the change has been tested reasonably well.

I don't think it's reasonable to ask Pali to test every single filesystem.
That's something the maintainer should do, as you're more likely to have
the infrastructure already set up to do testing of your filesystem and
be aware of fun corner cases and use cases than someone who's working
across all filesystems.