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Date:   Mon, 6 Apr 2020 15:04:16 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jeremy Kerr <jk@ozlabs.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/6] binfmt_elf: open code copy_siginfo_to_user to
 kernelspace buffer
Message-ID: <20200406130416.GB16479@lst.de>
References: <20200406120312.1150405-1-hch@lst.de> <20200406120312.1150405-3-hch@lst.de> <CAK8P3a02LQNOehukgaCj81wg1D2XhW1=_mQZ72cT6nQdO=mhOw@mail.gmail.com>
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On Mon, Apr 06, 2020 at 03:01:24PM +0200, Arnd Bergmann wrote:
> >  static void fill_siginfo_note(struct memelfnote *note, user_siginfo_t *csigdata,
> >                 const kernel_siginfo_t *siginfo)
> >  {
> > -       mm_segment_t old_fs = get_fs();
> > -       set_fs(KERNEL_DS);
> > -       copy_siginfo_to_user((user_siginfo_t __user *) csigdata, siginfo);
> > -       set_fs(old_fs);
> > +       memcpy(csigdata, siginfo, sizeof(struct kernel_siginfo));
> > +       memset((char *)csigdata + sizeof(struct kernel_siginfo), 0,
> > +               SI_EXPANSION_SIZE);
> >         fill_note(note, "CORE", NT_SIGINFO, sizeof(*csigdata), csigdata);
> >  }
> 
> I think this breaks compat binfmt-elf mode, which relies on this trick:
> 
> fs/compat_binfmt_elf.c:#define copy_siginfo_to_user     copy_siginfo_to_user32
> fs/compat_binfmt_elf.c#include "binfmt_elf.c"
> 
> At least we seem to only have one remaining implementation of
> __copy_siginfo_to_user32(), so fixing this won't require touching all
> architectures, but I don't see an obvious way to do it right. Maybe
> compat-binfmt-elf.c should just override fill_siginfo_note() itself
> rather than overriding copy_siginfo_to_user().

Ooops.  Yes, this will need some manual handling.