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Subject: Re: in_compat_syscall() returns from kernel thread for X86_32.
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <87o9bi6632.fsf@notabene.neil.brown.name>
Date:   Wed, 24 Oct 2018 21:45:30 -0700
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Andy Lutomirski <luto@kernel.org>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Peter Zijlstra <peterz@infradead.org>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-tip-commits@vger.kernel.org, jsimmons@infradead.org
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References: <1460987025-30360-1-git-send-email-dsafonov@virtuozzo.com> <tip-abfb9498ee1327f534df92a7ecaea81a85913bae@git.kernel.org> <87h8hkc9fd.fsf@notabene.neil.brown.name> <CALCETrXB8PE4Ue0BeD0wUS9psLLjpv_pt+nj=SEaj+Nb_dZzUA@mail.gmail.com> <871s8ndg6a.fsf@notabene.neil.brown.name> <CALCETrWH342Y7jKHsYdT=_3fuyDkWhvQLK9z4PTs9jzbfoCVwg@mail.gmail.com> <871s8g6roy.fsf@notabene.neil.brown.name> <20181024131534.GD11606@thunk.org> <87o9bi6632.fsf@notabene.neil.brown.name>
To:     NeilBrown <neilb@suse.com>
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> On Oct 24, 2018, at 8:46 PM, NeilBrown <neilb@suse.com> wrote:
>=20
>> On Wed, Oct 24 2018, Theodore Y. Ts'o wrote:
>>=20
>>> On Wed, Oct 24, 2018 at 12:47:57PM +1100, NeilBrown wrote:
>>>=20
>>> I doubt it was copied - more likely independent evolution.
>>> But on reflection, I see that it is probably reasonable that it
>>> shouldn't be used this way - or at all in this context.
>>> I'll try to understand what the issues really are and see if I can
>>> find a solution that doesn't depend on this interface.
>>> Thanks for your help.
>>=20
>> At least for ext4, the primary problem is that we want to use a 64-bit
>> telldir/seekdir cookie if all 64-bits will make it to user space, and
>> a 32-bit telldir cookie if only 32 bits will make it userspace.  This
>> impacts NFS as well because if there are people who are still using
>> NFSv2, which has 32-bit directory offsets, we need to constrain the
>> telldir/seekdir cookies we give to NFS to be a 32 has as opposed to a
>> 64-bit hash.
>=20
> NFSd uses FMODE_32BITHASH or FMODE64BITHASH to allow ext4 to do the
> right thing.  FMODE_32BITHASH is set for NFSv2 only.
>=20
> Maybe sys_getdents needs to set FMODE_32BITHASH, and sys_getdent64 needs
> to set FMODE_64BITHASH - or something like that.

It=E2=80=99s possible for a 32-bit process and a 64-bit process to share a d=
irectory fd, so I don=E2=80=99t think it=E2=80=99s quite that simple.

One option would be to add .llseek and .getdents flags or entire new compat o=
perations to indicate that the caller expects 32-bit offsets.

I wonder how overlayfs interacts with this whole mess.=