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Sat, 19 Sep 2020 14:53:12 +0000 (UTC) Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-123-Q8cAoOVtN9-3ia4vwC15wg-1; Sat, 19 Sep 2020 15:53:09 +0100 X-MC-Unique: Q8cAoOVtN9-3ia4vwC15wg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Sat, 19 Sep 2020 15:53:08 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Sat, 19 Sep 2020 15:53:08 +0100 From: David Laight To: 'Al Viro' , Christoph Hellwig CC: Andrew Morton , Jens Axboe , Arnd Bergmann , David Howells , "linux-arm-kernel@lists.infradead.org" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-parisc@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-s390@vger.kernel.org" , "sparclinux@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-aio@kvack.org" , "io-uring@vger.kernel.org" , "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "keyrings@vger.kernel.org" , "linux-security-module@vger.kernel.org" Subject: RE: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag Thread-Topic: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag Thread-Index: AQHWjcPPLxbJUITJXkeWJwtHmAdwxKlwCErw Date: Sat, 19 Sep 2020 14:53:08 +0000 Message-ID: <6d064d8688324279af89152a8da22d69@AcuMS.aculab.com> References: <20200918124533.3487701-1-hch@lst.de> <20200918124533.3487701-2-hch@lst.de> <20200918134012.GY3421308@ZenIV.linux.org.uk> <20200918134406.GA17064@lst.de> <20200918135822.GZ3421308@ZenIV.linux.org.uk> In-Reply-To: <20200918135822.GZ3421308@ZenIV.linux.org.uk> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-US X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: From: Al Viro > Sent: 18 September 2020 14:58 >=20 > On Fri, Sep 18, 2020 at 03:44:06PM +0200, Christoph Hellwig wrote: > > On Fri, Sep 18, 2020 at 02:40:12PM +0100, Al Viro wrote: > > > > =09/* Vector 0x110 is LINUX_32BIT_SYSCALL_TRAP */ > > > > -=09return pt_regs_trap_type(current_pt_regs()) =3D=3D 0x110; > > > > +=09return pt_regs_trap_type(current_pt_regs()) =3D=3D 0x110 || > > > > +=09=09(current->flags & PF_FORCE_COMPAT); > > > > > > Can't say I like that approach ;-/ Reasoning about the behaviour is = much > > > harder when it's controlled like that - witness set_fs() shite... > > > > I don't particularly like it either. But do you have a better idea > > how to deal with io_uring vs compat tasks? >=20 > git rm fs/io_uring.c would make a good starting point > Yes, I know it's not going to happen, but one can dream... Maybe the io_uring code needs some changes to make it vaguely safe. - No support for 32-bit compat mixed working (or at all?). Plausibly a special worker could do 32bit work. - ring structure (I'm assuming mapped by mmap()) never mapped in more than one process (not cloned by fork()). - No implicit handover of files to another process. Would need an munmap, handover, mmap sequence. In any case the io_ring rather abuses the import_iovec() interface. The canonical sequence is (types from memory): =09struct iovec cache[8], *iov =3D cache; =09struct iter iter; =09... =09rval =3D import_iovec(..., &iov, 8, &iter); =09// Do read/write user using 'iter' =09free(iov); I don't think there is any strict requirement that iter.iov is set to either 'cache' or 'iov' (it probably must point into one of them.) But the io_uring code will make that assumption because the actual copies can be done much later and it doesn't save 'iter'. It gets itself in a right mess because it doesn't separate the 'address I need to free' from 'the iov[] for any transfers'. io_uring is also the only code that relies on import_iovec() returning the iter.count on success. It would be much better to have: =09iov =3D import_iovec(..., &cache, ...); =09free(iov); and use ERR_PTR() et al for error detectoion. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)