From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Sun, 20 Sep 2020 07:52:38 +1000 (AEST) From: Finn Thain Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag In-Reply-To: Message-ID: 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> <20200918151615.GA23432@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII List-ID: To: Arnd Bergmann Cc: Peter Maydell , Andy Lutomirski , Christoph Hellwig , Al Viro , Andrew Morton , Jens Axboe , David Howells , linux-arm-kernel , X86 ML , LKML , "open list:MIPS" , Parisc List , linuxppc-dev , linux-s390 , sparclinux , linux-block , Linux SCSI List , Linux FS Devel , linux-aio , io-uring@vger.kernel.org, linux-arch , Linux-MM , Network Development , keyrings@vger.kernel.org, LSM List On Sat, 19 Sep 2020, Arnd Bergmann wrote: > On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski wrote: > > On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig wrote: > > > On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote: > > > > Said that, why not provide a variant that would take an explicit > > > > "is it compat" argument and use it there? And have the normal one > > > > pass in_compat_syscall() to that... > > > > > > That would help to not introduce a regression with this series yes. > > > But it wouldn't fix existing bugs when io_uring is used to access > > > read or write methods that use in_compat_syscall(). One example > > > that I recently ran into is drivers/scsi/sg.c. > > Ah, so reading /dev/input/event* would suffer from the same issue, and > that one would in fact be broken by your patch in the hypothetical case > that someone tried to use io_uring to read /dev/input/event on x32... > > For reference, I checked the socket timestamp handling that has a number > of corner cases with time32/time64 formats in compat mode, but none of > those appear to be affected by the problem. > > > Aside from the potentially nasty use of per-task variables, one thing > > I don't like about PF_FORCE_COMPAT is that it's one-way. If we're > > going to have a generic mechanism for this, shouldn't we allow a full > > override of the syscall arch instead of just allowing forcing compat > > so that a compat syscall can do a non-compat operation? > > The only reason it's needed here is that the caller is in a kernel > thread rather than a system call. Are there any possible scenarios where > one would actually need the opposite? > Quite possibly. The ext4 vs. compat getdents bug is still unresolved. Please see, https://lore.kernel.org/lkml/CAFEAcA9W+JK7_TrtTnL1P2ES1knNPJX9wcUvhfLwxLq9augq1w@mail.gmail.com/ > Arnd >