From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass Date: Wed, 29 Apr 2020 14:52:25 +0100 Message-ID: <20200429135224.GB10651@gaia> References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-21-catalin.marinas@arm.com> <20200429102650.GC30377@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from foss.arm.com ([217.140.110.172]:39594 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726885AbgD2Nwa (ORCPT ); Wed, 29 Apr 2020 09:52:30 -0400 Content-Disposition: inline In-Reply-To: <20200429102650.GC30377@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Dave Martin Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, Richard Earnshaw , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , Peter Collingbourne , linux-mm@kvack.org, Alexander Viro , Vincenzo Frascino , Will Deacon On Wed, Apr 29, 2020 at 11:26:51AM +0100, Dave P Martin wrote: > On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote: > > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h > > index 32fc8061aa76..566da441eba2 100644 > > --- a/arch/arm64/include/asm/uaccess.h > > +++ b/arch/arm64/include/asm/uaccess.h > > @@ -416,6 +416,17 @@ extern unsigned long __must_check __arch_copy_in_user(void __user *to, const voi > > #define INLINE_COPY_TO_USER > > #define INLINE_COPY_FROM_USER > > > > +static inline bool arch_has_exact_copy_from_user(unsigned long n) > > +{ > > + /* > > + * copy_from_user() aligns the source pointer if the size is greater > > + * than 15. Since all the loads are naturally aligned, they can only > > + * fail on the first byte. > > + */ > > + return n > 15; > > +} > > +#define arch_has_exact_copy_from_user > > Did you mean: > > #define arch_has_exact_copy_from_user arch_has_exact_copy_from_user Yes (and I shouldn't write patches late in the day). > Mind you, if this expands to 1 I'd have expected copy_mount_options() > not to compile, so I may be missing something. I think arch_has_exact_copy_from_user() (with the braces) is looked up in the function namespace, so the macro isn't expanded. So arguably the patch is correct but pretty dodgy ;). I scrapped this in my second attempt in reply to Kevin. > > diff --git a/fs/namespace.c b/fs/namespace.c > > index a28e4db075ed..8febc50dfc5d 100644 > > --- a/fs/namespace.c > > +++ b/fs/namespace.c > > @@ -3025,13 +3025,16 @@ void *copy_mount_options(const void __user * data) > > [ Is this applying a band-aid to duct tape? > > The fs presumably knows ahead of time whether it's expecting a string or > a fixed-size blob for data, so I'd hope we could just DTRT rather than > playing SEGV roulette here. > > This might require more refactoring than makes sense for this series > though. ] That's possible but it means moving the copy from sys_mount() to the specific places where it has additional information (the filesystems). I'm not even sure it's guaranteed to be strings. If it is, we could just replace all this with a strncpy_from_user(). -- Catalin