From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH v2 09/12] arm64: traps: Fix inconsistent faulting instruction skipping Date: Tue, 15 Oct 2019 17:49:05 +0100 Message-ID: <20191015164904.GY27757@arm.com> References: <1570733080-21015-1-git-send-email-Dave.Martin@arm.com> <1570733080-21015-10-git-send-email-Dave.Martin@arm.com> <20191011152453.GF33537@lakrids.cambridge.arm.com> <20191015152108.GX27757@arm.com> <20191015164204.GC24604@lakrids.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20191015164204.GC24604@lakrids.cambridge.arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Mark Rutland Cc: Paul Elliott , Peter Zijlstra , Catalin Marinas , Will Deacon , Andrew Jones , Amit Kachhap , Vincenzo Frascino , linux-arch@vger.kernel.org, Eugene Syromiatnikov , Szabolcs Nagy , "H.J. Lu" , Yu-cheng Yu , Kees Cook , Arnd Bergmann , Jann Horn , Richard Henderson , Kristina =?utf-8?Q?Mart=C5=A1enko?= , Mark Brown , Thomas Gleixner , linux-arm-kernel@lists.infr List-Id: linux-arch.vger.kernel.org On Tue, Oct 15, 2019 at 05:42:04PM +0100, Mark Rutland wrote: > On Tue, Oct 15, 2019 at 04:21:09PM +0100, Dave Martin wrote: > > On Fri, Oct 11, 2019 at 04:24:53PM +0100, Mark Rutland wrote: > > > On Thu, Oct 10, 2019 at 07:44:37PM +0100, Dave Martin wrote: > > > > Correct skipping of an instruction on AArch32 works a bit > > > > differently from AArch64, mainly due to the different CPSR/PSTATE > > > > semantics. > > > > > > > > There have been various attempts to get this right. Currenty > > > > arm64_skip_faulting_instruction() mostly does the right thing, but > > > > does not advance the IT state machine for the AArch32 case. > > > > > > > > arm64_compat_skip_faulting_instruction() handles the IT state > > > > machine but is local to traps.c, and porting other code to use it > > > > will make a mess since there are some call sites that apply for > > > > both the compat and native cases. > > > > > > > > Since manual instruction skipping implies a trap, it's a relatively > > > > slow path. > > > > > > > > So, make arm64_skip_faulting_instruction() handle both compat and > > > > native, and get rid of the arm64_compat_skip_faulting_instruction() > > > > special case. > > > > > > > > Fixes: 32a3e635fb0e ("arm64: compat: Add CNTFRQ trap handler") > > > > Fixes: 1f1c014035a8 ("arm64: compat: Add condition code checks and IT advance") > > > > Fixes: 6436beeee572 ("arm64: Fix single stepping in kernel traps") > > > > Fixes: bd35a4adc413 ("arm64: Port SWP/SWPB emulation support from arm") > > > > Signed-off-by: Dave Martin > > > > --- > > > > arch/arm64/kernel/traps.c | 18 ++++++++---------- > > > > 1 file changed, 8 insertions(+), 10 deletions(-) > > > > > > This looks good to me; it's certainly easier to reason about. > > > > > > I couldn't spot a place where we do the wrong thing today, given AFAICT > > > all the instances in arch/arm64/kernel/armv8_deprecated.c would be > > > UNPREDICTABLE within an IT block. > > > > > > It might be worth calling out an example in the commit message to > > > justify the fixes tags. > > > > IIRC I found no bug here; rather we have pointlessly fragmented code, > > so I followed the "if fixing the same bug in multiple places, merge > > those places so you need only fix it in one place next time" rule. > > Sure thing, that makes sense to me. > > > Since arm64_skip_faulting_instruction() is most of the way to being > > generically usable anyway, this series merges all the special-case > > handling into it. > > > > I could add something like > > > > --8<-- > > > > This allows this fiddly operation to be maintained in a single > > place rather than trying to maintain fragmented versions spread > > around arch/arm64. > > > > -->8-- > > > > Any good? > > My big concern is that the commit message reads as a fix, implying that > there's an existing correctness bug. I think that simplifying it to make > it clearer that it's a cleanup/improvement would be preferable. > > How about: > > | arm64: unify native/compat instruction skipping > | > | Skipping of an instruction on AArch32 works a bit differently from > | AArch64, mainly due to the different CPSR/PSTATE semantics. > | > | Currently arm64_skip_faulting_instruction() is only suitable for > | AArch64, and arm64_compat_skip_faulting_instruction() handles the IT > | state machine but is local to traps.c. > | > | Since manual instruction skipping implies a trap, it's a relatively > | slow path. > | > | So, make arm64_skip_faulting_instruction() handle both compat and > | native, and get rid of the arm64_compat_skip_faulting_instruction() > | special case. > | > | Signed-off-by: Dave Martin And drop the Fixes tags. Yes, I think that's reasonable. I think I was probably glossing over the fact that we don't need to get the ITSTATE machine advance correct for the compat insn emulation; as you say, I can't see what else this patch "fixes". > With that, feel free to add: > > Reviewed-by: Mark Rutland Thanks! > We could even point out that the armv8_deprecated cases are > UNPREDICTABLE in an IT block, and correctly emulated either way. Yes, I'll add something along those lines. Cheers ---Dave From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.110.172]:43176 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726362AbfJOQtL (ORCPT ); Tue, 15 Oct 2019 12:49:11 -0400 Date: Tue, 15 Oct 2019 17:49:05 +0100 From: Dave Martin Subject: Re: [PATCH v2 09/12] arm64: traps: Fix inconsistent faulting instruction skipping Message-ID: <20191015164904.GY27757@arm.com> References: <1570733080-21015-1-git-send-email-Dave.Martin@arm.com> <1570733080-21015-10-git-send-email-Dave.Martin@arm.com> <20191011152453.GF33537@lakrids.cambridge.arm.com> <20191015152108.GX27757@arm.com> <20191015164204.GC24604@lakrids.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191015164204.GC24604@lakrids.cambridge.arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Mark Rutland Cc: Paul Elliott , Peter Zijlstra , Catalin Marinas , Will Deacon , Andrew Jones , Amit Kachhap , Vincenzo Frascino , linux-arch@vger.kernel.org, Eugene Syromiatnikov , Szabolcs Nagy , "H.J. Lu" , Yu-cheng Yu , Kees Cook , Arnd Bergmann , Jann Horn , Richard Henderson , Kristina =?utf-8?Q?Mart=C5=A1enko?= , Mark Brown , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Florian Weimer , linux-kernel@vger.kernel.org, Sudakshina Das Message-ID: <20191015164905.kqVID3I3VeL_lHLGwbe29MZaWcMSfXz96ZLdNIVfeA8@z> On Tue, Oct 15, 2019 at 05:42:04PM +0100, Mark Rutland wrote: > On Tue, Oct 15, 2019 at 04:21:09PM +0100, Dave Martin wrote: > > On Fri, Oct 11, 2019 at 04:24:53PM +0100, Mark Rutland wrote: > > > On Thu, Oct 10, 2019 at 07:44:37PM +0100, Dave Martin wrote: > > > > Correct skipping of an instruction on AArch32 works a bit > > > > differently from AArch64, mainly due to the different CPSR/PSTATE > > > > semantics. > > > > > > > > There have been various attempts to get this right. Currenty > > > > arm64_skip_faulting_instruction() mostly does the right thing, but > > > > does not advance the IT state machine for the AArch32 case. > > > > > > > > arm64_compat_skip_faulting_instruction() handles the IT state > > > > machine but is local to traps.c, and porting other code to use it > > > > will make a mess since there are some call sites that apply for > > > > both the compat and native cases. > > > > > > > > Since manual instruction skipping implies a trap, it's a relatively > > > > slow path. > > > > > > > > So, make arm64_skip_faulting_instruction() handle both compat and > > > > native, and get rid of the arm64_compat_skip_faulting_instruction() > > > > special case. > > > > > > > > Fixes: 32a3e635fb0e ("arm64: compat: Add CNTFRQ trap handler") > > > > Fixes: 1f1c014035a8 ("arm64: compat: Add condition code checks and IT advance") > > > > Fixes: 6436beeee572 ("arm64: Fix single stepping in kernel traps") > > > > Fixes: bd35a4adc413 ("arm64: Port SWP/SWPB emulation support from arm") > > > > Signed-off-by: Dave Martin > > > > --- > > > > arch/arm64/kernel/traps.c | 18 ++++++++---------- > > > > 1 file changed, 8 insertions(+), 10 deletions(-) > > > > > > This looks good to me; it's certainly easier to reason about. > > > > > > I couldn't spot a place where we do the wrong thing today, given AFAICT > > > all the instances in arch/arm64/kernel/armv8_deprecated.c would be > > > UNPREDICTABLE within an IT block. > > > > > > It might be worth calling out an example in the commit message to > > > justify the fixes tags. > > > > IIRC I found no bug here; rather we have pointlessly fragmented code, > > so I followed the "if fixing the same bug in multiple places, merge > > those places so you need only fix it in one place next time" rule. > > Sure thing, that makes sense to me. > > > Since arm64_skip_faulting_instruction() is most of the way to being > > generically usable anyway, this series merges all the special-case > > handling into it. > > > > I could add something like > > > > --8<-- > > > > This allows this fiddly operation to be maintained in a single > > place rather than trying to maintain fragmented versions spread > > around arch/arm64. > > > > -->8-- > > > > Any good? > > My big concern is that the commit message reads as a fix, implying that > there's an existing correctness bug. I think that simplifying it to make > it clearer that it's a cleanup/improvement would be preferable. > > How about: > > | arm64: unify native/compat instruction skipping > | > | Skipping of an instruction on AArch32 works a bit differently from > | AArch64, mainly due to the different CPSR/PSTATE semantics. > | > | Currently arm64_skip_faulting_instruction() is only suitable for > | AArch64, and arm64_compat_skip_faulting_instruction() handles the IT > | state machine but is local to traps.c. > | > | Since manual instruction skipping implies a trap, it's a relatively > | slow path. > | > | So, make arm64_skip_faulting_instruction() handle both compat and > | native, and get rid of the arm64_compat_skip_faulting_instruction() > | special case. > | > | Signed-off-by: Dave Martin And drop the Fixes tags. Yes, I think that's reasonable. I think I was probably glossing over the fact that we don't need to get the ITSTATE machine advance correct for the compat insn emulation; as you say, I can't see what else this patch "fixes". > With that, feel free to add: > > Reviewed-by: Mark Rutland Thanks! > We could even point out that the armv8_deprecated cases are > UNPREDICTABLE in an IT block, and correctly emulated either way. Yes, I'll add something along those lines. Cheers ---Dave