From: Catalin Marinas <catalin.marinas@arm.com> To: Szabolcs Nagy <szabolcs.nagy@arm.com> Cc: Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Dave P Martin <Dave.Martin@arm.com>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Kevin Brodsky <kevin.brodsky@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Peter Collingbourne <pcc@google.com>, Andrew Morton <akpm@linux-foundation.org>, libc-alpha@sourceware.org Subject: Re: [PATCH v9 29/29] arm64: mte: Add Memory Tagging Extension documentation Date: Tue, 22 Sep 2020 17:55:29 +0100 [thread overview] Message-ID: <20200922165529.GH15643@gaia> (raw) In-Reply-To: <20200922155248.GA16385@arm.com> Hi Szabolcs, On Tue, Sep 22, 2020 at 04:52:49PM +0100, Szabolcs Nagy wrote: > The 09/17/2020 10:02, Catalin Marinas wrote: > > On Thu, Sep 17, 2020 at 09:11:08AM +0100, Will Deacon wrote: > > > On Fri, Sep 04, 2020 at 11:30:29AM +0100, Catalin Marinas wrote: > > > > From: Vincenzo Frascino <vincenzo.frascino@arm.com> > ... > > > > Acked-by: Szabolcs Nagy <szabolcs.nagy@arm.com> > > > > > > I'm taking this to mean that Szabolcs is happy with the proposed ABI -- > > > please shout if that's not the case! > > > > I think Szabolcs is still on holiday. To summarise the past threads, > > AFAICT he's happy with this per-thread control ABI but the discussion > > went on whether to expand it in the future (with a new bit) to > > synchronise the tag checking mode across all threads of a process. This > > adds some complications for the kernel as it needs an IPI to the other > > CPUs to set SCTLR_EL1 and it's also racy with multiple threads > > requesting different modes. > > > > Now, in the glibc land, if the tag check mode is controlled via > > environment variables, the dynamic loader can set this at process start > > while still in single-threaded mode and not touch it at run-time. The > > MTE checking can still be enabled at run-time, per mapped memory range > > via the PROT_MTE flag. This approach doesn't require any additional > > changes to the current patches. But it's for Szabolcs to confirm once > > he's back. > > my thinking now is that for PROT_MTE use outside of libc we will need > a way to enable tag checks early so user code does not have to worry > about tag check settings across threads (coordinating the setting at > runtime seems problematic, same for the irg exclusion set). Yeah, such settings are better set at process start time. We can explore synchronising across threads with an additional PR_* flag but given the interaction with stack tagging and other potential races, it will need better coordination with user space and agree on which settings can be changed (e.g. exclusion mask may not be allowed). However, at this point, I don't see a strong case for such ABI addition as long as the application starts with some sane defaults, potentially driven by the user. > if we add a kernel level opt-in mechanism for tag checks later (e.g. > elf marking) or if the settings are exclusively owned by early libc > code then i think the proposed abi is ok (this is our current > agreement and works as long as no late runtime change is needed to the > settings). In the Android case, run-time changes to the tag checking mode I think are expected (usually via signal handlers), though per-thread. > i'm now wondering about the default tag check mode: it may be better > to enable sync tag checks in the kernel. it's not clear to me what > would break with that. this is probably late to discuss now and libc > would need ways to override the default no matter what, but i'd like > to know if somebody sees problems or risks with unconditional sync tag > checks turned on (sorry i don't remember if we went through this > before). i assume it would have no effect on a process that never uses > PROT_MTE. I don't think it helps much. We already have a requirement that to be able to pass tagged pointers to kernel syscalls, the user needs a prctl(PR_TAGGED_ADDR_ENABLE) call (code already in mainline). Using PROT_MTE without tagged pointers won't be of much use. So if we are to set different tag check defaults, we should also enable the tagged addr ABI automatically. That said, I still have a preference for MTE and tagged addr ABI to be explicitly requested by the (human) user either via environment variables or marked in an ELF note as "safe with/using tags". Given the recent mremap() issue we caused in glibc, I'm worried that other things may break with enabling the tagged addr ABI everywhere. Another aspect is that sync mode by default in a distro where glibc is MTE-aware will lead to performance regressions. That's another case in favour of the user explicitly asking for tag checking. Anyway, I'm open to having a debate on changing the defaults. -- Catalin
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From: Catalin Marinas <catalin.marinas@arm.com> To: Szabolcs Nagy <szabolcs.nagy@arm.com> Cc: linux-arch@vger.kernel.org, libc-alpha@sourceware.org, Peter Collingbourne <pcc@google.com>, Andrey Konovalov <andreyknvl@google.com>, Kevin Brodsky <kevin.brodsky@arm.com>, linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Will Deacon <will@kernel.org>, Dave P Martin <Dave.Martin@arm.com>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v9 29/29] arm64: mte: Add Memory Tagging Extension documentation Date: Tue, 22 Sep 2020 17:55:29 +0100 [thread overview] Message-ID: <20200922165529.GH15643@gaia> (raw) In-Reply-To: <20200922155248.GA16385@arm.com> Hi Szabolcs, On Tue, Sep 22, 2020 at 04:52:49PM +0100, Szabolcs Nagy wrote: > The 09/17/2020 10:02, Catalin Marinas wrote: > > On Thu, Sep 17, 2020 at 09:11:08AM +0100, Will Deacon wrote: > > > On Fri, Sep 04, 2020 at 11:30:29AM +0100, Catalin Marinas wrote: > > > > From: Vincenzo Frascino <vincenzo.frascino@arm.com> > ... > > > > Acked-by: Szabolcs Nagy <szabolcs.nagy@arm.com> > > > > > > I'm taking this to mean that Szabolcs is happy with the proposed ABI -- > > > please shout if that's not the case! > > > > I think Szabolcs is still on holiday. To summarise the past threads, > > AFAICT he's happy with this per-thread control ABI but the discussion > > went on whether to expand it in the future (with a new bit) to > > synchronise the tag checking mode across all threads of a process. This > > adds some complications for the kernel as it needs an IPI to the other > > CPUs to set SCTLR_EL1 and it's also racy with multiple threads > > requesting different modes. > > > > Now, in the glibc land, if the tag check mode is controlled via > > environment variables, the dynamic loader can set this at process start > > while still in single-threaded mode and not touch it at run-time. The > > MTE checking can still be enabled at run-time, per mapped memory range > > via the PROT_MTE flag. This approach doesn't require any additional > > changes to the current patches. But it's for Szabolcs to confirm once > > he's back. > > my thinking now is that for PROT_MTE use outside of libc we will need > a way to enable tag checks early so user code does not have to worry > about tag check settings across threads (coordinating the setting at > runtime seems problematic, same for the irg exclusion set). Yeah, such settings are better set at process start time. We can explore synchronising across threads with an additional PR_* flag but given the interaction with stack tagging and other potential races, it will need better coordination with user space and agree on which settings can be changed (e.g. exclusion mask may not be allowed). However, at this point, I don't see a strong case for such ABI addition as long as the application starts with some sane defaults, potentially driven by the user. > if we add a kernel level opt-in mechanism for tag checks later (e.g. > elf marking) or if the settings are exclusively owned by early libc > code then i think the proposed abi is ok (this is our current > agreement and works as long as no late runtime change is needed to the > settings). In the Android case, run-time changes to the tag checking mode I think are expected (usually via signal handlers), though per-thread. > i'm now wondering about the default tag check mode: it may be better > to enable sync tag checks in the kernel. it's not clear to me what > would break with that. this is probably late to discuss now and libc > would need ways to override the default no matter what, but i'd like > to know if somebody sees problems or risks with unconditional sync tag > checks turned on (sorry i don't remember if we went through this > before). i assume it would have no effect on a process that never uses > PROT_MTE. I don't think it helps much. We already have a requirement that to be able to pass tagged pointers to kernel syscalls, the user needs a prctl(PR_TAGGED_ADDR_ENABLE) call (code already in mainline). Using PROT_MTE without tagged pointers won't be of much use. So if we are to set different tag check defaults, we should also enable the tagged addr ABI automatically. That said, I still have a preference for MTE and tagged addr ABI to be explicitly requested by the (human) user either via environment variables or marked in an ELF note as "safe with/using tags". Given the recent mremap() issue we caused in glibc, I'm worried that other things may break with enabling the tagged addr ABI everywhere. Another aspect is that sync mode by default in a distro where glibc is MTE-aware will lead to performance regressions. That's another case in favour of the user explicitly asking for tag checking. Anyway, I'm open to having a debate on changing the defaults. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2020-09-22 16:55 UTC|newest] Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-09-04 10:30 [PATCH v9 00/29] arm64: Memory Tagging Extension user-space support Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 01/29] arm64: mte: system register definitions Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 02/29] arm64: mte: Use Normal Tagged attributes for the linear map Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 03/29] arm64: mte: CPU feature detection and initial sysreg configuration Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 04/29] arm64: kvm: mte: Hide the MTE CPUID information from the guests Catalin Marinas 2020-09-04 10:46 ` Marc Zyngier 2020-09-04 10:46 ` Marc Zyngier 2020-09-04 10:30 ` [PATCH v9 05/29] arm64: mte: Add specific SIGSEGV codes Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 06/29] arm64: mte: Handle synchronous and asynchronous tag check faults Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 07/29] mm: Add PG_arch_2 page flag Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 08/29] mm: Preserve the PG_arch_2 flag in __split_huge_page_tail() Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 09/29] arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE Catalin Marinas 2020-09-10 10:23 ` Steven Price 2020-09-10 10:23 ` Steven Price 2020-09-10 10:52 ` Catalin Marinas 2020-09-10 10:52 ` Catalin Marinas 2020-09-10 11:12 ` Steven Price 2020-09-10 11:12 ` Steven Price 2020-09-10 11:55 ` Catalin Marinas 2020-09-10 11:55 ` Catalin Marinas 2020-09-10 12:43 ` Steven Price 2020-09-10 12:43 ` Steven Price 2020-09-04 10:30 ` [PATCH v9 10/29] arm64: mte: Tags-aware copy_{user_,}highpage() implementations Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 11/29] arm64: Avoid unnecessary clear_user_page() indirection Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 12/29] arm64: mte: Tags-aware aware memcmp_pages() implementation Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 13/29] mm: Introduce arch_calc_vm_flag_bits() Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 14/29] arm64: mte: Add PROT_MTE support to mmap() and mprotect() Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 15/29] mm: Introduce arch_validate_flags() Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 16/29] arm64: mte: Validate the PROT_MTE request via arch_validate_flags() Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 17/29] mm: Allow arm64 mmap(PROT_MTE) on RAM-based files Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 18/29] arm64: mte: Allow user control of the tag check mode via prctl() Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 19/29] arm64: mte: Allow user control of the generated random tags " Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 20/29] arm64: mte: Restore the GCR_EL1 register after a suspend Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 21/29] arm64: mte: Allow {set,get}_tagged_addr_ctrl() on non-current tasks Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 22/29] arm64: mte: ptrace: Add PTRACE_{PEEK,POKE}MTETAGS support Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 23/29] arm64: mte: ptrace: Add NT_ARM_TAGGED_ADDR_CTRL regset Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 24/29] fs: Handle intra-page faults in copy_mount_options() Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 25/29] mm: Add arch hooks for saving/restoring tags Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 26/29] arm64: mte: Enable swap of tagged pages Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 27/29] arm64: mte: Save tags when hibernating Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 28/29] arm64: mte: Kconfig entry Catalin Marinas 2020-09-04 10:30 ` [PATCH v9 29/29] arm64: mte: Add Memory Tagging Extension documentation Catalin Marinas 2020-09-17 8:11 ` Will Deacon 2020-09-17 8:11 ` Will Deacon 2020-09-17 9:02 ` Catalin Marinas 2020-09-17 9:02 ` Catalin Marinas 2020-09-17 16:15 ` Dave Martin 2020-09-17 16:15 ` Dave Martin 2020-09-18 8:30 ` Will Deacon 2020-09-18 8:30 ` Will Deacon 2020-10-14 23:43 ` Peter Collingbourne 2020-10-14 23:43 ` Peter Collingbourne 2020-10-14 23:43 ` Peter Collingbourne 2020-10-15 8:57 ` Will Deacon 2020-10-15 8:57 ` Will Deacon 2020-10-15 11:14 ` Szabolcs Nagy 2020-10-15 11:14 ` Szabolcs Nagy 2020-09-22 16:04 ` Catalin Marinas 2020-09-22 16:04 ` Catalin Marinas 2020-09-22 15:52 ` Szabolcs Nagy 2020-09-22 15:52 ` Szabolcs Nagy 2020-09-22 16:55 ` Catalin Marinas [this message] 2020-09-22 16:55 ` Catalin Marinas 2020-09-23 9:10 ` Szabolcs Nagy 2020-09-23 9:10 ` Szabolcs Nagy 2020-09-22 12:22 ` Andrey Konovalov 2020-09-22 12:22 ` Andrey Konovalov 2020-09-22 12:22 ` Andrey Konovalov
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