From: Catalin Marinas <catalin.marinas@arm.com> To: Dave Martin <Dave.Martin@arm.com> Cc: Mark Brown <broonie@kernel.org>, linux-arch@vger.kernel.org, libc-alpha@sourceware.org, Szabolcs Nagy <szabolcs.nagy@arm.com>, Jeremy Linton <jeremy.linton@arm.com>, Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter Date: Mon, 7 Jun 2021 19:12:13 +0100 [thread overview] Message-ID: <20210607181212.GD17957@arm.com> (raw) In-Reply-To: <20210607112536.GI4187@arm.com> On Mon, Jun 07, 2021 at 12:25:38PM +0100, Dave P Martin wrote: > On Thu, Jun 03, 2021 at 07:04:31PM +0100, Catalin Marinas via Libc-alpha wrote: > > On Thu, Jun 03, 2021 at 05:51:34PM +0100, Mark Brown wrote: > > > On Thu, Jun 03, 2021 at 04:40:35PM +0100, Dave Martin wrote: > > > > Do we know how libcs will detect that they don't need to do the > > > > mprotect() calls? Do we need a detection mechanism at all? > > > > > > > > Ignoring certain errors from mprotect() when ld.so is trying to set > > > > PROT_BTI on the main executable's code pages is probably a reasonable, > > > > backwards-compatible compromise here, but it seems a bit wasteful. > > > > > > I think the theory was that they would just do the mprotect() calls and > > > ignore any errors as they currently do, or declare that they depend on a > > > new enough kernel version I guess (not an option for glibc but might be > > > for others which didn't do BTI yet). > > > > I think we discussed the possibility of an AT_FLAGS bit. Until recently, > > this field was 0 but it gained a new bit now. If we are to expose this > > to arch-specific things, it may need some reservations. Anyway, that's > > an optimisation that can be added subsequently. > > I suppose so, but AT_FLAGS doesn't seem appropriate somehow. > > I wonder why we suddenly start considering adding a flag to AT_FLAGS > every few months, when it had sat empty for decades. This may say > something about the current health of the kernel ABI, but I'm not sure > exactly what. > > I think having mprotect() fail in a predictable way may be preferable > for now: glibc still only needs to probe with a single call and could > cache the knowledge after that. Code outside libc / ld.so seems quite > unlikely to care about this. I think that's the expected approach for now. If anyone complains about an extra syscall, we can look into options but I wouldn't rush on doing something. > Any ideas on how we would document this behaviour? The kernel and libc > behaviour are 100% clear: you _are_ allowed to twiddle PROT_BTI on > executable mappings, and there is no legitimate (or even useful) reason > to disallow this. It's only systemd deliberately breaking the API that > causes the behaviour seem by "userspace" to vary. I don't think we can document all the filters that can be added on top various syscalls, so I'd leave it undocumented (or part of the systemd documentation). It was a user space program (systemd) breaking another user space program (well, anything with a new enough glibc). The kernel ABI was still valid when /sbin/init started ;). -- Catalin
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From: Catalin Marinas <catalin.marinas@arm.com> To: Dave Martin <Dave.Martin@arm.com> Cc: Mark Brown <broonie@kernel.org>, linux-arch@vger.kernel.org, libc-alpha@sourceware.org, Szabolcs Nagy <szabolcs.nagy@arm.com>, Jeremy Linton <jeremy.linton@arm.com>, Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter Date: Mon, 7 Jun 2021 19:12:13 +0100 [thread overview] Message-ID: <20210607181212.GD17957@arm.com> (raw) In-Reply-To: <20210607112536.GI4187@arm.com> On Mon, Jun 07, 2021 at 12:25:38PM +0100, Dave P Martin wrote: > On Thu, Jun 03, 2021 at 07:04:31PM +0100, Catalin Marinas via Libc-alpha wrote: > > On Thu, Jun 03, 2021 at 05:51:34PM +0100, Mark Brown wrote: > > > On Thu, Jun 03, 2021 at 04:40:35PM +0100, Dave Martin wrote: > > > > Do we know how libcs will detect that they don't need to do the > > > > mprotect() calls? Do we need a detection mechanism at all? > > > > > > > > Ignoring certain errors from mprotect() when ld.so is trying to set > > > > PROT_BTI on the main executable's code pages is probably a reasonable, > > > > backwards-compatible compromise here, but it seems a bit wasteful. > > > > > > I think the theory was that they would just do the mprotect() calls and > > > ignore any errors as they currently do, or declare that they depend on a > > > new enough kernel version I guess (not an option for glibc but might be > > > for others which didn't do BTI yet). > > > > I think we discussed the possibility of an AT_FLAGS bit. Until recently, > > this field was 0 but it gained a new bit now. If we are to expose this > > to arch-specific things, it may need some reservations. Anyway, that's > > an optimisation that can be added subsequently. > > I suppose so, but AT_FLAGS doesn't seem appropriate somehow. > > I wonder why we suddenly start considering adding a flag to AT_FLAGS > every few months, when it had sat empty for decades. This may say > something about the current health of the kernel ABI, but I'm not sure > exactly what. > > I think having mprotect() fail in a predictable way may be preferable > for now: glibc still only needs to probe with a single call and could > cache the knowledge after that. Code outside libc / ld.so seems quite > unlikely to care about this. I think that's the expected approach for now. If anyone complains about an extra syscall, we can look into options but I wouldn't rush on doing something. > Any ideas on how we would document this behaviour? The kernel and libc > behaviour are 100% clear: you _are_ allowed to twiddle PROT_BTI on > executable mappings, and there is no legitimate (or even useful) reason > to disallow this. It's only systemd deliberately breaking the API that > causes the behaviour seem by "userspace" to vary. I don't think we can document all the filters that can be added on top various syscalls, so I'd leave it undocumented (or part of the systemd documentation). It was a user space program (systemd) breaking another user space program (well, anything with a new enough glibc). The kernel ABI was still valid when /sbin/init started ;). -- 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:[~2021-06-07 18:12 UTC|newest] Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-21 14:46 [PATCH v1 0/2] arm64: Enable BTI for the executable as well as the interpreter Mark Brown 2021-05-21 14:46 ` Mark Brown 2021-05-21 14:46 ` [PATCH v1 1/2] elf: Allow architectures to parse properties on the main executable Mark Brown 2021-05-21 14:46 ` Mark Brown 2021-06-03 15:40 ` Dave Martin 2021-06-03 15:40 ` Dave Martin 2021-06-03 18:52 ` Mark Brown 2021-06-03 18:52 ` Mark Brown 2021-05-21 14:46 ` [PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter Mark Brown 2021-05-21 14:46 ` Mark Brown 2021-06-03 15:40 ` Dave Martin 2021-06-03 15:40 ` Dave Martin 2021-06-03 16:51 ` Mark Brown 2021-06-03 16:51 ` Mark Brown 2021-06-03 18:04 ` Catalin Marinas 2021-06-03 18:04 ` Catalin Marinas 2021-06-07 11:25 ` Dave Martin 2021-06-07 11:25 ` Dave Martin 2021-06-07 18:12 ` Catalin Marinas [this message] 2021-06-07 18:12 ` Catalin Marinas 2021-06-08 11:33 ` Mark Brown 2021-06-08 11:33 ` Mark Brown 2021-06-08 15:19 ` Dave Martin 2021-06-08 15:19 ` Dave Martin 2021-06-08 15:42 ` Jeremy Linton 2021-06-08 15:42 ` Jeremy Linton 2021-06-10 10:33 ` Dave Martin 2021-06-10 10:33 ` Dave Martin
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