From: Dave Martin <Dave.Martin@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arch@vger.kernel.org,
Richard Earnshaw <Richard.Earnshaw@arm.com>,
Will Deacon <will@kernel.org>,
Szabolcs Nagy <szabolcs.nagy@arm.com>,
Andrey Konovalov <andreyknvl@google.com>,
Kevin Brodsky <kevin.brodsky@arm.com>,
linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Peter Collingbourne <pcc@google.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass
Date: Wed, 29 Apr 2020 11:28:06 +0100 [thread overview]
Message-ID: <20200429102806.GD30377@arm.com> (raw)
In-Reply-To: <20200428140626.GJ3868@gaia>
On Tue, Apr 28, 2020 at 03:06:27PM +0100, Catalin Marinas wrote:
> On Mon, Apr 27, 2020 at 05:56:42PM +0100, Dave P Martin wrote:
> > On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote:
> > > The copy_mount_options() function takes a user pointer argument but not
> > > a size. It tries to read up to a PAGE_SIZE. However, copy_from_user() is
> > > not guaranteed to return all the accessible bytes if, for example, the
> > > access crosses a page boundary and gets a fault on the second page. To
> > > work around this, the current copy_mount_options() implementations
> > > performs to copy_from_user() passes, first to the end of the current
> > > page and the second to what's left in the subsequent page.
> > >
> > > Some architectures like arm64 can guarantee an exact copy_from_user()
> > > depending on the size (since the arch function performs some alignment
> > > on the source register). Introduce an arch_has_exact_copy_from_user()
> > > function and allow copy_mount_options() to perform the user access in a
> > > single pass.
> > >
> > > While this function is not on a critical path, the single-pass behaviour
> > > is required for arm64 MTE (memory tagging) support where a uaccess can
> > > trigger intra-page faults (tag not matching). With the current
> > > implementation, if this happens during the first page, the function will
> > > return -EFAULT.
> >
> > Do you know how much extra overhead we'd incur if we read at must one
> > tag granule at a time, instead of PAGE_SIZE?
>
> Our copy routines already read 16 bytes at a time, so that's the tag
> granule. With current copy_mount_options() we have the issue that it
> assumes a fault in the first page is fatal.
>
> Even if we change it to a loop of smaller uaccess, we still have the
> issue of unaligned accesses which can fail without reading all that's
> possible (i.e. the access goes across a tag granule boundary).
>
> The previous copy_mount_options() implementation (from couple of months
> ago I think) had a fallback to byte-by-byte, didn't have this issue.
>
> > I'm guessing that in practice strcpy_from_user() type operations copy
> > much less than a page most of the time, so what we lose in uaccess
> > overheads we _might_ regain in less redundant copying.
>
> strncpy_from_user() has a fallback to byte by byte, so we don't have an
> issue here.
>
> The above is only for synchronous accesses. For async, in v3 I disabled
> such checks for the uaccess routines.
Fair enough, I hadn't fully got my head around what's going on here.
(But see my other reply.)
I was suspicious about the WARN_ON(), but I see people are on top of
that.
Cheers
---Dave
next prev parent reply other threads:[~2020-04-29 10:28 UTC|newest]
Thread overview: 81+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-21 14:25 [PATCH v3 00/23] arm64: Memory Tagging Extension user-space support Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 01/23] arm64: alternative: Allow alternative_insn to always issue the first instruction Catalin Marinas
2020-04-27 16:57 ` Dave Martin
2020-04-28 11:43 ` Catalin Marinas
2020-04-29 10:26 ` Dave Martin
2020-04-29 14:04 ` Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 02/23] arm64: mte: system register definitions Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 03/23] arm64: mte: CPU feature detection and initial sysreg configuration Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 04/23] arm64: mte: Use Normal Tagged attributes for the linear map Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 05/23] arm64: mte: Assembler macros and default architecture for .S files Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 06/23] arm64: mte: Tags-aware clear_page() implementation Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 07/23] arm64: mte: Tags-aware copy_page() implementation Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 08/23] arm64: Tags-aware memcmp_pages() implementation Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 09/23] arm64: mte: Add specific SIGSEGV codes Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 10/23] arm64: mte: Handle synchronous and asynchronous tag check faults Catalin Marinas
2020-04-23 10:38 ` Catalin Marinas
2020-04-27 16:58 ` Dave Martin
2020-04-28 13:43 ` Catalin Marinas
2020-04-29 10:26 ` Dave Martin
2020-04-21 14:25 ` [PATCH v3 11/23] mm: Introduce arch_calc_vm_flag_bits() Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 12/23] arm64: mte: Add PROT_MTE support to mmap() and mprotect() Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 13/23] mm: Introduce arch_validate_flags() Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 14/23] arm64: mte: Validate the PROT_MTE request via arch_validate_flags() Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 15/23] mm: Allow arm64 mmap(PROT_MTE) on RAM-based files Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 16/23] arm64: mte: Allow user control of the tag check mode via prctl() Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 17/23] arm64: mte: Allow user control of the generated random tags " Catalin Marinas
2020-04-21 14:25 ` [PATCH v3 18/23] arm64: mte: Restore the GCR_EL1 register after a suspend Catalin Marinas
2020-04-23 15:23 ` Lorenzo Pieralisi
2020-04-21 14:25 ` [PATCH v3 19/23] arm64: mte: Add PTRACE_{PEEK,POKE}MTETAGS support Catalin Marinas
2020-04-24 23:28 ` Peter Collingbourne
2020-04-29 10:27 ` Kevin Brodsky
2020-04-29 15:24 ` Catalin Marinas
2020-04-29 16:46 ` Dave Martin
2020-04-30 10:21 ` Catalin Marinas
2020-05-04 16:40 ` Dave Martin
2020-05-05 18:03 ` Luis Machado
2020-05-12 19:05 ` Luis Machado
2020-05-13 10:48 ` Catalin Marinas
2020-05-13 12:52 ` Luis Machado
2020-05-13 14:11 ` Catalin Marinas
2020-05-13 15:09 ` Luis Machado
2020-05-13 16:45 ` Luis Machado
2020-05-13 17:11 ` Catalin Marinas
2020-05-18 16:47 ` Dave Martin
2020-05-18 17:12 ` Luis Machado
2020-05-19 16:10 ` Catalin Marinas
2020-04-21 14:26 ` [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass Catalin Marinas
2020-04-21 15:29 ` Al Viro
2020-04-21 16:45 ` Catalin Marinas
2020-04-27 16:56 ` Dave Martin
2020-04-28 14:06 ` Catalin Marinas
2020-04-29 10:28 ` Dave Martin [this message]
2020-04-28 18:16 ` Kevin Brodsky
2020-04-28 19:40 ` Catalin Marinas
2020-04-29 11:58 ` Catalin Marinas
2020-04-28 19:36 ` Catalin Marinas
2020-04-29 10:26 ` Dave Martin
2020-04-29 13:52 ` Catalin Marinas
2020-05-04 16:40 ` Dave Martin
2020-04-21 14:26 ` [PATCH v3 21/23] arm64: mte: Check the DT memory nodes for MTE support Catalin Marinas
2020-04-24 13:57 ` Catalin Marinas
2020-04-24 16:17 ` Catalin Marinas
2020-04-27 11:14 ` Suzuki K Poulose
2020-04-21 14:26 ` [PATCH v3 22/23] arm64: mte: Kconfig entry Catalin Marinas
2020-04-21 14:26 ` [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation Catalin Marinas
2020-04-29 16:47 ` Dave Martin
2020-04-30 16:23 ` Catalin Marinas
2020-05-04 16:46 ` Dave Martin
2020-05-11 16:40 ` Catalin Marinas
2020-05-13 15:48 ` Dave Martin
2020-05-14 11:37 ` Catalin Marinas
2020-05-15 10:38 ` Catalin Marinas
2020-05-15 11:14 ` Szabolcs Nagy
2020-05-15 11:27 ` Catalin Marinas
2020-05-15 12:04 ` Szabolcs Nagy
2020-05-15 12:13 ` Catalin Marinas
2020-05-15 12:53 ` Szabolcs Nagy
2020-05-18 16:52 ` Dave Martin
2020-05-18 17:13 ` Catalin Marinas
2020-05-05 10:32 ` Szabolcs Nagy
2020-05-05 17:30 ` Catalin Marinas
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