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From: Eric Biggers <ebiggers@kernel.org>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	Will Deacon <will@kernel.org>,
	linux-toolchains@vger.kernel.org,
	Mark Rutland <mark.rutland@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: Aarch64 EXT4FS inode checksum failures - seems to be weak memory ordering issues
Date: Thu, 7 Jan 2021 14:41:15 -0800	[thread overview]
Message-ID: <X/eOC/tj3966kBRH@gmail.com> (raw)
In-Reply-To: <20210107221446.GS1551@shell.armlinux.org.uk>

On Thu, Jan 07, 2021 at 10:14:46PM +0000, Russell King - ARM Linux admin wrote:
> On Thu, Jan 07, 2021 at 10:48:05PM +0100, Arnd Bergmann wrote:
> > On Thu, Jan 7, 2021 at 5:27 PM Theodore Ts'o <tytso@mit.edu> wrote:
> > >
> > > On Thu, Jan 07, 2021 at 01:37:47PM +0000, Russell King - ARM Linux admin wrote:
> > > > > The gcc bugzilla mentions backports into gcc-linaro, but I do not see
> > > > > them in my git history.
> > > >
> > > > So, do we raise the minimum gcc version for the kernel as a whole to 5.1
> > > > or just for aarch64?
> > >
> > > Russell, Arnd, thanks so much for tracking down the root cause of the
> > > bug!
> > 
> > There is one more thing that I wondered about when looking through
> > the ext4 code: Should it just call the crc32c_le() function directly
> > instead of going through the crypto layer? It seems that with Ard's
> > rework from 2018, that can just call the underlying architecture specific
> > implementation anyway.
> 
> Yes, I've been wondering about that too. To me, it looks like the
> ext4 code performs a layering violation by going "under the covers"
> - there are accessor functions to set the CRC and retrieve it. ext4
> instead just makes the assumption that the CRC value is stored after
> struct shash_desc. Especially as the crypto/crc32c code references
> the value using:
> 
> 	struct chksum_desc_ctx *ctx = shash_desc_ctx(desc);
> 
> Not even crypto drivers are allowed to assume that desc+1 is where
> the CRC is stored.

It violates how the shash API is meant to be used in general, but there is a
test that enforces that the shash_desc_ctx for crc32c must be just the single
u32 crc value.  See alg_test_crc32c() in crypto/testmgr.c.  So it's apparently
intended to work.

> 
> However, struct shash_desc is already 128 bytes in size on aarch64,

Ard Biesheuvel recently sent a patch to reduce the alignment of struct
shash_desc to ARCH_SLAB_MINALIGN
(https://lkml.kernel.org/linux-crypto/20210107124128.19791-1-ardb@kernel.org/),
since apparently most of the bloat is from alignment for DMA, which isn't
necessary.  I think that reduces the size by a lot on arm64.

> and the proper way of doing it via SHASH_DESC_ON_STACK() is overkill,
> being strangely 2 * sizeof(struct shash_desc) + 360 (which looks like
> another bug to me!)

Are you referring to the '2 * sizeof(struct shash_desc)' rather than just
'sizeof(struct shash_desc)'?  As mentioned in the comment above
HASH_MAX_DESCSIZE, there can be a nested shash_desc due to HMAC.
So I believe the value is correct.

> So, I agree with you wrt crc32c_le(), especially as it would be more
> efficient, and as the use of crc32c is already hard coded in the ext4
> code - not only with crypto_alloc_shash("crc32c", 0, 0) but also with
> the fixed-size structure in ext4_chksum().
> 
> However, it's ultimately up to the ext4 maintainers to decide.

As I mentioned in my other response, crc32c_le() isn't a proper library API
(like some of the newer lib/crypto/ stuff) but rather just a wrapper for the
shash API, and it doesn't handle modules being dynamically loaded/unloaded.
So switching to it may cause a performance regression.

What I'd recommend is making crc32c_le() able to call architecture-speccific
implementations directly, similar to blake2s() and chacha20() in lib/crypto/.
Then there would be no concern about when modules get loaded, etc...

- Eric

WARNING: multiple messages have this Message-ID
From: Eric Biggers <ebiggers@kernel.org>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Arnd Bergmann <arnd@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	linux-toolchains@vger.kernel.org,
	Ext4 Developers List <linux-ext4@vger.kernel.org>,
	Will Deacon <will@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: Aarch64 EXT4FS inode checksum failures - seems to be weak memory ordering issues
Date: Thu, 7 Jan 2021 14:41:15 -0800	[thread overview]
Message-ID: <X/eOC/tj3966kBRH@gmail.com> (raw)
In-Reply-To: <20210107221446.GS1551@shell.armlinux.org.uk>

On Thu, Jan 07, 2021 at 10:14:46PM +0000, Russell King - ARM Linux admin wrote:
> On Thu, Jan 07, 2021 at 10:48:05PM +0100, Arnd Bergmann wrote:
> > On Thu, Jan 7, 2021 at 5:27 PM Theodore Ts'o <tytso@mit.edu> wrote:
> > >
> > > On Thu, Jan 07, 2021 at 01:37:47PM +0000, Russell King - ARM Linux admin wrote:
> > > > > The gcc bugzilla mentions backports into gcc-linaro, but I do not see
> > > > > them in my git history.
> > > >
> > > > So, do we raise the minimum gcc version for the kernel as a whole to 5.1
> > > > or just for aarch64?
> > >
> > > Russell, Arnd, thanks so much for tracking down the root cause of the
> > > bug!
> > 
> > There is one more thing that I wondered about when looking through
> > the ext4 code: Should it just call the crc32c_le() function directly
> > instead of going through the crypto layer? It seems that with Ard's
> > rework from 2018, that can just call the underlying architecture specific
> > implementation anyway.
> 
> Yes, I've been wondering about that too. To me, it looks like the
> ext4 code performs a layering violation by going "under the covers"
> - there are accessor functions to set the CRC and retrieve it. ext4
> instead just makes the assumption that the CRC value is stored after
> struct shash_desc. Especially as the crypto/crc32c code references
> the value using:
> 
> 	struct chksum_desc_ctx *ctx = shash_desc_ctx(desc);
> 
> Not even crypto drivers are allowed to assume that desc+1 is where
> the CRC is stored.

It violates how the shash API is meant to be used in general, but there is a
test that enforces that the shash_desc_ctx for crc32c must be just the single
u32 crc value.  See alg_test_crc32c() in crypto/testmgr.c.  So it's apparently
intended to work.

> 
> However, struct shash_desc is already 128 bytes in size on aarch64,

Ard Biesheuvel recently sent a patch to reduce the alignment of struct
shash_desc to ARCH_SLAB_MINALIGN
(https://lkml.kernel.org/linux-crypto/20210107124128.19791-1-ardb@kernel.org/),
since apparently most of the bloat is from alignment for DMA, which isn't
necessary.  I think that reduces the size by a lot on arm64.

> and the proper way of doing it via SHASH_DESC_ON_STACK() is overkill,
> being strangely 2 * sizeof(struct shash_desc) + 360 (which looks like
> another bug to me!)

Are you referring to the '2 * sizeof(struct shash_desc)' rather than just
'sizeof(struct shash_desc)'?  As mentioned in the comment above
HASH_MAX_DESCSIZE, there can be a nested shash_desc due to HMAC.
So I believe the value is correct.

> So, I agree with you wrt crc32c_le(), especially as it would be more
> efficient, and as the use of crc32c is already hard coded in the ext4
> code - not only with crypto_alloc_shash("crc32c", 0, 0) but also with
> the fixed-size structure in ext4_chksum().
> 
> However, it's ultimately up to the ext4 maintainers to decide.

As I mentioned in my other response, crc32c_le() isn't a proper library API
(like some of the newer lib/crypto/ stuff) but rather just a wrapper for the
shash API, and it doesn't handle modules being dynamically loaded/unloaded.
So switching to it may cause a performance regression.

What I'd recommend is making crc32c_le() able to call architecture-speccific
implementations directly, similar to blake2s() and chacha20() in lib/crypto/.
Then there would be no concern about when modules get loaded, etc...

- Eric

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2021-01-07 22:42 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-05 15:47 Russell King - ARM Linux admin
2021-01-05 15:47 ` Russell King - ARM Linux admin
2021-01-05 18:27 ` Darrick J. Wong
2021-01-05 18:27   ` Darrick J. Wong
2021-01-05 19:50   ` Russell King - ARM Linux admin
2021-01-05 19:50     ` Russell King - ARM Linux admin
2021-01-06 11:53 ` Mark Rutland
2021-01-06 11:53   ` Mark Rutland
2021-01-06 12:13   ` Russell King - ARM Linux admin
2021-01-06 12:13     ` Russell King - ARM Linux admin
2021-01-06 13:52   ` Russell King - ARM Linux admin
2021-01-06 17:20     ` Will Deacon
2021-01-06 17:20       ` Will Deacon
2021-01-06 17:46       ` Russell King - ARM Linux admin
2021-01-06 17:46         ` Russell King - ARM Linux admin
2021-01-06 21:04       ` Arnd Bergmann
2021-01-06 21:04         ` Arnd Bergmann
2021-01-06 22:00         ` Arnd Bergmann
2021-01-06 22:00           ` Arnd Bergmann
2021-01-06 22:32       ` Russell King - ARM Linux admin
2021-01-06 22:32         ` Russell King - ARM Linux admin
2021-01-07 11:18         ` Russell King - ARM Linux admin
2021-01-07 11:18           ` Russell King - ARM Linux admin
2021-01-07 12:45           ` Russell King - ARM Linux admin
2021-01-07 12:45             ` Russell King - ARM Linux admin
2021-01-07 13:16             ` Arnd Bergmann
2021-01-07 13:16               ` Arnd Bergmann
2021-01-07 13:37               ` Russell King - ARM Linux admin
2021-01-07 13:37                 ` Russell King - ARM Linux admin
2021-01-07 16:27                 ` Theodore Ts'o
2021-01-07 16:27                   ` Theodore Ts'o
2021-01-07 17:00                   ` Florian Weimer
2021-01-07 17:00                     ` Florian Weimer
2021-01-07 21:48                   ` Arnd Bergmann
2021-01-07 21:48                     ` Arnd Bergmann
2021-01-07 22:14                     ` Russell King - ARM Linux admin
2021-01-07 22:14                       ` Russell King - ARM Linux admin
2021-01-07 22:41                       ` Eric Biggers [this message]
2021-01-07 22:41                         ` Eric Biggers
2021-01-08  8:21                         ` Ard Biesheuvel
2021-01-08  8:21                           ` Ard Biesheuvel
2021-01-07 22:27                     ` Eric Biggers
2021-01-07 22:27                       ` Eric Biggers
2021-01-07 23:53                       ` Darrick J. Wong
2021-01-07 23:53                         ` Darrick J. Wong
2021-01-08  8:05                         ` Arnd Bergmann
2021-01-08  8:05                           ` Arnd Bergmann
2021-01-08  9:13                   ` Peter Zijlstra
2021-01-08  9:13                     ` Peter Zijlstra
2021-01-08 10:31                   ` Pavel Machek
2021-01-08 10:31                     ` Pavel Machek
2021-01-07 21:20                 ` Arnd Bergmann
2021-01-07 21:20                   ` Arnd Bergmann
2021-01-08  9:21                   ` Peter Zijlstra
2021-01-08  9:21                     ` Peter Zijlstra
2021-01-08  9:26                     ` Will Deacon
2021-01-08  9:26                       ` Will Deacon
2021-01-08 20:02                       ` Linus Torvalds
2021-01-08 20:02                         ` Linus Torvalds
2021-01-08 20:22                         ` Arnd Bergmann
2021-01-08 20:22                           ` Arnd Bergmann
2021-01-08 21:20                           ` Nick Desaulniers
2021-01-08 21:20                             ` Nick Desaulniers
2021-01-08 20:29                         ` Russell King - ARM Linux admin
2021-01-08 20:29                           ` Russell King - ARM Linux admin
2021-01-12 13:20                         ` Lukas Wunner
2021-01-12 13:31                           ` Florian Weimer
2021-01-12 13:31                             ` Florian Weimer
2021-01-12 13:46                             ` David Laight
2021-01-12 13:46                               ` David Laight
2021-01-12 17:28                           ` Linus Torvalds
2021-01-12 17:28                             ` Linus Torvalds
2021-01-14 13:13                             ` Lukas Wunner

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