From: Boris Burkov <firstname.lastname@example.org> To: Eric Biggers <email@example.com> Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH v4 4/4] generic: test fs-verity EFBIG scenarios Date: Fri, 10 Sep 2021 16:26:42 -0700 [thread overview] Message-ID: <YTvpsib6hrp/9ZPY@zen> (raw) In-Reply-To: <YK1c62bh1WQ/h45O@sol.localdomain> On Tue, May 25, 2021 at 01:24:11PM -0700, Eric Biggers wrote: > On Wed, May 05, 2021 at 02:04:46PM -0700, Boris Burkov wrote: > > diff --git a/tests/generic/632 b/tests/generic/632 > > new file mode 100755 > > index 00000000..5a5ed576 > > --- /dev/null > > +++ b/tests/generic/632 > > @@ -0,0 +1,86 @@ > > +#! /bin/bash > > +# SPDX-License-Identifier: GPL-2.0 > > +# Copyright (c) 2021 Facebook, Inc. All Rights Reserved. > > +# > > +# FS QA Test 632 > > +# > > +# Test some EFBIG scenarios with very large files. > > +# To create the files, use pwrite with an offset close to the > > +# file system's max file size. > > Can you please make this comment properly describe the purpose of this test? > As-is it doesn't mention that it is related to fs-verity at all, let alone to > specific filesystems' implementations of fs-verity. Sorry for disappearing on this one for a while. Oops, good point. In addressing your and Eryu's points, I realized that this isn't really a generic test, since as you say, it assumes the filesystem's implementation. Further, I think it is plausible for an fs to cache the Merkle tree pages some other way which wouldn't need to EFBIG for large files. With that said, I do think it's a useful test of an edge case I got wrong several times in the btrfs implementation. I am leaning towards making this a btrfs specific test. Just wanted to double check with you if you think ext4 and f2fs would benefit from running this test too.. > > > +max_sz=$(_get_max_file_size) > > +_fsv_scratch_begin_subtest "way too big: fail on first merkle block" > > +# have to go back by 4096 from max to not hit the fsverity MAX_DEPTH check. > > What is meant by the "fsverity MAX_DEPTH" check? If you use $max_sz or $max_sz-1 (or anything bigger than $max_sz-4096) the vfs fsverity code will conclude the tree will exceed MAX_LEVELS. I got LEVELS and DEPTH mixed up. > > > +$XFS_IO_PROG -fc "pwrite -q $(($max_sz - 4096)) 1" $fsv_file > > +_fsv_enable $fsv_file |& _filter_scratch > > Using the "truncate" xfs_io command instead of "pwrite" would probably make more > sense here, as the goal is to just create a file of a specific size. In my memory, truncate didn't work for btrfs, but it took me a while to get this to work, so I might have made some silly mistake early on with truncate. I'll try again to be sure. > > > + > > +# The goal of this second test is to make a big enough file that we trip the > > +# EFBIG codepath, but not so big that we hit it immediately as soon as we try > > +# to write a Merkle leaf. Because of the layout of the Merkle tree that > > +# fs-verity uses, this is a bit complicated to compute dynamically. > > + > > +# The layout of the Merkle tree has the leaf nodes last, but writes them first. > > +# To get an interesting overflow, we need the start of L0 to be < MAX but the > > +# end of the merkle tree (EOM) to be past MAX. Ideally, the start of L0 is only > > +# just smaller than MAX, so that we don't have to write many blocks to blow up. > > + > > +# 0 EOF round-to-64k L7L6L5 L4 L3 L2 L1 L0 MAX EOM > > +# |-------------------------| ||-|--|---|----|-----|------|--|!!!!!| > > + > > +# Given this structure, we can compute the size of the file that yields the > > +# desired properties: > > +# sz + 64k + sz/128^8 + sz/128^7 + ... + sz/128^2 < MAX > > +# (128^8)sz + (128^8)64k + sz + (128)sz + (128^2)sz + ... + (128^6)sz < (128^8)MAX > > +# sz(128^8 + 128^6 + 128^5 + 128^4 + 128^3 + 128^2 + 128 + 1) < (128^8)(MAX - 64k) > > +# sz < (128^8/(128^8 + (128^6 + ... 1))(MAX - 64k) > > +# > > +# Do the actual caclulation with 'bc' and 20 digits of precision. > > This calculation isn't completely accurate because it doesn't round the levels > to a block boundary. Nor does it consider that the 64K is an alignment rather > than a fixed amount added. > > But for the test you don't need the absolute largest file whose level 1 doesn't > exceed the limit, but rather just one almost that large. > > So it would be okay to add 64K as a fixed amount, along with 4K for every level > on top of the 'sz/128^(level+1)' you already have, to get an over-estimate of > the amount of extra space needed to cache the Merkle tree. > > But please make it clear that it's an over-estimate, and hence an under-estimate > of the file size desired for the test. > > Also please document that this is all assuming SHA-256 with 4K blocks, and also > that the maximum file size is assumed to fit in 64 bits; hence the consideration > of 8 levels is sufficient. Agreed with all of this, will do. > > - Eric
next prev parent reply other threads:[~2021-09-10 23:26 UTC|newest] Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-05 21:04 [PATCH v4 0/4] tests for btrfs fsverity Boris Burkov 2021-05-05 21:04 ` [PATCH v4 1/4] btrfs: test btrfs specific fsverity corruption Boris Burkov 2021-05-16 16:34 ` Eryu Guan 2021-05-05 21:04 ` [PATCH v4 2/4] generic/574: corrupt btrfs merkle tree data Boris Burkov 2021-05-16 16:38 ` Eryu Guan 2021-05-05 21:04 ` [PATCH v4 3/4] btrfs: test verity orphans with dmlogwrites Boris Burkov 2021-05-16 16:43 ` Eryu Guan 2021-05-05 21:04 ` [PATCH v4 4/4] generic: test fs-verity EFBIG scenarios Boris Burkov 2021-05-16 16:47 ` Eryu Guan 2021-05-25 20:24 ` Eric Biggers 2021-09-10 23:26 ` Boris Burkov [this message] 2021-09-10 23:32 ` Eric Biggers 2021-05-25 18:13 ` [PATCH v4 0/4] tests for btrfs fsverity Eric Biggers
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