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Filipe Manana 978b63f746 btrfs: fix race when detecting delalloc ranges during fiemap 
For fiemap we recently stopped locking the target extent range for the
whole duration of the fiemap call, in order to avoid a deadlock in a
scenario where the fiemap buffer happens to be a memory mapped range of
the same file. This use case is very unlikely to be useful in practice but
it may be triggered by fuzz testing (syzbot, etc).

This however introduced a race that makes us miss delalloc ranges for
file regions that are currently holes, so the caller of fiemap will not
be aware that there's data for some file regions. This can be quite
serious for some use cases - for example in coreutils versions before 9.0,
the cp program used fiemap to detect holes and data in the source file,
copying only regions with data (extents or delalloc) from the source file
to the destination file in order to preserve holes (see the documentation
for its --sparse command line option). This means that if cp was used
with a source file that had delalloc in a hole, the destination file could
end up without that data, which is effectively a data loss issue, if it
happened to hit the race described below.

The race happens like this:

1) Fiemap is called, without the FIEMAP_FLAG_SYNC flag, for a file that
   has delalloc in the file range [64M, 65M[, which is currently a hole;

2) Fiemap locks the inode in shared mode, then starts iterating the
   inode's subvolume tree searching for file extent items, without having
   the whole fiemap target range locked in the inode's io tree - the
   change introduced recently by commit b0ad381fa7 ("btrfs: fix
   deadlock with fiemap and extent locking"). It only locks ranges in
   the io tree when it finds a hole or prealloc extent since that
   commit;

3) Note that fiemap clones each leaf before using it, and this is to
   avoid deadlocks when locking a file range in the inode's io tree and
   the fiemap buffer is memory mapped to some file, because writing
   to the page with btrfs_page_mkwrite() will wait on any ordered extent
   for the page's range and the ordered extent needs to lock the range
   and may need to modify the same leaf, therefore leading to a deadlock
   on the leaf;

4) While iterating the file extent items in the cloned leaf before
   finding the hole in the range [64M, 65M[, the delalloc in that range
   is flushed and its ordered extent completes - meaning the corresponding
   file extent item is in the inode's subvolume tree, but not present in
   the cloned leaf that fiemap is iterating over;

5) When fiemap finds the hole in the [64M, 65M[ range by seeing the gap in
   the cloned leaf (or a file extent item with disk_bytenr == 0 in case
   the NO_HOLES feature is not enabled), it will lock that file range in
   the inode's io tree and then search for delalloc by checking for the
   EXTENT_DELALLOC bit in the io tree for that range and ordered extents
   (with btrfs_find_delalloc_in_range()). But it finds nothing since the
   delalloc in that range was already flushed and the ordered extent
   completed and is gone - as a result fiemap will not report that there's
   delalloc or an extent for the range [64M, 65M[, so user space will be
   mislead into thinking that there's a hole in that range.

This could actually be sporadically triggered with test case generic/094
from fstests, which reports a missing extent/delalloc range like this:

  generic/094 2s ... - output mismatch (see /home/fdmanana/git/hub/xfstests/results//generic/094.out.bad)
      --- tests/generic/094.out	2020-06-10 19:29:03.830519425 +0100
      +++ /home/fdmanana/git/hub/xfstests/results//generic/094.out.bad	2024-02-28 11:00:00.381071525 +0000
      @@ -1,3 +1,9 @@
       QA output created by 094
       fiemap run with sync
       fiemap run without sync
      +ERROR: couldn't find extent at 7
      +map is 'HHDDHPPDPHPH'
      +logical: [       5..       6] phys:   301517..  301518 flags: 0x800 tot: 2
      +logical: [       8..       8] phys:   301520..  301520 flags: 0x800 tot: 1
      ...
      (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/generic/094.out /home/fdmanana/git/hub/xfstests/results//generic/094.out.bad'  to see the entire diff)

So in order to fix this, while still avoiding deadlocks in the case where
the fiemap buffer is memory mapped to the same file, change fiemap to work
like the following:

1) Always lock the whole range in the inode's io tree before starting to
   iterate the inode's subvolume tree searching for file extent items,
   just like we did before commit b0ad381fa7 ("btrfs: fix deadlock with
   fiemap and extent locking");

2) Now instead of writing to the fiemap buffer every time we have an extent
   to report, write instead to a temporary buffer (1 page), and when that
   buffer becomes full, stop iterating the file extent items, unlock the
   range in the io tree, release the search path, submit all the entries
   kept in that buffer to the fiemap buffer, and then resume the search
   for file extent items after locking again the remainder of the range in
   the io tree.

   The buffer having a size of a page, allows for 146 entries in a system
   with 4K pages. This is a large enough value to have a good performance
   by avoiding too many restarts of the search for file extent items.
   In other words this preserves the huge performance gains made in the
   last two years to fiemap, while avoiding the deadlocks in case the
   fiemap buffer is memory mapped to the same file (useless in practice,
   but possible and exercised by fuzz testing and syzbot).

Fixes: b0ad381fa7 ("btrfs: fix deadlock with fiemap and extent locking")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
History 2024-03-05 18:12:37 +01:00
arch powerpc fixes for 6.8 #5 2024-03-03 09:47:19 -08:00
block block: sed-opal: handle empty atoms when parsing response 2024-02-16 15:52:45 -07:00
certs This update includes the following changes: 2023-11-02 16:15:30 -10:00
crypto crypto: lskcipher - Copy IV in lskcipher glue code always 2024-02-24 08:37:24 +08:00
Documentation Including fixes from bluetooth, WiFi and netfilter. 2024-02-29 12:40:20 -08:00
drivers phy second set of fixes for 6.8 2024-03-03 09:56:49 -08:00
fs btrfs: fix race when detecting delalloc ranges during fiemap 2024-03-05 18:12:37 +01:00
include btrfs: qgroup: validate btrfs_qgroup_inherit parameter 2024-03-05 17:13:24 +01:00
init update workarounds for gcc "asm goto" issue 2024-02-15 11:14:33 -08:00
io_uring io_uring/net: fix multishot accept overflow handling 2024-02-14 18:30:19 -07:00
ipc shm: Slim down dependencies 2023-12-20 19:26:31 -05:00
kernel Probes fixes for v6.8-rc5: 2024-03-01 11:17:23 -08:00
lib Including fixes from bluetooth, WiFi and netfilter. 2024-02-29 12:40:20 -08:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm 6 hotfixes. 3 are cc:stable and the remainder address post-6.7 issues 2024-02-27 16:44:15 -08:00
net bluetooth pull request for net: 2024-02-29 09:10:24 -08:00
rust Rust changes for v6.8 2024-01-11 13:05:41 -08:00
samples work around gcc bugs with 'asm goto' with outputs 2024-02-09 15:57:48 -08:00
scripts RISC-V Fixes for 6.8-rc7 2024-03-01 12:44:33 -08:00
security tomoyo: fix UAF write bug in tomoyo_write_control() 2024-03-01 11:14:00 -08:00
sound ASoC: Fixes for v6.8 2024-02-29 08:29:04 +01:00
tools powerpc fixes for 6.8 #5 2024-03-03 09:47:19 -08:00
usr Kbuild updates for v6.8 2024-01-18 17:57:07 -08:00
virt Generic: 2024-01-17 13:03:37 -08:00
.clang-format clang-format: Update with v6.7-rc4's for_each macro list 2023-12-08 23:54:38 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig Add .editorconfig file for basic formatting 2023-12-28 16:22:47 +09:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore Add .editorconfig file for basic formatting 2023-12-28 16:22:47 +09:00
.mailmap MAINTAINERS: mailmap: update Shakeel's email address 2024-02-20 14:20:50 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: supplement of zswap maintainers update 2024-01-25 23:52:21 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS RISC-V Fixes for 6.8-rc7 2024-03-01 12:44:33 -08:00
Makefile Linux 6.8-rc7 2024-03-03 13:02:52 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.