Upstream fix is:
https://gitlab.com/nbdkit/nbdkit/-/commit/c0b15574647672cb5c48178333acdd07424692ef

Waiting on RHEL AV 8.6 branch to open before I can push this.

Test the bug with below builds:
nbdkit-1.24.0-3.module+el8.6.0+12512+6129c95d.x86_64
libnbd-1.6.0-4.module+el8.6.0+12490+ec3e565c.x86_64
python3-libnbd-1.6.0-4.module+el8.6.0+12490+ec3e565c.x86_64

Steps:
1.# nbdkit --filter=cow data "33 * 100000" --run 'nbdsh -u $uri -c "h.trim(100000, 0)" ; nbdcopy $uri - | hexdump -C'
nbdsh: command line script failed: nbd_trim: server does not support trim operations: Invalid argument
00000000  21 21 21 21 21 21 21 21  21 21 21 21 21 21 21 21  |!!!!!!!!!!!!!!!!|
*
00018000


Result:
   Got unexpected result, bug is not fixed.

Actually the problem is with the test itself:

  nbdsh: command line script failed: nbd_trim: server does not support trim operations: Invalid argument

Replacing ";" with "&&" shows that the script failed:

  $ nbdkit --filter=cow data "33 * 100000" --run 'nbdsh -u $uri -c "h.trim(100000, 0)" && nbdcopy $uri - | hexdump -C'
  nbdsh: command line script failed: nbd_trim: server does not support trim operations: Invalid argument
  $ echo $?
  1

I believe it's not possible to hit this code path with nbdkit 1.24.
This is because trim is not support in the cache and cow filters in
1.24.  zero is supported, but is not reachable because:

  https://gitlab.com/nbdkit/nbdkit/-/blob/7f4f80fd418452ffcc29e3f4d99e7ae90880dbde/filters/cow/cow.c#L415

I still want to keep this patch in 1.24 because it is possible
that future changes on this branch (eg. enabling trim support)
could trigger this bug.

May be best to check that the patch is applied in build.log?
In other words, simple sanity only check.

Rich - assigning to you directly since we have an ITM/release+ and you've made the change. Feel free to move to eblake if he's doing the downstream work for nbdkit.

Verify the bug with nbdkit-1.24.0-3.module+el8.6.0+12861+13975d62.x86_64

Steps:
1.According to comment10, check if the patch 'https://gitlab.com/nbdkit/nbdkit/-/commit/c0b15574647672cb5c48178333acdd07424692ef' is added to nbdkit-1.24.0-3

# rpmbuild -rp nbdkit-1.24.0-3.module+el8.6.0+12861+13975d62.src.rpm

# cat /root/rpmbuild/BUILD/nbdkit-1.24.0/filters/cache/cache.c |grep memset
      memset (&block[blkoffs], 0, n);
    memset (block, 0, blksize);
      memset (block, 0, count);

# cat /root/rpmbuild/BUILD/nbdkit-1.24.0/filters/cow/cow.c |grep memset
      memset (&block[blkoffs], 0, n);
    memset (block, 0, BLKSIZE);
      memset (block, 0, count);


Hi Richard,

    I found the code changes of https://gitlab.com/nbdkit/nbdkit/-/commit/c0b15574647672cb5c48178333acdd07424692ef are different with the changes of https://gitlab.com/nbdkit/nbdkit/-/commit/a0ae7b2158598ce48ac31706319007f716d01c87, which commit is expected for the bug?

Check the code changes for the bug in nbdkit-1.28.0-1.el9.src.rpm, the result of cow.c is different with nbdkit-1.24.0-3

# cat /root/rpmbuild/BUILD/nbdkit-1.28.0/filters/cow/cow.c |grep memset
      memset (&block[blkoffs], 0, n);
    memset (block, 0, blksize);
      memset (block, 0, count);
      memset (&block[blkoffs], 0, n);
      memset (block, 0, count);

The reason for the difference is because nbdkit 1.24 cow/cache
filters did not support the TRIM operation.  So the whole function
(cow_trim, cache_trim) is missing in 1.24.  This leads to fewer
calls to memset (only cow_zero/cow_trim).  In addition the *_zero
functions call memset three times because they actually zero the
head, main body of data, and tail.  But the *_trim functions call
memset twice because they only zero the head and tail (there's a trim-
specific operation for the main body of data).

Anyway, these match the patched files, so the verification is
correct, thanks.

Thanks Richard, move the bug from ON_QA to VERIFIED according to comment14 and comment15

*** Bug 2040775 has been marked as a duplicate of this bug. ***

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: virt:rhel and virt-devel:rhel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2022:1759