[I am not sure why this error only popped up after I merged Andrew's
patch set ...]

After merging the btrfs tree, today's linux-next build (x86_64
allmodconfig) failed like this:

In file included from include/linux/string.h:253,
                 from include/linux/bitmap.h:11,
                 from include/linux/cpumask.h:12,
                 from arch/x86/include/asm/cpumask.h:5,
                 from arch/x86/include/asm/msr.h:11,
                 from arch/x86/include/asm/processor.h:22,
                 from arch/x86/include/asm/cpufeature.h:5,
                 from arch/x86/include/asm/thread_info.h:53,
                 from include/linux/thread_info.h:60,
                 from arch/x86/include/asm/preempt.h:7,
                 from include/linux/preempt.h:78,
                 from include/linux/spinlock.h:55,
                 from include/linux/wait.h:9,
                 from include/linux/mempool.h:8,
                 from include/linux/bio.h:8,
                 from fs/btrfs/ioctl.c:7:
In function 'memcpy',
    inlined from '_btrfs_ioctl_send' at fs/btrfs/ioctl.c:4846:3:
include/linux/fortify-string.h:219:4: error: call to '__write_overflow' declared with attribute error: detected write beyond size of object (1st parameter)
  219 |    __write_overflow();
      |    ^~~~~~~~~~~~~~~~~~

Caused by commit

  c8d9cdfc766d ("btrfs: send: prepare for v2 protocol")

This changes the "reserved" field of struct btrfs_ioctl_send_args from 4 u64's to 3, but the above memcpy is copying the "reserved" filed from a struct btrfs_ioctl_send_args_32 (4 u64s) into it.

All I could really do at this point was mark BTRFS_FS as BROKEN
(TEST_KMOD selects BTRFS_FS):

From: Stephen Rothwell <redacted>
Date: Wed, 27 Oct 2021 20:53:24 +1100
Subject: [PATCH] make btrfs as BROKEN due to an inconsistent API change

Signed-off-by: Stephen Rothwell <redacted>
---
 fs/btrfs/Kconfig  | 1 +
 lib/Kconfig.debug | 1 +
 2 files changed, 2 insertions(+)

diff --git a/fs/btrfs/Kconfig b/fs/btrfs/Kconfig
index 520a0f6a7d9e..f7dd994a88af 100644
--- a/fs/btrfs/Kconfig
+++ b/fs/btrfs/Kconfig

@@ -20,6 +20,7 @@ config BTRFS_FS
 	select SRCU
 	depends on !PPC_256K_PAGES	# powerpc
 	depends on !PAGE_SIZE_256KB	# hexagon
+	depends on BROKEN
 
 	help
 	  Btrfs is a general purpose copy-on-write filesystem with extents,

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 0104cafd403f..44a6df361016 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug

@@ -2491,6 +2491,7 @@ config TEST_KMOD
 	depends on m
 	depends on NETDEVICES && NET_CORE && INET # for TUN
 	depends on BLOCK
+	depends on BROKEN
 	select TEST_LKM
 	select XFS_FS
 	select TUN

-- 
2.33.0