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Bug#1130781: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL)

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  Bug#1130781: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL) Steven Shiau <steven.shiau@niar.org.tw> - 2026-03-15 07:20 +0100

#91669 — Bug#1130781: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL)

Package: src:linux
Version: 6.19.6+deb14-amd64
Severity: important

Description:
We have identified a forward-compatibility regression in the md 
subsystem introduced between Linux Kernel 6.18 and 6.19. When creating a 
standard RAID 1 array using mdadm, the resulting superblock generated 
under Kernel 6.19 is violently rejected by older enterprise Linux 
kernels (e.g., older LTS kernels with version 6.1) with an -EINVAL 
argument error during assembly.

A strict A/B test was performed using Debian Live (based on Sid) 
environments. The exact same version of mdadm (4.5-5) and the exact same 
creation command was used in both environments:
mdadm --create "/dev/md127" --force --run --level=raid1 --raid-devices=2 
--metadata=1.2 --data-offset=34816 --bitmap=internal --assume-clean 
/dev/nvme0n1p2 /dev/nvme1n1p2

Test 1 (Success):
* Environment: Kernel 6.18.15+deb14-amd64
* Result: Array creates successfully. When the machine reboots into the 
older guest OS, the legacy kernel assembles the array normally.

Test 2 (Failure/Regression):
* Environment: Kernel 6.19.6+deb14-amd64
* Result: Array creates successfully in the live environment. However, 
when the machine reboots into the older guest OS, the legacy kernel 
rejects the component drives with:
   mdadm: failed to add /dev/nvme1n1p2 to /dev/md/0: Invalid argument.
   mdadm: failed to add /dev/nvme0n1p2 to /dev/md/0: Invalid argument.
   mdadm: /dev/md0 assembled from 0 drives

Conclusion:
Kernel 6.19 appears to be injecting a new feature flag, modifying the 
bitmap format, or altering the superblock structure during creation in a 
way that older md_mod drivers cannot parse, breaking backward 
compatibility for disaster recovery tools restoring older operating systems.

Steven

-- 
Steven Shiau <steven _at_ stevenshiau org>
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