linux-kernel - Cache coherency bug: stale reads on /dev/sda1

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Message-ID: <CA+jjjYTk=5wn2o46uNB+bJYX8xLgMP==dsJuvC94DvtN2f_6Yw@mail.gmail.com>
Date:   Sat, 26 Aug 2023 14:59:45 -0700
From:   Joshua Hudson <joshudson@...il.com>
To:     linux-kernel <linux-kernel@...r.kernel.org>
Subject: Cache coherency bug: stale reads on /dev/sda1

uname -a
Linux nova 6.4.12 #1 SMP PREEMPT_DYNAMIC Sat Aug 26 09:11:27 PDT 2023
x86_64 GNU/Linux

Kernel source is
https://mirrors.edge.kernel.org/pub/linux/kernel/v6.x/linux-6.4.12.tar.xz

Can reliably reproduce as follows:

# hexedit /dev/sda1 (this is my EFI partition)
PgDn
# hexedit /dev/sda
G 100000  (your value may vary--we want to seek to the start of the
EFI partition)
PgDn
PgDn
^C
write a marker to the padding between the BPB and the first FAT sector.
(If your system doesn't have one, edit an error message in the boot sector)
^X
# hexedit /dev/sda1
PgDn
Look for marker written above, find it's not there !!!
^C

I discovered this one trying to defragment my EFI partition after a
grub upgrade left it very fragmented due to replacing logos.