linux-kernel - Re: [PATCH] tmpfs: Disallow CONFIG_TMPFS

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Date:   Fri, 5 Feb 2021 16:05:51 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Seth Forshee <seth.forshee@...onical.com>
Cc:     Hugh Dickins <hughd@...gle.com>, Chris Down <chris@...isdown.name>,
        Amir Goldstein <amir73il@...il.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tmpfs: Disallow CONFIG_TMPFS_INODE64 on s390

On Fri,  5 Feb 2021 17:06:20 -0600 Seth Forshee <seth.forshee@...onical.com> wrote:

> This feature requires ino_t be 64-bits, which is true for every
> 64-bit architecture but s390, so prevent this option from being
> selected there.
> 

The previous patch nicely described the end-user impact of the bug. 
This is especially important when requesting a -stable backport.

Here's what I ended up with:


From: Seth Forshee <seth.forshee@...onical.com>
Subject: tmpfs: disallow CONFIG_TMPFS_INODE64 on s390

Currently there is an assumption in tmpfs that 64-bit architectures also
have a 64-bit ino_t.  This is not true on s390 which has a 32-bit ino_t. 
With CONFIG_TMPFS_INODE64=y tmpfs mounts will get 64-bit inode numbers and
display "inode64" in the mount options, but passing the "inode64" mount
option will fail.  This leads to the following behavior:

 # mkdir mnt
 # mount -t tmpfs nodev mnt
 # mount -o remount,rw mnt
 mount: /home/ubuntu/mnt: mount point not mounted or bad option.

As mount sees "inode64" in the mount options and thus passes it in the
options for the remount.


So prevent CONFIG_TMPFS_INODE64 from being selected on s390.

Link: https://lkml.kernel.org/r/20210205230620.518245-1-seth.forshee@canonical.com
Fixes: ea3271f7196c ("tmpfs: support 64-bit inums per-sb")
Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
Cc: Chris Down <chris@...isdown.name>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Amir Goldstein <amir73il@...il.com>
Cc: Heiko Carstens <hca@...ux.ibm.com>
Cc: Vasily Gorbik <gor@...ux.ibm.com>
Cc: Christian Borntraeger <borntraeger@...ibm.com>
Cc: <stable@...r.kernel.org>	[5.9+]
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 fs/Kconfig |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/Kconfig~tmpfs-disallow-config_tmpfs_inode64-on-s390
+++ a/fs/Kconfig
@@ -203,7 +203,7 @@ config TMPFS_XATTR
 
 config TMPFS_INODE64
 	bool "Use 64-bit ino_t by default in tmpfs"
-	depends on TMPFS && 64BIT
+	depends on TMPFS && 64BIT && !S390
 	default n
 	help
 	  tmpfs has historically used only inode numbers as wide as an unsigned
_