Patchwork [v3] Make initramfs honor CONFIG_DEVTMPFS_MOUNT

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Submitter Rob Landley
Date Sept. 13, 2017, 11:51 p.m.
Message ID <4aa9cb26-f868-0720-e5c8-3c4e08afad20@landley.net>
Download mbox | patch
Permalink /patch/9952289/
State New
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Comments

Rob Landley - Sept. 13, 2017, 11:51 p.m.
From: Rob Landley <rob@landley.net>

Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
/dev/console open after devtmpfs mount.

Add workaround for Debian bug that was copied by Ubuntu.

Signed-off-by: Rob Landley <rob@landley.net>
---

v2 discussion: http://lkml.iu.edu/hypermail/linux/kernel/1705.2/05611.html

 drivers/base/Kconfig |   14 ++++----------
 fs/namespace.c       |   14 ++++++++++++++
 init/main.c          |   15 +++++++++------
 3 files changed, 27 insertions(+), 16 deletions(-)

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LEROY Christophe - Sept. 14, 2017, 9:17 a.m.
Le 14/09/2017 à 01:51, Rob Landley a écrit :
> From: Rob Landley <rob@landley.net>
> 
> Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
> /dev/console open after devtmpfs mount.
> 
> Add workaround for Debian bug that was copied by Ubuntu.

Is that a bug only for Debian ? Why ?
Why should a Debian bug be fixed by a workaround in the mainline kernel ?

> 
> Signed-off-by: Rob Landley <rob@landley.net>
> ---
> 
> v2 discussion: http://lkml.iu.edu/hypermail/linux/kernel/1705.2/05611.html
> 
>   drivers/base/Kconfig |   14 ++++----------
>   fs/namespace.c       |   14 ++++++++++++++
>   init/main.c          |   15 +++++++++------
>   3 files changed, 27 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> index f046d21..97352d4 100644
> --- a/drivers/base/Kconfig
> +++ b/drivers/base/Kconfig
> @@ -48,16 +48,10 @@ config DEVTMPFS_MOUNT
>   	bool "Automount devtmpfs at /dev, after the kernel mounted the rootfs"
>   	depends on DEVTMPFS
>   	help
> -	  This will instruct the kernel to automatically mount the
> -	  devtmpfs filesystem at /dev, directly after the kernel has
> -	  mounted the root filesystem. The behavior can be overridden
> -	  with the commandline parameter: devtmpfs.mount=0|1.
> -	  This option does not affect initramfs based booting, here
> -	  the devtmpfs filesystem always needs to be mounted manually
> -	  after the rootfs is mounted.
> -	  With this option enabled, it allows to bring up a system in
> -	  rescue mode with init=/bin/sh, even when the /dev directory
> -	  on the rootfs is completely empty.
> +	  Automatically mount devtmpfs at /dev on the root filesystem, which
> +	  lets the system to come up in rescue mode with [rd]init=/bin/sh.
> +	  Override with devtmpfs.mount=0 on the commandline. Initramfs can
> +	  create a /dev dir as needed, other rootfs needs the mount point.

Why modifying the initial text ?
Why talking about rescue mode only, whereas this feature mainly concerns 
the standard mode.

>   
>   config STANDALONE
>   	bool "Select only drivers that don't need compile-time external firmware"
> diff --git a/fs/namespace.c b/fs/namespace.c
> index f8893dc..06057d7 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2417,7 +2417,21 @@ static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
>   	err = -EBUSY;
>   	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
>   	    path->mnt->mnt_root == path->dentry)
> +	{
> +		if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT) &&
> +		    !strcmp(path->mnt->mnt_sb->s_type->name, "devtmpfs"))
> +		{
> +			/* Debian's kernel config enables DEVTMPFS_MOUNT, then
> +			   its initramfs setup script tries to mount devtmpfs
> +			   again, and if the second mount-over-itself fails
> +			   the script overmounts a tmpfs on /dev to hide the
> +			   existing contents, then boot fails with empty /dev. */

Does it matter for the kernel code what Debian's kernel config does ?

> +			printk(KERN_WARNING "Debian bug workaround for devtmpfs overmount.");

Is this log message worth it when this modification goes in mainline 
kernel ?

If so, pr_err() should be used instead.

> +
> +			err = 0;
> +		}
>   		goto unlock;
> +	}
>   
>   	err = -EINVAL;
>   	if (d_is_symlink(newmnt->mnt.mnt_root))
> diff --git a/init/main.c b/init/main.c
> index 0ee9c686..0d8e5ec 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1065,12 +1065,6 @@ static noinline void __init kernel_init_freeable(void)
>   
>   	do_basic_setup();
>   
> -	/* Open the /dev/console on the rootfs, this should never fail */
> -	if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
> -		pr_err("Warning: unable to open an initial console.\n");
> -
> -	(void) sys_dup(0);
> -	(void) sys_dup(0);
>   	/*
>   	 * check if there is an early userspace init.  If yes, let it do all
>   	 * the work
> @@ -1082,8 +1076,17 @@ static noinline void __init kernel_init_freeable(void)
>   	if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
>   		ramdisk_execute_command = NULL;
>   		prepare_namespace();
> +	} else if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT)) {
> +		sys_mkdir("/dev", 0755);

Why not, but couldn't we also expect the initramfs to already contains 
that mountpoint ?

> +		devtmpfs_mount("/dev");
>   	}
>   
> +	/* Open the /dev/console on the rootfs, this should never fail */
> +	if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
> +		pr_err("Warning: unable to open an initial console.\n");
> +	(void) sys_dup(0);
> +	(void) sys_dup(0);
> +
>   	/*
>   	 * Ok, we have completed the initial bootup, and
>   	 * we're essentially up and running. Get rid of the
> 


Christophe
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gregkh@linuxfoundation.org - Sept. 14, 2017, 5:45 p.m.
On Wed, Sep 13, 2017 at 06:51:25PM -0500, Rob Landley wrote:
> From: Rob Landley <rob@landley.net>
> 
> Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
> /dev/console open after devtmpfs mount.
> 
> Add workaround for Debian bug that was copied by Ubuntu.
> 
> Signed-off-by: Rob Landley <rob@landley.net>
> ---
> 
> v2 discussion: http://lkml.iu.edu/hypermail/linux/kernel/1705.2/05611.html
> 
>  drivers/base/Kconfig |   14 ++++----------
>  fs/namespace.c       |   14 ++++++++++++++
>  init/main.c          |   15 +++++++++------
>  3 files changed, 27 insertions(+), 16 deletions(-)

Always run scripts/checkpatch.pl so you don't get grumpy emails from
reviewers telling you to run scripts/checkpatch.pl... telling you to run
scripts/checkpatch.pl... telling you to run scripts/checkpatch.pl...
telling you to run scripts/checkpatch.pl...
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Rob Landley - Sept. 17, 2017, 4:03 a.m.
On 09/14/2017 04:17 AM, Christophe LEROY wrote:
> Le 14/09/2017 à 01:51, Rob Landley a écrit :
>> From: Rob Landley <rob@landley.net>
>>
>> Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
>> /dev/console open after devtmpfs mount.
>>
>> Add workaround for Debian bug that was copied by Ubuntu.
> 
> Is that a bug only for Debian ? Why ?

Look down, specifically this bit:

>> v2 discussion:
>> http://lkml.iu.edu/hypermail/linux/kernel/1705.2/05611.html

That's some discussion of version 2 of this patch, which was merged for
a while last dev cycle, then backed out again because it triggered the
same bug in a number of system init scripts:

  http://lkml.iu.edu/hypermail/linux/kernel/1705.2/07072.html
  http://lkml.iu.edu/hypermail/linux/kernel/1705.3/01182.html
  http://lkml.iu.edu/hypermail/linux/kernel/1705.3/01505.html
  http://lkml.iu.edu/hypermail/linux/kernel/1705.3/01320.html

All of whom copied the broken error "recovery" path from debian. If they
checked whether it was already mounted, or didn't _blank_ the /dev
directory in response to mounting the exact same filesystem over itself
giving -EBUSY, the system would work fine. Heck, if you built a kernel
with a static /dev in initramfs and no devtmpfs configured in, the
script would break things exactly the same way. The breakage is that
script takes a hammer to a perfectly functional /dev directory and then
continues the boot with an empty /dev. That's bonkers.

> Why should a Debian bug be fixed by a workaround in the mainline kernel ?

That was my argument last time, and the answer was "Breaking userspace
is bad, mmmkay." Even when userspace is doing something REALLY OBVIOUSLY
STUPID and it is _clearly_ their fault, as long as they got there first
they've established the status quo and it doesn't matter how silly it is.

This was explicitly stated to me here:

  http://lkml.iu.edu/hypermail/linux/kernel/1705.3/03292.html

I.E. don't argue with me, argue with him. :)

So, I added a workaround with a printk in hopes of embarassing them into
someday fixing it.

Rob
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Henrique de Moraes Holschuh - Sept. 17, 2017, 1:51 p.m.
On Sat, 16 Sep 2017, Rob Landley wrote:
> So, I added a workaround with a printk in hopes of embarassing them into
> someday fixing it.

Oh, it will be fixed in Debian alright.  I am just waiting the issue to
settle a bit to file the bug reports, or maybe even send in the Debian
patches myself (note that I am not responsible for the code in question,
so I am not wearing a brown paperbag at this time).  Even if I didn't do
it, there are several other Debian Developers reading LKML that could do
it (provided they noticed this specific thread and are aware of the
situation) :p

I can even push for the fixes to be accepted into the stable and
oldstable branches of Debian, but that can take anything from a few
weeks to several months, due to the way our stable releases work.  But
it would eventually happen.

Whether such fixes will ever make it to LTS branches, especially
Ubuntu's, *that* I don't know.
Rob Landley - Sept. 20, 2017, 3:29 a.m.
On 09/17/2017 08:51 AM, Henrique de Moraes Holschuh wrote:
> On Sat, 16 Sep 2017, Rob Landley wrote:
>> So, I added a workaround with a printk in hopes of embarassing them into
>> someday fixing it.
> 
> Oh, it will be fixed in Debian alright.

Cool!

But part of the problem is people upgrade the kernel on existing
deployed root filesystems, some of which are a fork off of a fork off of
debian, so we won't exhaust the broken userspace for probably a couple
years.

I'd put it in feature-removal-schedule.txt but Linus zapped that, so...

> I am just waiting the issue to
> settle a bit to file the bug reports, or maybe even send in the Debian
> patches myself (note that I am not responsible for the code in question,
> so I am not wearing a brown paperbag at this time).  Even if I didn't do
> it, there are several other Debian Developers reading LKML that could do
> it (provided they noticed this specific thread and are aware of the
> situation) :p

There was a previous thread last merge window they didn't notice. I was
hoping the warning would be obvious enough. :)

> I can even push for the fixes to be accepted into the stable and
> oldstable branches of Debian, but that can take anything from a few
> weeks to several months, due to the way our stable releases work.  But
> it would eventually happen.
> 
> Whether such fixes will ever make it to LTS branches, especially
> Ubuntu's, *that* I don't know.

I have no idea what that powerpc system was, the guy didn't say...

Rob
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Michael Ellerman - Sept. 21, 2017, 10:13 a.m.
Rob Landley <rob@landley.net> writes:

> On 09/14/2017 04:17 AM, Christophe LEROY wrote:
>> Le 14/09/2017 à 01:51, Rob Landley a écrit :
>>> From: Rob Landley <rob@landley.net>
>>>
>>> Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
>>> /dev/console open after devtmpfs mount.
>>>
>>> Add workaround for Debian bug that was copied by Ubuntu.
>> 
>> Is that a bug only for Debian ? Why ?
>
> Look down, specifically this bit:
>
>>> v2 discussion:
>>> http://lkml.iu.edu/hypermail/linux/kernel/1705.2/05611.html
>
> That's some discussion of version 2 of this patch, which was merged for
> a while last dev cycle, then backed out again because it triggered the
> same bug in a number of system init scripts:
>
>   http://lkml.iu.edu/hypermail/linux/kernel/1705.2/07072.html
>   http://lkml.iu.edu/hypermail/linux/kernel/1705.3/01182.html
>   http://lkml.iu.edu/hypermail/linux/kernel/1705.3/01505.html
>   http://lkml.iu.edu/hypermail/linux/kernel/1705.3/01320.html
>
> All of whom copied the broken error "recovery" path from debian. If they
> checked whether it was already mounted, or didn't _blank_ the /dev
> directory in response to mounting the exact same filesystem over itself
> giving -EBUSY, the system would work fine. Heck, if you built a kernel
> with a static /dev in initramfs and no devtmpfs configured in, the
> script would break things exactly the same way. The breakage is that
> script takes a hammer to a perfectly functional /dev directory and then
> continues the boot with an empty /dev. That's bonkers.
>
>> Why should a Debian bug be fixed by a workaround in the mainline kernel ?
>
> That was my argument last time, and the answer was "Breaking userspace
> is bad, mmmkay." Even when userspace is doing something REALLY OBVIOUSLY
> STUPID and it is _clearly_ their fault, as long as they got there first
> they've established the status quo and it doesn't matter how silly it is.
>
> This was explicitly stated to me here:
>
>   http://lkml.iu.edu/hypermail/linux/kernel/1705.3/03292.html
>
> I.E. don't argue with me, argue with him. :)

I'm still here. And I'm still right :)

No one wants their system to stop booting because of this obscure
functionality.

Just put it behind a new config option which defaults off. No
workarounds required, no broken systems, no long email threads required.

cheers
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Patch

diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index f046d21..97352d4 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -48,16 +48,10 @@  config DEVTMPFS_MOUNT
 	bool "Automount devtmpfs at /dev, after the kernel mounted the rootfs"
 	depends on DEVTMPFS
 	help
-	  This will instruct the kernel to automatically mount the
-	  devtmpfs filesystem at /dev, directly after the kernel has
-	  mounted the root filesystem. The behavior can be overridden
-	  with the commandline parameter: devtmpfs.mount=0|1.
-	  This option does not affect initramfs based booting, here
-	  the devtmpfs filesystem always needs to be mounted manually
-	  after the rootfs is mounted.
-	  With this option enabled, it allows to bring up a system in
-	  rescue mode with init=/bin/sh, even when the /dev directory
-	  on the rootfs is completely empty.
+	  Automatically mount devtmpfs at /dev on the root filesystem, which
+	  lets the system to come up in rescue mode with [rd]init=/bin/sh.
+	  Override with devtmpfs.mount=0 on the commandline. Initramfs can
+	  create a /dev dir as needed, other rootfs needs the mount point.
 
 config STANDALONE
 	bool "Select only drivers that don't need compile-time external firmware"
diff --git a/fs/namespace.c b/fs/namespace.c
index f8893dc..06057d7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2417,7 +2417,21 @@  static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
 	err = -EBUSY;
 	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
 	    path->mnt->mnt_root == path->dentry)
+	{
+		if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT) &&
+		    !strcmp(path->mnt->mnt_sb->s_type->name, "devtmpfs"))
+		{
+			/* Debian's kernel config enables DEVTMPFS_MOUNT, then
+			   its initramfs setup script tries to mount devtmpfs
+			   again, and if the second mount-over-itself fails
+			   the script overmounts a tmpfs on /dev to hide the
+			   existing contents, then boot fails with empty /dev. */
+			printk(KERN_WARNING "Debian bug workaround for devtmpfs overmount.");
+
+			err = 0;
+		}
 		goto unlock;
+	}
 
 	err = -EINVAL;
 	if (d_is_symlink(newmnt->mnt.mnt_root))
diff --git a/init/main.c b/init/main.c
index 0ee9c686..0d8e5ec 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1065,12 +1065,6 @@  static noinline void __init kernel_init_freeable(void)
 
 	do_basic_setup();
 
-	/* Open the /dev/console on the rootfs, this should never fail */
-	if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
-		pr_err("Warning: unable to open an initial console.\n");
-
-	(void) sys_dup(0);
-	(void) sys_dup(0);
 	/*
 	 * check if there is an early userspace init.  If yes, let it do all
 	 * the work
@@ -1082,8 +1076,17 @@  static noinline void __init kernel_init_freeable(void)
 	if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
 		ramdisk_execute_command = NULL;
 		prepare_namespace();
+	} else if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT)) {
+		sys_mkdir("/dev", 0755);
+		devtmpfs_mount("/dev");
 	}
 
+	/* Open the /dev/console on the rootfs, this should never fail */
+	if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
+		pr_err("Warning: unable to open an initial console.\n");
+	(void) sys_dup(0);
+	(void) sys_dup(0);
+
 	/*
 	 * Ok, we have completed the initial bootup, and
 	 * we're essentially up and running. Get rid of the