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[PATCH] Make initramfs honor CONFIG_DEVTMPFS_MOUNT

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  [PATCH] Make initramfs honor CONFIG_DEVTMPFS_MOUNT Rob Landley <rob@landley.net> - 2017-05-04 23:10 +0200
    Re: [PATCH] Make initramfs honor CONFIG_DEVTMPFS_MOUNT Kees Cook <keescook@chromium.org> - 2017-05-05 01:40 +0200
    Re: [PATCH] Make initramfs honor CONFIG_DEVTMPFS_MOUNT Andrew Morton <akpm@linux-foundation.org> - 2017-05-09 23:40 +0200
      Re: [PATCH] Make initramfs honor CONFIG_DEVTMPFS_MOUNT Rob Landley <rob@landley.net> - 2017-05-11 19:10 +0200
      [PATCHv2] Make initramfs honor CONFIG_DEVTMPFS_MOUNT Rob Landley <rob@landley.net> - 2017-05-11 19:10 +0200

#1636047 — [PATCH] Make initramfs honor CONFIG_DEVTMPFS_MOUNT

From: Rob Landley <rob@landley.net>

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

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

 init/main.c |   15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/init/main.c b/init/main.c
index 2858be7..71ed0d7 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1016,12 +1016,6 @@ 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
@@ -1033,8 +1027,17 @@ 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);
+		sys_mount("dev", "dev", "devtmpfs", MS_SILENT, NULL);
 	}
 
+	/* 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

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#1636086

On Thu, May 4, 2017 at 2:09 PM, Rob Landley <rob@landley.net> wrote:
> From: Rob Landley <rob@landley.net>
>
> Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
> /dev/console open after devtmpfs mount.
>
> Signed-off-by: Rob Landley <rob@landley.net>

Seems sensible.

Reviewed-by: Kees Cook <keescook@chromium.org>


-Kees

> ---
>
>  init/main.c |   15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/init/main.c b/init/main.c
> index 2858be7..71ed0d7 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1016,12 +1016,6 @@ 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
> @@ -1033,8 +1027,17 @@ 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);
> +               sys_mount("dev", "dev", "devtmpfs", MS_SILENT, NULL);
>         }
>
> +       /* 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



-- 
Kees Cook
Pixel Security

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#1638410

On Thu, 4 May 2017 16:09:06 -0500 Rob Landley <rob@landley.net> wrote:

> From: Rob Landley <rob@landley.net>
> 
> Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
> /dev/console open after devtmpfs mount.


Could we please see complete description of the runtime effects of this
change?  How does it affect users?  How does it benefit users?

The DEVTMPFS_MOUNT Kconfig help (drivers/base/Kconfig) says:

	  This option does not affect initramfs based booting, here
	  the devtmpfs filesystem always needs to be mounted manually
	  after the rootfs is mounted.

which seems to no longer be correct?

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#1639900

On 05/09/2017 04:31 PM, Andrew Morton wrote:
> On Thu, 4 May 2017 16:09:06 -0500 Rob Landley <rob@landley.net> wrote:
> 
>> From: Rob Landley <rob@landley.net>
>>
>> Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
>> /dev/console open after devtmpfs mount.
> 
> 
> Could we please see complete description of the runtime effects of this
> change?  How does it affect users?  How does it benefit users?

It makes the behavior consistent. If you're going to have the config
symbol anyway, why is initramfs a second class citizen?

That said, I was fixing a specific bug when I started the patch: when
you statically link in an initramfs by pointing the kernel build at a
directory (so it makes its own cpio archive from that), if you're not
running the build as root you can't create dev/console in there and
there's no obvious way to add nodes (like you can editing the
gen_initramfs_list) output.

This means there's no /dev/console when init gets launched, so PID 1's
stdin/stdout/stderr go nowhere, and until your init script can open its
own and redirect you get no output if something goes wrong, so debugging
is fiddly and there's a hole where output gets lost. Userspace can't
close that hole.

When making the patch I did a version that mounted /proc /sys and
/dev/pts too, so rdinit=/bin/sh had pretty much its full environment
without an init script just like the DEVTMPFS_MOUNT option's help text
implied... but that seemed unlikely to be accepted. The console gap is a
problem userspace can't fix, the rest userspace can, so I did the
minimal thing.

> The DEVTMPFS_MOUNT Kconfig help (drivers/base/Kconfig) says:
> 
> 	  This option does not affect initramfs based booting, here
> 	  the devtmpfs filesystem always needs to be mounted manually
> 	  after the rootfs is mounted.
> 
> which seems to no longer be correct?

Ah, sorry. I rewrote the help text and didn't include that file in the
diff. And rechecking I see the override part wasn't implemented by my
patch, I'll send a new one.

Rob

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#1639903 — [PATCHv2] Make initramfs honor CONFIG_DEVTMPFS_MOUNT

From: Rob Landley <rob@landley.net>

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

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

 drivers/base/Kconfig |   14 ++++----------
 init/main.c          |   15 +++++++++------
 2 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index d718ae4..74779ee 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 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/init/main.c b/init/main.c
index f866510..9ec09ff 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1038,12 +1038,6 @@ 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
@@ -1055,8 +1049,17 @@ 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

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