[Buildroot] [PATCH] fs/cpio/init: unbreak ttyname_r() on glibc after dropping /dev/console exec

[Buildroot] [PATCH] fs/cpio/init: unbreak ttyname_r() on glibc after dropping /dev/console exec

[Peter Korsgaard]

Commit 98a6f1fc02e41 (fs/cpio: make initramfs init script survive 'console='
kernel argument) dropped the explicit /dev/console execs for fd 0,1,2, as
they fail when booted with console= and aren't really needed as the kernel
will setup fd 0,1,2 from /dev/console before executing the initramfs anyway.

Not doing this unfortunately confuses glibc's ttyname_r(3) implementation
(used by E.G.  busybox/coreutils 'tty'), causing it to fail with ENOENT as
it does a fstat on fd 0 and tries to match up st_ino / st_dev against the
entries in /dev (since glibc 2.26):

 commit 15e9a4f378c8607c2ae1aa465436af4321db0e23
 Author: Christian Brauner <christian.brauner@canonical.com>
 Date:   Fri Jan 27 15:59:59 2017 +0100

    linux ttyname and ttyname_r: do not return wrong results

    If a link (say /proc/self/fd/0) pointing to a device, say /dev/pts/2, in a
    parent mount namespace is passed to ttyname, and a /dev/pts/2 exists (in a
    different devpts) in the current namespace, then it returns /dev/pts/2.
    But /dev/pts/2 is NOT the current tty, it is a different file and device.

    Detect this case and return ENODEV.  Userspace can choose to take this as a hint
    that the fd points to a tty device but to act on the fd rather than the link.

    Signed-off-by: Serge Hallyn <serge@hallyn.com>
    Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

The reason it fails is that we manually mount devtmpfs on /dev in /init, so
the /dev/console used by the kernel (in rootfs) is not the same file as
/dev/console at runtime (in devtmpfs).

Notice: Once logged in, tty does work correctly.  Presumably login reopens
stdin/stdout/stderr.

To fix this, re-add the exec of /dev/console for fd 0,1,2, but only do so if
possible.  Because of the above mentioned shell behaviour (specified by
POSIX [0]), perform this check in a subshell.

[0] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_20_01

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
---
 fs/cpio/init | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/cpio/init b/fs/cpio/init
index b0af18b67a..f74ef7e15f 100755
--- a/fs/cpio/init
+++ b/fs/cpio/init
@@ -1,4 +1,15 @@
 #!/bin/sh
 # devtmpfs does not get automounted for initramfs
 /bin/mount -t devtmpfs devtmpfs /dev
+
+# use the /dev/console device node from devtmpfs if possible to not
+# confuse glibc's ttyname_r().
+# This may fail (E.G. booted with console=), and errors from exec will
+# terminate the shell, so use a subshell for the test
+if (exec 0</dev/console) 2>/dev/null; then
+    exec 0</dev/console
+    exec 1>/dev/console
+    exec 2>/dev/console
+fi
+
 exec /sbin/init "$@"
-- 
2.20.1

[Buildroot] [PATCH] fs/cpio/init: unbreak ttyname_r() on glibc after dropping /dev/console exec

[Yann E. MORIN]

Peter, All,

On 2020-08-29 15:09 +0200, Peter Korsgaard spake thusly:
> Commit 98a6f1fc02e41 (fs/cpio: make initramfs init script survive 'console='
> kernel argument) dropped the explicit /dev/console execs for fd 0,1,2, as
> they fail when booted with console= and aren't really needed as the kernel
> will setup fd 0,1,2 from /dev/console before executing the initramfs anyway.
> 
> Not doing this unfortunately confuses glibc's ttyname_r(3) implementation
> (used by E.G.  busybox/coreutils 'tty'), causing it to fail with ENOENT as
> it does a fstat on fd 0 and tries to match up st_ino / st_dev against the
> entries in /dev (since glibc 2.26):
> 
>  commit 15e9a4f378c8607c2ae1aa465436af4321db0e23
>  Author: Christian Brauner <christian.brauner@canonical.com>
>  Date:   Fri Jan 27 15:59:59 2017 +0100
> 
>     linux ttyname and ttyname_r: do not return wrong results
> 
>     If a link (say /proc/self/fd/0) pointing to a device, say /dev/pts/2, in a
>     parent mount namespace is passed to ttyname, and a /dev/pts/2 exists (in a
>     different devpts) in the current namespace, then it returns /dev/pts/2.
>     But /dev/pts/2 is NOT the current tty, it is a different file and device.
> 
>     Detect this case and return ENODEV.  Userspace can choose to take this as a hint
>     that the fd points to a tty device but to act on the fd rather than the link.
> 
>     Signed-off-by: Serge Hallyn <serge@hallyn.com>
>     Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> 
> The reason it fails is that we manually mount devtmpfs on /dev in /init, so
> the /dev/console used by the kernel (in rootfs) is not the same file as
> /dev/console at runtime (in devtmpfs).
> 
> Notice: Once logged in, tty does work correctly.  Presumably login reopens
> stdin/stdout/stderr.
> 
> To fix this, re-add the exec of /dev/console for fd 0,1,2, but only do so if
> possible.  Because of the above mentioned shell behaviour (specified by
> POSIX [0]), perform this check in a subshell.
> 
> [0] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_20_01
> 
> Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
> ---
>  fs/cpio/init | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/cpio/init b/fs/cpio/init
> index b0af18b67a..f74ef7e15f 100755
> --- a/fs/cpio/init
> +++ b/fs/cpio/init
> @@ -1,4 +1,15 @@
>  #!/bin/sh
>  # devtmpfs does not get automounted for initramfs
>  /bin/mount -t devtmpfs devtmpfs /dev
> +
> +# use the /dev/console device node from devtmpfs if possible to not
> +# confuse glibc's ttyname_r().
> +# This may fail (E.G. booted with console=), and errors from exec will
> +# terminate the shell, so use a subshell for the test
> +if (exec 0</dev/console) 2>/dev/null; then
> +    exec 0</dev/console
> +    exec 1>/dev/console
> +    exec 2>/dev/console
> +fi

You forgot the else part, which should redirect to /dev/null, so that we
do have valid fds in the case /dev/console does not really exist, no?

Regards,
Yann E. MORIN.

>  exec /sbin/init "$@"
> -- 
> 2.20.1
> 

-- 
.-----------------.--------------------.------------------.--------------------.
|  Yann E. MORIN  | Real-Time Embedded | /"\ ASCII RIBBON | Erics' conspiracy: |
| +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___               |
| +33 561 099 427 `------------.-------:  X  AGAINST      |  \e/  There is no  |
| http://ymorin.is-a-geek.org/ | _/*\_ | / \ HTML MAIL    |   v   conspiracy.  |
'------------------------------^-------^------------------^--------------------'

[Buildroot] [PATCH] fs/cpio/init: unbreak ttyname_r() on glibc after dropping /dev/console exec

[Peter Korsgaard]

>>>>> "Yann" == Yann E MORIN <yann.morin.1998@free.fr> writes:

Hi,

 >> +# use the /dev/console device node from devtmpfs if possible to not
 >> +# confuse glibc's ttyname_r().
 >> +# This may fail (E.G. booted with console=), and errors from exec will
 >> +# terminate the shell, so use a subshell for the test
 >> +if (exec 0</dev/console) 2>/dev/null; then
 >> +    exec 0</dev/console
 >> +    exec 1>/dev/console
 >> +    exec 2>/dev/console
 >> +fi

 > You forgot the else part, which should redirect to /dev/null, so that we
 > do have valid fds in the case /dev/console does not really exist, no?

Well, the kernel already takes care of that since commit 2bd3a997befc2
(E.G. as explained by buildroot commit 98a6f1fc02e41), so I am not sure
what explicitly reopening /dev/null would achive?

ttyname_r() / tty already returns ENOENT (not a tty) for this case
without reopening /dev/null:

tty </dev/null
not a tty

-- 
Bye, Peter Korsgaard

[Buildroot] [PATCH] fs/cpio/init: unbreak ttyname_r() on glibc after dropping /dev/console exec

[Yann E. MORIN]

Peter, All,

On 2020-08-29 15:09 +0200, Peter Korsgaard spake thusly:
> Commit 98a6f1fc02e41 (fs/cpio: make initramfs init script survive 'console='
> kernel argument) dropped the explicit /dev/console execs for fd 0,1,2, as
> they fail when booted with console= and aren't really needed as the kernel
> will setup fd 0,1,2 from /dev/console before executing the initramfs anyway.
> 
> Not doing this unfortunately confuses glibc's ttyname_r(3) implementation
> (used by E.G.  busybox/coreutils 'tty'), causing it to fail with ENOENT as
> it does a fstat on fd 0 and tries to match up st_ino / st_dev against the
> entries in /dev (since glibc 2.26):
> 
>  commit 15e9a4f378c8607c2ae1aa465436af4321db0e23
>  Author: Christian Brauner <christian.brauner@canonical.com>
>  Date:   Fri Jan 27 15:59:59 2017 +0100
> 
>     linux ttyname and ttyname_r: do not return wrong results
> 
>     If a link (say /proc/self/fd/0) pointing to a device, say /dev/pts/2, in a
>     parent mount namespace is passed to ttyname, and a /dev/pts/2 exists (in a
>     different devpts) in the current namespace, then it returns /dev/pts/2.
>     But /dev/pts/2 is NOT the current tty, it is a different file and device.
> 
>     Detect this case and return ENODEV.  Userspace can choose to take this as a hint
>     that the fd points to a tty device but to act on the fd rather than the link.
> 
>     Signed-off-by: Serge Hallyn <serge@hallyn.com>
>     Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> 
> The reason it fails is that we manually mount devtmpfs on /dev in /init, so
> the /dev/console used by the kernel (in rootfs) is not the same file as
> /dev/console at runtime (in devtmpfs).
> 
> Notice: Once logged in, tty does work correctly.  Presumably login reopens
> stdin/stdout/stderr.
> 
> To fix this, re-add the exec of /dev/console for fd 0,1,2, but only do so if
> possible.  Because of the above mentioned shell behaviour (specified by
> POSIX [0]), perform this check in a subshell.
> 
> [0] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_20_01
> 
> Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Applied to master, thanks.

Regards,
Yann E. MORIN.

> ---
>  fs/cpio/init | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/cpio/init b/fs/cpio/init
> index b0af18b67a..f74ef7e15f 100755
> --- a/fs/cpio/init
> +++ b/fs/cpio/init
> @@ -1,4 +1,15 @@
>  #!/bin/sh
>  # devtmpfs does not get automounted for initramfs
>  /bin/mount -t devtmpfs devtmpfs /dev
> +
> +# use the /dev/console device node from devtmpfs if possible to not
> +# confuse glibc's ttyname_r().
> +# This may fail (E.G. booted with console=), and errors from exec will
> +# terminate the shell, so use a subshell for the test
> +if (exec 0</dev/console) 2>/dev/null; then
> +    exec 0</dev/console
> +    exec 1>/dev/console
> +    exec 2>/dev/console
> +fi
> +
>  exec /sbin/init "$@"
> -- 
> 2.20.1
> 

-- 
.-----------------.--------------------.------------------.--------------------.
|  Yann E. MORIN  | Real-Time Embedded | /"\ ASCII RIBBON | Erics' conspiracy: |
| +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___               |
| +33 561 099 427 `------------.-------:  X  AGAINST      |  \e/  There is no  |
| http://ymorin.is-a-geek.org/ | _/*\_ | / \ HTML MAIL    |   v   conspiracy.  |
'------------------------------^-------^------------------^--------------------'

[Buildroot] [PATCH] fs/cpio/init: unbreak ttyname_r() on glibc after dropping /dev/console exec

[Peter Korsgaard]

>>>>> "Peter" == Peter Korsgaard <peter@korsgaard.com> writes:

 > Commit 98a6f1fc02e41 (fs/cpio: make initramfs init script survive 'console='
 > kernel argument) dropped the explicit /dev/console execs for fd 0,1,2, as
 > they fail when booted with console= and aren't really needed as the kernel
 > will setup fd 0,1,2 from /dev/console before executing the initramfs anyway.

 > Not doing this unfortunately confuses glibc's ttyname_r(3) implementation
 > (used by E.G.  busybox/coreutils 'tty'), causing it to fail with ENOENT as
 > it does a fstat on fd 0 and tries to match up st_ino / st_dev against the
 > entries in /dev (since glibc 2.26):

 >  commit 15e9a4f378c8607c2ae1aa465436af4321db0e23
 >  Author: Christian Brauner <christian.brauner@canonical.com>
 >  Date:   Fri Jan 27 15:59:59 2017 +0100

 >     linux ttyname and ttyname_r: do not return wrong results

 >     If a link (say /proc/self/fd/0) pointing to a device, say /dev/pts/2, in a
 >     parent mount namespace is passed to ttyname, and a /dev/pts/2 exists (in a
 >     different devpts) in the current namespace, then it returns /dev/pts/2.
 >     But /dev/pts/2 is NOT the current tty, it is a different file and device.

 >     Detect this case and return ENODEV.  Userspace can choose to take this as a hint
 >     that the fd points to a tty device but to act on the fd rather than the link.

 >     Signed-off-by: Serge Hallyn <serge@hallyn.com>
 >     Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

 > The reason it fails is that we manually mount devtmpfs on /dev in /init, so
 > the /dev/console used by the kernel (in rootfs) is not the same file as
 > /dev/console at runtime (in devtmpfs).

 > Notice: Once logged in, tty does work correctly.  Presumably login reopens
 > stdin/stdout/stderr.

 > To fix this, re-add the exec of /dev/console for fd 0,1,2, but only do so if
 > possible.  Because of the above mentioned shell behaviour (specified by
 > POSIX [0]), perform this check in a subshell.

 > [0] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_20_01

 > Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Committed to 2020.02.x and 2020.05.x, thanks.

-- 
Bye, Peter Korsgaard