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

[Peter Korsgaard]

>>>>> "Peter" == Peter Korsgaard <peter at 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 at 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 at hallyn.com>
 >     Signed-off-by: Christian Brauner <christian.brauner at 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 at korsgaard.com>

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

-- 
Bye, Peter Korsgaard