nsenter: fix parsing of -t, -S and -G options
-t, -S and -G each take mandatory integer arguments. getopt32long()'s
option string syntax for this type of argument is 'c:+', however nsenter's
opt_str uses 'c+', which specifies two options 'c' and '+' which do not
take arguments. This means that giving a target PID causes nsenter to
exit and print the usage string:
# nsenter -t1 sh
nsenter: unrecognized option: 1
BusyBox v1.27.2 (2017-12-12 10:41:50 GMT) multi-call binary.
...
The long form options are also broken:
# nsenter --setuid=1000 --setgid=1000 sh
BusyBox v1.29.0.git (2018-05-04 13:56:49 UTC) multi-call binary.
...
`nsenter --target=<pid> sh` parses correctly and appears to work, but
<pid> is ignored and set to 0. This doesn't raise an error unless one
of the namespace arguments is also given:
# ./busybox_unstripped nsenter --target=42 sh
# exit
# ./busybox_unstripped nsenter -n --target=42 sh
BusyBox v1.29.0.git (2018-05-04 13:56:49 UTC) multi-call binary.
...
This has caused problems in a couple of places:
https://github.com/linuxkit/linuxkit/issues/567
https://github.com/gliderlabs/docker-alpine/issues/359
https://github.com/kontena/pharos-cluster/pull/81
Signed-off-by: Euan Harris <euan.harris@docker.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
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