Read mount id also on legacy kernels

[smitsohu]

Recursive remounting (noexec, read-only, read-write) currently needs a mount id. So far this id could not be obtained on kernels before 3.15.

Add support for recursive remounting also on legacy kernels, like frequently found on mobile devices. Should help SailfishOS community ports.

[kmk3]

@smitsohu commented on Oct 5:

Recursive remounting (noexec, read-only, read-write) currently needs a
mount id. So far this id could not be obtained on kernels before 3.15.

Add support for recursive remounting also on legacy kernels, like frequently
found on mobile devices. Should help SailfishOS community ports.

Interesting, I've always wondered why firejail explicitly aims to support old
kernels ("3.x or newer") and mobile phones would explain it all.

[smitsohu]

Interesting, I've always wondered why firejail explicitly aims to support old
kernels ("3.x or newer") and mobile phones would explain it all.

Just checked, this 3.x support statement dates back to the very first Firejail release in 2015, when 3.x kernels were quite new.

Six years later Firejail is still doing ok on these outdated kernels, and this has found (new) users. I guess that's the story more or less.

[smitsohu]

I updated the pull request to remove what was probably unnecessary root permissions.

As these changes are quite boring in its current form, I'll go ahead and merge.

[kmk3]

@smitsohu commented on Oct 6:

Interesting, I've always wondered why firejail explicitly aims to support
old kernels ("3.x or newer") and mobile phones would explain it all.

Just checked, this 3.x support statement dates back to the very first
Firejail release in 2015, when 3.x kernels were quite new.

Six years later Firejail is still doing ok on these outdated kernels, and
this has found (new) users. I guess that's the story more or less.

Quite fascinating that firejail apparently only started to be used on mobile on
these kernels after they were closer to being legacy.

IIRC Android started to become a big deal around the times of Linux 3.x, so
that makes a lot of sense as well.

Cc: @spiiroin @Tomin1 (from SailfishOS)

[Tomin1]

The oldest kernels we use in Sailfish OS are 3.10 and 3.18.