NOFORK and NOEXEC without PREFER_APPLETS

[Denys Vlasenko]

On Wed, Jul 20, 2016 at 3:01 PM, Bruno Chevalier
<bmgg.chevalier at gmail.com> wrote:
> Hi,
>
> I am wondering why FEATURE_PREFER_APPLETS needs to be enabled for the
> FEATURE_SH_STANDALONE (to do the NOEXEC trick) and FEATURE_SH_NOFORK (to do
> the NOFORK trick).
>
> If the applet tables would always contain the NOFORK/NOEXEC bits, the
> FEATURE_PREFER_APPLETS wouldn't be mandatory anymore.
> You can look in the path and if the executable that gets found for a
> specific command is busybox, then we can do the NOEXEC or NOFORK trick.
> Otherwise, we just execute the program that was found using PATH in the
> normal way.
>
> A use case for this is the following:
> Let's say we have 2 buildroot configurations.
> We als have one common busybox configuration that contains the hexdump
> applet and that gets used for both buildroot configurations.
>
> 1 buildroot configuration contains a version of hexdump that overwrites the
> busybox implementation, the other doesn't.
>
> We notice that hexdump is a NOEXEC app and we want to take advantage of
> this.
>
> In the current implementation of busybox we have two scenarios:
> 1. We enable FEATURE_PREFER_APPLETS and FEATURE_SH_STANDALONE. This way, the
> hexdump that gets executed is always the busybox implementation and it makes
> use of the NOEXEC trick.
> We can make use of the NOEXEC trick, but the buildroot configuration that
> included hexdump does not execute its own hexdump by default unless you
> specify the complete path.
>
> 2. We don't enable FEATURE_PREFER_APPLETS. Now we execute the hexdump from
> buildroot in the buildroot config that enabled it and we execute the hexdump
> from busybox in the buildroot config that didn't include hexdump.
> However, the busybox version does not make use of the NOEXEC trick because
> that feature cannot be enabled without FEATURE_PREFER_APPLETS.
>
>
> Now we would like to be able to do scenario 2, but when the busybox version
> gets executed, we still want the NOEXEC trick to be used.

Enabling FEATURE_SH_STANDALONE does not give you case 2: hexdump
from buildroot won't be executed, internal applet takes precedence. IOW:
" Now we execute the hexdump from
> buildroot in the buildroot config that enabled it and we execute the hexdump
> from busybox in the buildroot config that didn't include hexdump."
will not be true, AFAICS.

I committed a fix which makes it possible to choose FEATURE_SH_STANDALONE
and/or NOFORK without FEATURE_PREFER_APPLETS. Please try current git.