Re: [OE-core] [PATCH] systemd: Fix build on musl

["Andre McCurdy" <armccurdy@gmail.com>]

On Fri, Aug 6, 2021 at 7:12 AM Luca Boccassi <luca.boccassi@gmail.com> wrote:
>
> On Thu, 29 Jul 2021 at 20:11, Andre McCurdy <armccurdy@gmail.com> wrote:
> >
> > On Thu, Jul 29, 2021 at 6:49 AM Luca Bocassi <luca.boccassi@gmail.com> wrote:
> > >
> > > Having a look at the patches, a few comments:
> > >
> > > - 0012-don-t-pass-AT_SYMLINK_NOFOLLOW-flag-to-faccessat.patch I find
> > > quite worrying, as it fundamentally changes access patterns, some of
> > > which are done for security reasons. At best, this will cause
> > > completely different runtime behaviours for the same filesystem
> > > depending on the libc implementation, which doesn't sound great?
> >
> > I wrote a long and verbose comment when I created the patch which
> > tries to document any differences in runtime behaviour.
> >
> >   ----
> >   Avoid using AT_SYMLINK_NOFOLLOW flag. It doesn't seem like the right thing to
> >   do and it's not portable (not supported by musl). See:
> >
> >     http://lists.landley.net/pipermail/toybox-landley.net/2014-September/003610.html
> >     http://www.openwall.com/lists/musl/2015/02/05/2
> >
> >   Note that laccess() is never passing AT_EACCESS so a lot of the discussion in
> >   the links above doesn't apply. Note also that (currently) all systemd callers
> >   of laccess() pass mode as F_OK, so only check for existence of a file, not
> >   access permissions. Therefore, in this case, the only distiction between
> >   faccessat() with (flag == 0) and (flag == AT_SYMLINK_NOFOLLOW) is the
> >   behaviour for broken symlinks; laccess() on a broken symlink will succeed
> >   with (flag == AT_SYMLINK_NOFOLLOW) and fail (flag == 0).
> >
> >   The laccess() macros was added to systemd some time ago and it's not clear if
> >   or why it needs to return success for broken symlinks. Maybe just historical
> >   and not actually necessary or desired behaviour?
> >   ----
> >
> > If that comment is now out of date or something is missing then please
> > send a patch to update it.
> >
> > However looking at this patch again now, it appears to have got broken
> > during a past rebase:
> >
> >   https://git.openembedded.org/openembedded-core/commit/?id=e8dd5a36bf2f1e645fb2ff15eb3b5e97c04776e6
> >
> > The upstream code changed from:
> >
> >   #define laccess(path, mode) faccessat(AT_FDCWD, (path), (mode),
> > AT_SYMLINK_NOFOLLOW)
> >
> > to
> >
> >   #define laccess(path, mode)                                             \
> >           (faccessat(AT_FDCWD, (path), (mode), AT_SYMLINK_NOFOLLOW) <
> > 0 ? -errno : 0)
> >
> > but the replacement version in the patch still returns the raw result
> > from faccessat(). That looks like an issue.
>
> If you think the flag is unnecessary (I don't, we use these for a
> reason, but that's not important right now), the correct action is to
> send a PR upstream to discuss removing it. Patching it out for one
> build case of many is just going to be a source of incompatibility and
> surprises for users, as the behaviour on the same filesystem changes
> depending on the build option. Having said that, I don't use musl so
> all of this is really not a problem for me, just providing some
> feedback as upstream maintainer, in case it can be useful.

I don't have any interest in systemd + musl anymore either. I did an
initial port as a proof of concept and sent patches to Khem off
list... and was somewhat surprised when they showed up some time later
in oe-core.

Note that there have been long discussions here previously about
whether OE should claim to support systemd + musl. The effort to
support it properly (including clarifying questions like this with
upstream as you suggest) doesn't seem huge but so far no one seems to
care enough about systemd + musl to do it. We rebase and tweak the
patches but guidance to potential users should still be "use at your
own risk".