Re: [Acl-devel] libattr 2.4.48 change in syscalls.c breaks fakechroot

[Andreas Grünbacher]

Am Mi., 15. Aug. 2018 um 22:16 Uhr schrieb Dmitry V. Levin <address@hidden>:
> On Wed, Aug 15, 2018 at 09:50:14PM +0200, Andreas Grünbacher wrote:
> > Am Mi., 15. Aug. 2018 um 18:36 Uhr schrieb Filipe Brandenburger 
> > <address@hidden>:
> > > On Tue, Aug 14, 2018 at 11:00 AM Dmitry V. Levin <address@hidden> wrote:
> > > > On Mon, Aug 13, 2018 at 10:12:57PM -0700, Filipe Brandenburger wrote:
> > > > > See here for more details:
> > > > > https://github.com/dex4er/fakechroot/issues/57
> > > >
> > > > I suppose fakechroot has to be fixed to remain useful.
> > >
> > > I'm proposing using explicit symbol versions (dlvsym) in fakechroot to 
> > > fix this:
> > > https://github.com/dex4er/fakechroot/pull/58
> > >
> > > > > Any chance you'd consider going back to an implementation directly
> > > > > using SYSCALL(...) in libattr?
> > > >
> > > > I believe commit v2.4.48~17 was the right change in this respect.
> > > > Note that software has been using glibc functions to access acl syscalls
> > > > for quite a long time, so changing libacl won't help fakechroot to 
> > > > handle
> > > > that software.
> > >
> > > I somewhat agree, but the way v2.4.48~11 reintroduces them is not
> > > ideal, since it interacts with LD_PRELOAD and dlsym() in unexpected
> > > ways. (Again, I'm not saying libattr is at fault here, just that
> > > LD_PRELOAD and dlsym is a somewhat incomplete API and leads to this
> > > kind of issues.)
> > >
> > > In a perfect world (or, at least, slightly better than this own),
> > > instead of libattr exporting wrappers (like libattr_lsetxattr) which
> > > call the glibc function (lsetxattr), I think it would be good if they
> > > could import the glibc symbol (address@hidden) and re-export that
> > > (as address@hidden, non-default symbol.)
> > >
> > > I don't really know if it's even possible to do this with linker
> > > scripts or, if it is, how to do that... But I think that would be an
> > > improvement.
> >
> > Hmm, commit efa0b1ea (v2.4.48~11) was not such a good idea.
> >
> > I wonder if there's a way to leave older versions of a symbol around
> > in a library, but to un-export it in the default version so that
> > programs newly built against the library will skip it. I couldn't
> > figure out a way to do that though.
>
> This is exactly what commit v2.4.48~11 does: it brings versioned symbols
> back but does not allow to link with them.  As result, every program
> or library linked with libattr >= v2.4.48~11 will automatically switch
> to *attr symbols provided by glibc.

It does seem that fakeroot ends up calling the libattr_*xattr
trampoline functions.

> > Alternatively, we could add a second libattr with a new major version.
>
> Just to strip these compatibility symbols?  Does it worth the trouble
> caused by a soname change?

To fix something we've screwed up instead of forcing a second hack on
top of our hack into fakeroot?

Andreas