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[PATCH] elf: Don't load archive element after dynamic definition

H.J. Lu hjl.tools@gmail.com
Thu Sep 3 02:16:14 GMT 2020 
On Wed, Sep 2, 2020 at 6:31 PM Alan Modra <amodra@gmail.com> wrote:
>
> On Wed, Sep 02, 2020 at 07:35:58AM -0700, H.J. Lu wrote:
> > On Wed, Sep 2, 2020 at 7:29 AM Alan Modra <amodra@gmail.com> wrote:
> > >
> > > On Wed, Sep 02, 2020 at 06:22:08AM -0700, H.J. Lu wrote:
> > > > > It's reasonably obvious that we need to load archive elements when
> > > > > they define IR referenced symbols, because the archive element might
> > > > > be an LTO object.  What's not so obvious is whether loading of shared
> > > > > libraries should follow the same rule.  I think they should, in order
> > > > > for LTO to get symbol resolution correct in corner cases.  Basically
> > > > > LTO needs to know what shared libraries are loaded before
> > > > > recompilation.  See commit a896df97b952 log comments.
> > > >
> > > > There is dynamic_def for this purpose.
> > >
> > > Your patch doesn't make changes to ld/plugin.c to inform LTO of the
> > > availability of these symbols.  And if you did, then how does the
> > > linker work out whether or not the LTO recompilation depended on those
> > > symbols?  If it did change LTO recompilation then you had better
> > > ensure the library really is loaded.  By the time you work all of that
> > > out, if it is even possible, your patch will likely be very
> > > complicated indeed.
> >
> > A testcase?
>
> What don't you understand from my emails in this thread?  I suggest
> you look at what happened with the testcase in PR26314, in regard to
> my comment
>     The lto recompilation didn't see symbol references from libbfd.so and
>     variables like _xexit_cleanup are made local in the recompiled
>     objects.  Oops, two copies of them.

A testcase?

> That's critical in understanding why LTO needs to know about symbols
> from shared libraries before LTO recompilation.
>

Linker is complicated and LTO makes it even more complicated.
Without a testcase, we can't be sure that the bug is really fixed
nor we can be sure it won't regress.


-- 
H.J.
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