Re: [PATCH] AArch64: Add support for -mdirect-extern-access

[Fangrui Song <maskray@google.com>]

On Thu, Nov 17, 2022 at 1:37 PM Andrew Pinski <pinskia@gmail.com> wrote:
>
> On Thu, Nov 17, 2022 at 1:21 PM maskray--- via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > > +.. option:: -mdirect-extern-access, -mno-direct-extern-access
> > > +
> > > +  Use direct accesses for external data symbols.  It avoids a GOT indirection
> > > +  on all external data symbols with :option:`-fpie` or :option:`-fPIE`.  This is
> > > +  useful for executables linked with :option:`-static` or :option:`-static-pie`.
> > > +  With :option:`-fpic` or :option:`-fPIC`, it only affects accesses to protected
> > > +  data symbols.  It has no effect on non-position independent code.  The default
> > > +  is :option:`-mno-direct-extern-access`.
> > > +
> > > +  .. warning::
> > > +
> > > +    Use :option:`-mdirect-extern-access` either in shared libraries or in
> > > +    executables, but not in both.  Protected symbols used both in a shared
> > > +    library and executable may cause linker errors or fail to work correctly.
> >
> > I think current GCC and Clang's behavior is:
> >
> > * -mdirect-extern-access is the default for -fno-pic. This is to enable optimizations for -static programs but may introduce copy relocations.
> > * -mno-direct-extern-access is the default for -fpie and -fpic. This uses some GOT-generating relocations which can be optimized out (lld, see https://maskray.me/blog/2021-08-29-all-about-global-offset-table) but the instruction is nevertheless slightly longer.
> >
> > (-mdirect-extern-access for -fpic probably doesn't make sense.)
> >
> > The option I introduced to Clang is -fdirect-access-external-data
> > (see https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected).
> > If -mdirect-extern-access gets more popular, I can add a Clang alias.
> > But I am opposed to forcing a GNU property for -mdirect-extern-access/-mno-direct-extern-access.
> >
> > FWIW I used https://gist.github.com/MaskRay/c03a90922003df666551589f1629df22 to test my Clang changes related to -fno-semantic-interposition
> > on various visibility attributes x non-weak/weak x nopic/pie/pic x dllimport/not x ...
>
>
> The x86_64 discussion about this is here
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112 .
> I think clang changing the ABI is just broken and should think twice
> before we do it for GCC.
>
> And there is a lot of visibility protected issues filed in GCC bug
> databases specifically about copy relocs too.
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56527
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37611
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19520
> https://sourceware.org/bugzilla/show_bug.cgi?id=28875
> https://sourceware.org/bugzilla/show_bug.cgi?id=28877
> I also suspect clang's behavior is still broken too.
>
> Thanks,
> Andrew

Well, I don't think Clang changed ABI regarding -fno-pic/-fpie/-fpic.
As I did archaeology on
https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected
"Reflection on protected data symbols and copy relocations"
GCC 5 x86-64 made a change and GCC aarch64 accidentally picked up the change.

"""
On the GCC side, in -fpic mode, using GOT-generating relocations when
accessing a protected variable subverts the point using the protected
visibility. The unneeded pessimization is the foremost complaint. The
pessimization applies to all ports with #define TARGET_BINDS_LOCAL_P
default_binds_local_p_2. aarch64 moved to default_binds_local_p_2
accidentally by
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cbddf64c0243816b45e6680754a251c603245dbc.

For GCC<5 (and all versions of Clang), direct accesses to protected
variables are produced in -fpic code. Mixing such object files can
still silently break copy relocations on protected data symbols.
Therefore, GNU ld made the controversial change
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=ca3fe95e469b9daec153caa2c90665f5daaec2b5
to error in -shared mode.
"""


> >
> > On 2022-11-17, Ramana Radhakrishnan wrote:
> > >On Thu, Nov 17, 2022 at 5:30 PM Richard Sandiford via Gcc-patches
> > ><gcc-patches@gcc.gnu.org> wrote:
> > >>
> > >> Wilco Dijkstra <Wilco.Dijkstra@arm.com> writes:
> > >> > Hi Richard,
> > >> >
> > >> >> Can you go into more detail about:
> > >> >>
> > >> >>    Use :option:`-mdirect-extern-access` either in shared libraries or in
> > >> >>    executables, but not in both.  Protected symbols used both in a shared
> > >> >>    library and executable may cause linker errors or fail to work correctly
> > >> >>
> > >> >> If this is LLVM's default for PIC (and by assumption shared libraries),
> > >> >> is it then invalid to use -mdirect-extern-access for any PIEs that
> > >> >> are linked against those shared libraries and use protected symbols
> > >> >> from those libraries?  How would a user know that one of the shared
> > >> >> libraries they're linking against was built in this way?
> > >> >
> > >> > Yes, the usage model is that you'd either use it for static PIE or only on
> > >> > data that is not shared. If you get it wrong them you'll get the copy
> > >> > relocation error.
> > >>
> > >> Thanks.  I think I'm still missing something though.  If, for the
> > >> non-executable case, people should only use the feature on data that
> > >> is not shared, why do we need to relax the binds-local condition for
> > >> protected symbols on -fPIC?  Oughtn't the symbol to be hidden rather
> > >> than protected if the data isn't shared?
> > >>
> > >> I can understand the reasoning for the PIE changes but I'm still
> > >> struggling with the PIC-but-not-PIE bits.
> > >
> > >I think I'm with Richard S on hidden vs protected on first reading. I
> > >can see why this works out of the box and can even be default for
> > >static-pie.
> > >
> > >Any reason why this is not on by default - it's early enough in the
> > >stage3 cycle and we can always flip the defaults if there are more
> > >problems found.
> > >
> > >You probably need a rebase for the documentation bits,.
> > >
> > >regards
> > >Ramana
> > >
> > >
> > >Ramana
> >
> >
> > +  is :option:`-mno-direct-extern-access`.



-- 
宋方睿