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From:   =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= <maskray@google.com>
Date:   Mon, 10 Feb 2020 10:41:29 -0800
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Subject: Re: [PATCH] powerpc/vdso32: mark __kernel_datapage_offset as STV_PROTECTED
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
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On Mon, Feb 10, 2020 at 3:01 AM Michael Ellerman <mpe@ellerman.id.au> wrote=
:
>
> Fangrui Song <maskray@google.com> writes:
> > A PC-relative relocation (R_PPC_REL16_LO in this case) referencing a
> > preemptible symbol in a -shared link is not allowed.  GNU ld's powerpc
> > port is permissive and allows it [1], but lld will report an error afte=
r
> > https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git/commi=
t/?id=3Dec0895f08f99515194e9fcfe1338becf6f759d38
> >
> > Make the symbol protected so that it is non-preemptible but still
> > exported.
>
> "preemptible" means something different to me, and I assume we're not
> using it to mean the same thing.
>
> Can you explain it using small words that a kernel developer can
> understand? :)
>
> cheers

The term used in the ELF specification is "preemptable". I heard from
Roland McGrathr that "preemptable" was a typo. The correct term is
"preemptible".
On a random article I found, it mentions that "preemptible" is used
more than "preemptable". So now I stick with "preemptible".

The word is overloaded and has a different meaning in the kernel, but
here we refer to within the ELF binary format context.

From http://www.sco.com/developers/gabi/latest/ch4.symtab.html
"The visibility of symbols with the STV_DEFAULT attribute is as
specified by the symbol's binding type. That is, global and weak
symbols are visible outside of their defining component (executable
file or shared object). Local symbols are hidden, as described below.
Global and weak symbols are also preemptable, that is, they may by
preempted by definitions of the same name in another component."

__kernel_datapage_offset is a STB_GLOBAL STV_DEFAULT symbol. In a
-shared link, it is considered preemptible. There are some methods
that make such symbols non-preemptible but none is used in this
context.

* -Bsymbolic
* -Bsymbolic-functions if STT_FUNC
* --dynamic-list is specified but the dynamic list does not name this symbo=
l
* A --version-script makes the symbol local

__kernel_datapage_offset is accessed via some mechanism similar to
dlsym, so it has to be exported.

Given all the above, I chose STV_PROTECTED, which is the simplest and
least intrusive approach.

> > [1]: https://sourceware.org/bugzilla/show_bug.cgi?id=3D25500
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/851
> > Signed-off-by: Fangrui Song <maskray@google.com>
>
> > ---
> >  arch/powerpc/kernel/vdso32/datapage.S | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/kernel/vdso32/datapage.S b/arch/powerpc/kerne=
l/vdso32/datapage.S
> > index 217bb630f8f9..2831a8676365 100644
> > --- a/arch/powerpc/kernel/vdso32/datapage.S
> > +++ b/arch/powerpc/kernel/vdso32/datapage.S
> > @@ -13,7 +13,8 @@
> >  #include <asm/vdso_datapage.h>
> >
> >       .text
> > -     .global __kernel_datapage_offset;
> > +     .global __kernel_datapage_offset
> > +     .protected      __kernel_datapage_offset
> >  __kernel_datapage_offset:
> >       .long   0
> >
> > --
> > 2.25.0.341.g760bfbb309-goog


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