Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime relocations

[Ard Biesheuvel]

On Wed, 1 Jul 2020 at 01:28, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> On Tue, Jun 30, 2020 at 03:00:43PM -0700, Fangrui Song wrote:
> > * Ard Biesheuvel
> > > On Tue, 30 Jun 2020 at 01:34, Fangrui Song <maskray@xxxxxxxxxx> wrote:
> >
> > If the executable is purely static, it does not need to have PLT. All
> > calls to a PLT can be redirected to the function itself.  Some range
> > extension thunks (other terms: stub groups, veneers, etc) may still be
> > needed if the distance is too large.
> >
> > There are cases where a GOT cannot be avoided, e.g.
> >
> > extern char foo[] __attribute__((weak, visibility("hidden")));
> > char *fun() { return foo; }
> >
> > If foo is a SHN_ABS, `movq foo@GOTPCREL(%rip), %rax` can't be optimized
> > by GOTPCRELX (https://sourceware.org/bugzilla/show_bug.cgi?id=25749 binutils>=2.35 will be good)
> > Many other architectures don't even have a GOT optimization.
>
> Urk -- the example given in that bug report isn't even weak. Are you
> guys proposing to pessimize every access to a global symbol, regardless
> of visibility, by going through the GOT on the off chance that somebody
> might define one of them as SHN_ABS? Can we at least gate it behind
> something like __attribute__((might_be_shn_abs))?
>

SHN_ABS is typically only used for constants emitted by the linker
script, so I don't think this is a huge deal.

The example above is not that different from having a statically
initialized function pointer in your object (which might be NULL), and
that is something we already have to deal with anyway.

What I was talking about is the tendency of the compiler to assume
that every function symbol with external linkage is preemptible, and
the only way to suppress this behavior is to issue a #pragma that can
be done in code only, not on the compiler command line.

> >
> > >I don't think it's really relevant for the kernel build -- all we get is
> > >ld -static --no-dynamic-linker, all -static does is prevent searching
> > >shared libraries, and we already pass --no-dynamic-linker if it's
> > >supported.
> > >
> > >[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81498