Re: [PATCH] module: use hidden visibility for weak symbol references

[Will Deacon]

On Wed, Oct 28, 2020 at 01:27:01PM +0100, Ard Biesheuvel wrote:
> On Wed, 28 Oct 2020 at 11:00, Will Deacon <will@xxxxxxxxxx> wrote:
> > On Tue, Oct 27, 2020 at 04:11:32PM +0100, Ard Biesheuvel wrote:
> > > Geert reports that commit be2881824ae9eb92 ("arm64/build: Assert for
> > > unwanted sections") results in build errors on arm64 for configurations
> > > that have CONFIG_MODULES disabled.
> > >
> > > The commit in question added ASSERT()s to the arm64 linker script to
> > > ensure that linker generated sections such as .got, .plt etc are empty,
> > > but as it turns out, there are corner cases where the linker does emit
> > > content into those sections. More specifically, weak references to
> > > function symbols (which can remain unsatisfied, and can therefore not
> > > be emitted as relative references) will be emitted as GOT and PLT
> > > entries when linking the kernel in PIE mode (which is the case when
> > > CONFIG_RELOCATABLE is enabled, which is on by default).
> > >
> > > What happens is that code such as
> > >
> > >       struct device *(*fn)(struct device *dev);
> > >       struct device *iommu_device;
> > >
> > >       fn = symbol_get(mdev_get_iommu_device);
> > >       if (fn) {
> > >               iommu_device = fn(dev);
> > >
> > > essentially gets converted into the following when CONFIG_MODULES is off:
> > >
> > >       struct device *iommu_device;
> > >
> > >       if (&mdev_get_iommu_device) {
> > >               iommu_device = mdev_get_iommu_device(dev);
> > >
> > > where mdev_get_iommu_device is emitted as a weak symbol reference into
> > > the object file. The first reference is decorated with an ordinary
> > > ABS64 data relocation (which yields 0x0 if the reference remains
> > > unsatisfied). However, the indirect call is turned into a direct call
> > > covered by a R_AARCH64_CALL26 relocation, which is converted into a
> > > call via a PLT entry taking the target address from the associated
> > > GOT entry.
> > >
> > > Given that such GOT and PLT entries are unnecessary for fully linked
> > > binaries such as the kernel, let's give these weak symbol references
> > > hidden visibility, so that the linker knows that the weak reference
> > > via R_AARCH64_CALL26 can simply remain unsatisfied.
> > >
> > > Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > > Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > > Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > ---
> > >  include/linux/module.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > Cheers. I gave this a spin, but I unfortunately still see the following
> > linker warning with allnoconfig:
> >
> >   aarch64-linux-gnu-ld: warning: orphan section `.igot.plt' from `arch/arm64/kernel/head.o' being placed in section `.igot.plt'
> >
> > which looks unrelated to symbol_get(), but maybe it's worth knocking these
> > things on the head (no pun intended) at the same time?
> >
> 
> Yeah, that is just one of those spurious sections that turns up empty
> anyway. The head.o is a red herring, it is simply the first file
> appearing in the link.
> 
> This should fix it
> 
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 6567d80dd15f..48b222f1c700 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -278,7 +278,7 @@ SECTIONS
>          * explicitly check instead of blindly discarding.
>          */
>         .plt : {
> -               *(.plt) *(.plt.*) *(.iplt) *(.igot)
> +               *(.plt) *(.plt.*) *(.iplt) *(.igot .igot.plt)
>         }
>         ASSERT(SIZEOF(.plt) == 0, "Unexpected run-time procedure
> linkages detected!")

Cheers, that fixes the extra warning for me. If you could send a proper
patch, I'm happy to queue as an arm64 fix! (I'm assuming the former is going
via Jessica, but I can also take that with her Ack).

Will