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

[Fangrui Song]

One nit about ".got" in the message:

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

On 2020-10-27, Nick Desaulniers wrote:
> + Fangrui
>
> On Tue, Oct 27, 2020 at 8:11 AM Ard Biesheuvel <ardb@xxxxxxxxxx> 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,

.got -> .got.plt

be2881824ae9eb92 does not ASSERT on .got (it can).

Strangely *(.got) is placed in .text in arch/arm64/kernel/vmlinux.lds.S
I think that line can be removed. On x86, aarch64 and many other archs,
the start of .got.plt is the GOT base. .got is not needed (ppc/arm/riscv
use .got instead of .got.plt as the GOT base anchor).

> > 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).

Confirmed.

> > 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.

Yes, the R_AARCH64_CALL26 relocation referencing an undefined weak
symbol causes one .plt entry and one .got.plt 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(-)
> >
> > diff --git a/include/linux/module.h b/include/linux/module.h
> > index 7ccdf87f376f..6264617bab4d 100644
> > --- a/include/linux/module.h
> > +++ b/include/linux/module.h
> > @@ -740,7 +740,7 @@ static inline bool within_module(unsigned long addr, const struct module *mod)
> >  }
> >
> >  /* Get/put a kernel symbol (calls should be symmetric) */
> > -#define symbol_get(x) ({ extern typeof(x) x __attribute__((weak)); &(x); })
> > +#define symbol_get(x) ({ extern typeof(x) x __attribute__((weak,visibility("hidden"))); &(x); })
> >  #define symbol_put(x) do { } while (0)
> >  #define symbol_put_addr(x) do { } while (0)
> >
> > --
> > 2.17.1
>
>
> --
> Thanks,
> ~Nick Desaulniers