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

[Nick Desaulniers <ndesaulniers@google.com>]

+ Fangrui

On Tue, Oct 27, 2020 at 8:11 AM Ard Biesheuvel <ardb@kernel.org> 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@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  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