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

[Jessica Yu <jeyu@kernel.org>]

+++ Will Deacon [28/10/20 13:24 +0000]:
>On Wed, Oct 28, 2020 at 01:27:01PM +0100, Ard Biesheuvel wrote:
>> On Wed, 28 Oct 2020 at 11:00, Will Deacon <will@kernel.org> 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@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(-)
>> >
>> > 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).

Hi! Yes, please feel free to take this patch along with the other fix:

Acked-by: Jessica Yu <jeyu@kernel.org>

Thanks,

Jessica