Messages in this thread Patch in this message | | | Date | Wed, 28 Oct 2020 15:03:44 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH] module: use hidden visibility for weak symbol references |
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+++ 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
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