Messages in this thread | | | Date | Wed, 28 Oct 2020 14:07:48 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] module: use hidden visibility for weak symbol references |
| |
On Wed, Oct 28, 2020 at 03:03:44PM +0100, Jessica Yu wrote: > +++ 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>
Cheers, Jessica -- I'll queue them in a sec!
Will
| |