Messages in this thread | | | Date | Tue, 27 Oct 2020 15:18:14 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH] module: use hidden visibility for weak symbol references |
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One nit about ".got" in the message:
Reviewed-by: Fangrui Song <maskray@google.com>
On 2020-10-27, Nick Desaulniers wrote: >+ 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,
.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@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
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