Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 27 Oct 2020 11:27:33 -0700 | Subject | Re: [PATCH] module: use hidden visibility for weak symbol references |
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+ 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
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