From: Ard Biesheuvel <ardb@kernel.org> To: linux-kernel@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org, will@kernel.org, catalin.marinas@arm.com, Ard Biesheuvel <ardb@kernel.org>, Jessica Yu <jeyu@kernel.org>, Kees Cook <keescook@chromium.org>, Geert Uytterhoeven <geert@linux-m68k.org>, Nick Desaulniers <ndesaulniers@google.com> Subject: [PATCH] module: use hidden visibility for weak symbol references Date: Tue, 27 Oct 2020 16:11:32 +0100 [thread overview] Message-ID: <20201027151132.14066-1-ardb@kernel.org> (raw) 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
WARNING: multiple messages have this Message-ID (diff)
From: Ard Biesheuvel <ardb@kernel.org> To: linux-kernel@vger.kernel.org Cc: Kees Cook <keescook@chromium.org>, catalin.marinas@arm.com, Nick Desaulniers <ndesaulniers@google.com>, Geert Uytterhoeven <geert@linux-m68k.org>, Jessica Yu <jeyu@kernel.org>, will@kernel.org, Ard Biesheuvel <ardb@kernel.org>, linux-arm-kernel@lists.infradead.org Subject: [PATCH] module: use hidden visibility for weak symbol references Date: Tue, 27 Oct 2020 16:11:32 +0100 [thread overview] Message-ID: <20201027151132.14066-1-ardb@kernel.org> (raw) 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next reply other threads:[~2020-10-27 15:43 UTC|newest] Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-10-27 15:11 Ard Biesheuvel [this message] 2020-10-27 15:11 ` [PATCH] module: use hidden visibility for weak symbol references Ard Biesheuvel 2020-10-27 17:55 ` Geert Uytterhoeven 2020-10-27 17:55 ` Geert Uytterhoeven 2020-10-27 18:27 ` Nick Desaulniers 2020-10-27 18:27 ` Nick Desaulniers 2020-10-27 22:18 ` Fangrui Song 2020-10-27 22:18 ` Fangrui Song 2020-10-28 10:00 ` Will Deacon 2020-10-28 10:00 ` Will Deacon 2020-10-28 12:27 ` Ard Biesheuvel 2020-10-28 12:27 ` Ard Biesheuvel 2020-10-28 13:24 ` Will Deacon 2020-10-28 13:24 ` Will Deacon 2020-10-28 14:03 ` Jessica Yu 2020-10-28 14:03 ` Jessica Yu 2020-10-28 14:07 ` Will Deacon 2020-10-28 14:07 ` Will Deacon 2020-10-28 15:12 ` Will Deacon 2020-10-28 15:12 ` Will Deacon
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