| Message ID | 20170903120757.14968-20-ard.biesheuvel@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Sun, 3 Sep 2017, Ard Biesheuvel wrote: > Update the build flags and linker script to allow vmlinux to be built > as a PIE binary, which retains relocation information about absolute > symbol references so that they can be fixed up at runtime. This will > be used for implementing KASLR, > > Cc: Russell King <linux@armlinux.org.uk> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> > --- > arch/arm/Kconfig | 4 ++++ > arch/arm/Makefile | 5 +++++ > arch/arm/include/asm/assembler.h | 2 +- > arch/arm/kernel/vmlinux.lds.S | 9 +++++++++ > include/linux/hidden.h | 20 ++++++++++++++++++++ > scripts/module-common.lds | 1 + > 6 files changed, 40 insertions(+), 1 deletion(-) > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig > index 61a0cb15067e..300add3b8023 100644 > --- a/arch/arm/Kconfig > +++ b/arch/arm/Kconfig > @@ -2085,6 +2085,10 @@ config DMI > firmware need to be enabled. This would require the DMI subsystem > to be enabled much earlier than we do on ARM, which is non-trivial. > > +config RELOCATABLE > + bool > + select HAVE_ARCH_PREL32_RELOCATIONS > + You should probably make it depend on !XIP_KERNEL. Other than that Acked-by: Nicolas Pitre <nico@linaro.org> > > menu "CPU Power Management" > diff --git a/arch/arm/Makefile b/arch/arm/Makefile > index 47d3a1ab08d2..8544c8148b9f 100644 > --- a/arch/arm/Makefile > +++ b/arch/arm/Makefile > @@ -52,6 +52,11 @@ AS += -EL > LD += -EL > endif > > +ifeq ($(CONFIG_RELOCATABLE),y) > +KBUILD_CFLAGS += -fpic -include $(srctree)/include/linux/hidden.h > +LDFLAGS_vmlinux += -pie -shared -Bsymbolic > +endif > + > # > # The Scalar Replacement of Aggregates (SRA) optimization pass in GCC 4.9 and > # later may result in code being generated that handles signed short and signed > diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h > index ae43859a9000..0586e1c77a80 100644 > --- a/arch/arm/include/asm/assembler.h > +++ b/arch/arm/include/asm/assembler.h > @@ -555,7 +555,7 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) > * mov_l - move a constant value or [relocated] address into a register > */ > .macro mov_l, dst:req, imm:req > - .if __LINUX_ARM_ARCH__ < 7 > + .if CONFIG_RELOCATABLE == 1 || __LINUX_ARM_ARCH__ < 7 > ldr \dst, =\imm > .else > movw \dst, #:lower16:\imm > diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S > index c83a7ba737d6..5853d4be2067 100644 > --- a/arch/arm/kernel/vmlinux.lds.S > +++ b/arch/arm/kernel/vmlinux.lds.S > @@ -89,6 +89,9 @@ SECTIONS > #endif > *(.discard) > *(.discard.*) > + *(.ARM.exidx.discard.text) > + *(.interp .dynamic) > + *(.dynsym .dynstr .hash) > } > > . = PAGE_OFFSET + TEXT_OFFSET; > @@ -209,6 +212,12 @@ SECTIONS > __smpalt_end = .; > } > #endif > + .rel.dyn : ALIGN(8) { > + __rel_begin = .; > + *(.rel .rel.* .rel.dyn) > + } > + __rel_end = ADDR(.rel.dyn) + SIZEOF(.rel.dyn); > + > .init.pv_table : { > __pv_table_begin = .; > *(.pv_table) > diff --git a/include/linux/hidden.h b/include/linux/hidden.h > new file mode 100644 > index 000000000000..781fb6f0c742 > --- /dev/null > +++ b/include/linux/hidden.h > @@ -0,0 +1,20 @@ > +/* > + * GCC assumes that we are building shared libraries or hosted binaries > + * when the -fpic or -fpie switches are used. This results in all references > + * to symbols with external linkage to be redirected via entries in the global > + * offset table (GOT), which keeps .text pages clean and reduces the footprint > + * of CoWed dirty pages to the GOT itself. It also allows symbol preemption, > + * which is mandatory under ELF rules for shared libraries. > + * > + * For the kernel, we use PIC so that we can relocate the executable image at > + * runtime. This does not involve CoW or symbol preemption, and we'd rather > + * have relative references instead of absolute ones whenever possible. > + * So set the default visibility to hidden: this informs the compiler that > + * none of our symbols will ever be exported from a shared library, allowing > + * it to use relative references where possible. > + * > + * Note that simply passing -fvisibility=hidden is not sufficient to achieve > + * this: In that case, definitions will be marked hidden, but declarations > + * will not, and we still end up with GOT entries unnecessarily. > + */ > +#pragma GCC visibility push(hidden) > diff --git a/scripts/module-common.lds b/scripts/module-common.lds > index d61b9e8678e8..55bb41f32c8a 100644 > --- a/scripts/module-common.lds > +++ b/scripts/module-common.lds > @@ -7,6 +7,7 @@ SECTIONS { > /DISCARD/ : { > *(.discard) > *(.discard.*) > + *(*.discard.*) > } > > __ksymtab 0 : { *(SORT(___ksymtab+*)) } > -- > 2.11.0 > >
On 4 September 2017 at 19:11, Nicolas Pitre <nicolas.pitre@linaro.org> wrote: > On Sun, 3 Sep 2017, Ard Biesheuvel wrote: > >> Update the build flags and linker script to allow vmlinux to be built >> as a PIE binary, which retains relocation information about absolute >> symbol references so that they can be fixed up at runtime. This will >> be used for implementing KASLR, >> >> Cc: Russell King <linux@armlinux.org.uk> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> >> --- >> arch/arm/Kconfig | 4 ++++ >> arch/arm/Makefile | 5 +++++ >> arch/arm/include/asm/assembler.h | 2 +- >> arch/arm/kernel/vmlinux.lds.S | 9 +++++++++ >> include/linux/hidden.h | 20 ++++++++++++++++++++ >> scripts/module-common.lds | 1 + >> 6 files changed, 40 insertions(+), 1 deletion(-) >> >> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig >> index 61a0cb15067e..300add3b8023 100644 >> --- a/arch/arm/Kconfig >> +++ b/arch/arm/Kconfig >> @@ -2085,6 +2085,10 @@ config DMI >> firmware need to be enabled. This would require the DMI subsystem >> to be enabled much earlier than we do on ARM, which is non-trivial. >> >> +config RELOCATABLE >> + bool >> + select HAVE_ARCH_PREL32_RELOCATIONS >> + > > You should probably make it depend on !XIP_KERNEL. > Indeed. > Other than that > > Acked-by: Nicolas Pitre <nico@linaro.org> > Thanks.
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 61a0cb15067e..300add3b8023 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig @@ -2085,6 +2085,10 @@ config DMI firmware need to be enabled. This would require the DMI subsystem to be enabled much earlier than we do on ARM, which is non-trivial. +config RELOCATABLE + bool + select HAVE_ARCH_PREL32_RELOCATIONS + endmenu menu "CPU Power Management" diff --git a/arch/arm/Makefile b/arch/arm/Makefile index 47d3a1ab08d2..8544c8148b9f 100644 --- a/arch/arm/Makefile +++ b/arch/arm/Makefile @@ -52,6 +52,11 @@ AS += -EL LD += -EL endif +ifeq ($(CONFIG_RELOCATABLE),y) +KBUILD_CFLAGS += -fpic -include $(srctree)/include/linux/hidden.h +LDFLAGS_vmlinux += -pie -shared -Bsymbolic +endif + # # The Scalar Replacement of Aggregates (SRA) optimization pass in GCC 4.9 and # later may result in code being generated that handles signed short and signed diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h index ae43859a9000..0586e1c77a80 100644 --- a/arch/arm/include/asm/assembler.h +++ b/arch/arm/include/asm/assembler.h @@ -555,7 +555,7 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) * mov_l - move a constant value or [relocated] address into a register */ .macro mov_l, dst:req, imm:req - .if __LINUX_ARM_ARCH__ < 7 + .if CONFIG_RELOCATABLE == 1 || __LINUX_ARM_ARCH__ < 7 ldr \dst, =\imm .else movw \dst, #:lower16:\imm diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S index c83a7ba737d6..5853d4be2067 100644 --- a/arch/arm/kernel/vmlinux.lds.S +++ b/arch/arm/kernel/vmlinux.lds.S @@ -89,6 +89,9 @@ SECTIONS #endif *(.discard) *(.discard.*) + *(.ARM.exidx.discard.text) + *(.interp .dynamic) + *(.dynsym .dynstr .hash) } . = PAGE_OFFSET + TEXT_OFFSET; @@ -209,6 +212,12 @@ SECTIONS __smpalt_end = .; } #endif + .rel.dyn : ALIGN(8) { + __rel_begin = .; + *(.rel .rel.* .rel.dyn) + } + __rel_end = ADDR(.rel.dyn) + SIZEOF(.rel.dyn); + .init.pv_table : { __pv_table_begin = .; *(.pv_table) diff --git a/include/linux/hidden.h b/include/linux/hidden.h new file mode 100644 index 000000000000..781fb6f0c742 --- /dev/null +++ b/include/linux/hidden.h @@ -0,0 +1,20 @@ +/* + * GCC assumes that we are building shared libraries or hosted binaries + * when the -fpic or -fpie switches are used. This results in all references + * to symbols with external linkage to be redirected via entries in the global + * offset table (GOT), which keeps .text pages clean and reduces the footprint + * of CoWed dirty pages to the GOT itself. It also allows symbol preemption, + * which is mandatory under ELF rules for shared libraries. + * + * For the kernel, we use PIC so that we can relocate the executable image at + * runtime. This does not involve CoW or symbol preemption, and we'd rather + * have relative references instead of absolute ones whenever possible. + * So set the default visibility to hidden: this informs the compiler that + * none of our symbols will ever be exported from a shared library, allowing + * it to use relative references where possible. + * + * Note that simply passing -fvisibility=hidden is not sufficient to achieve + * this: In that case, definitions will be marked hidden, but declarations + * will not, and we still end up with GOT entries unnecessarily. + */ +#pragma GCC visibility push(hidden) diff --git a/scripts/module-common.lds b/scripts/module-common.lds index d61b9e8678e8..55bb41f32c8a 100644 --- a/scripts/module-common.lds +++ b/scripts/module-common.lds @@ -7,6 +7,7 @@ SECTIONS { /DISCARD/ : { *(.discard) *(.discard.*) + *(*.discard.*) } __ksymtab 0 : { *(SORT(___ksymtab+*)) }
Update the build flags and linker script to allow vmlinux to be built as a PIE binary, which retains relocation information about absolute symbol references so that they can be fixed up at runtime. This will be used for implementing KASLR, Cc: Russell King <linux@armlinux.org.uk> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> --- arch/arm/Kconfig | 4 ++++ arch/arm/Makefile | 5 +++++ arch/arm/include/asm/assembler.h | 2 +- arch/arm/kernel/vmlinux.lds.S | 9 +++++++++ include/linux/hidden.h | 20 ++++++++++++++++++++ scripts/module-common.lds | 1 + 6 files changed, 40 insertions(+), 1 deletion(-)