[PATCH v3 1/3] arm64: unwind: add asynchronous unwind tables to kernel and modules

[Ard Biesheuvel]

On Wed, 15 Jun 2022 at 18:50, Sami Tolvanen <samitolvanen at google.com> wrote:
>
> On Mon, Jun 13, 2022 at 03:40:06PM +0200, Ard Biesheuvel wrote:
> > Enable asynchronous unwind table generation for both the core kernel as
> > well as modules, and emit the resulting .eh_frame sections as init code
> > so we can use the unwind directives for code patching at boot or module
> > load time.
> >
> > This will be used by dynamic shadow call stack support, which will rely
> > on code patching rather than compiler codegen to emit the shadow call
> > stack push and pop instructions.
> >
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>
> > ---
> >  arch/arm64/Kconfig                    |  3 +++
> >  arch/arm64/Makefile                   |  5 +++++
> >  arch/arm64/include/asm/module.lds.h   |  8 ++++++++
> >  arch/arm64/kernel/vmlinux.lds.S       | 13 +++++++++++++
> >  arch/arm64/kvm/hyp/nvhe/Makefile      |  1 +
> >  drivers/firmware/efi/libstub/Makefile |  1 +
> >  6 files changed, 31 insertions(+)
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 1652a9800ebe..5f92344edff5 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -366,6 +366,9 @@ config KASAN_SHADOW_OFFSET
> >       default 0xeffffff800000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS
> >       default 0xffffffffffffffff
> >
> > +config UNWIND_TABLES
> > +     bool
> > +
> >  source "arch/arm64/Kconfig.platforms"
> >
> >  menu "Kernel Features"
> > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> > index 6d9d4a58b898..4fbca56fa602 100644
> > --- a/arch/arm64/Makefile
> > +++ b/arch/arm64/Makefile
> > @@ -45,8 +45,13 @@ KBUILD_CFLAGS      += $(call cc-option,-mabi=lp64)
> >  KBUILD_AFLAGS        += $(call cc-option,-mabi=lp64)
> >
> >  # Avoid generating .eh_frame* sections.
> > +ifneq ($(CONFIG_UNWIND_TABLES),y)
> >  KBUILD_CFLAGS        += -fno-asynchronous-unwind-tables -fno-unwind-tables
> >  KBUILD_AFLAGS        += -fno-asynchronous-unwind-tables -fno-unwind-tables
> > +else
> > +KBUILD_CFLAGS        += -fasynchronous-unwind-tables
> > +KBUILD_AFLAGS        += -fasynchronous-unwind-tables
> > +endif
> >
> >  ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> >  prepare: stack_protector_prepare
> > diff --git a/arch/arm64/include/asm/module.lds.h b/arch/arm64/include/asm/module.lds.h
> > index 094701ec5500..dbba4b7559aa 100644
> > --- a/arch/arm64/include/asm/module.lds.h
> > +++ b/arch/arm64/include/asm/module.lds.h
> > @@ -17,4 +17,12 @@ SECTIONS {
> >        */
> >       .text.hot : { *(.text.hot) }
> >  #endif
> > +
> > +#ifdef CONFIG_UNWIND_TABLES
> > +     /*
> > +      * Currently, we only use unwind info at module load time, so we can
> > +      * put it into the .init allocation.
> > +      */
> > +     .init.eh_frame : { *(.eh_frame) }
> > +#endif
> >  }
> > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> > index 2d4a8f995175..7bf4809f523d 100644
> > --- a/arch/arm64/kernel/vmlinux.lds.S
> > +++ b/arch/arm64/kernel/vmlinux.lds.S
> > @@ -120,6 +120,17 @@ jiffies = jiffies_64;
> >  #define TRAMP_TEXT
> >  #endif
> >
> > +#ifdef CONFIG_UNWIND_TABLES
> > +#define UNWIND_DATA_SECTIONS                         \
> > +     .eh_frame : {                                   \
> > +             __eh_frame_start = .;                   \
> > +             *(.eh_frame)                            \
> > +             __eh_frame_end = .;                     \
> > +     }
> > +#else
> > +#define UNWIND_DATA_SECTIONS
> > +#endif
>
> How does this work with SANITIZER_DISCARDS dropping .eh_frame in
> include/asm-generic/vmlinux.lds.h and scripts/module.lds.S? We would
> definitely want to enable this together with CONFIG_CFI_CLANG, so it
> seems like we'd have to drop the discard rules as well.
>

Good point, I had no idea that that existed.

Clang 13 should have the fix for the original issue, so we could make
this workaround specific to 12 and earlier.