Re: [PATCH v5 0/8] unwind, arm64: add sframe unwinder for kernel

Hi Mostafa,

On Fri, May 15, 2026 at 4:32 AM Mostafa Saleh <[email protected]> wrote:
>
> On Tue, Apr 28, 2026 at 06:36:35PM +0000, Dylan Hatch wrote:
> > Implement a generic kernel sframe-based [1] unwinder. The main goal is
> > to improve reliable stacktrace on arm64 by unwinding across exception
> > boundaries.
> >
> > On x86, the ORC unwinder provides reliable stacktrace through similar
> > methodology, but arm64 lacks the necessary support from objtool to
> > create ORC unwind tables.
> >
> > Currently, there's already a sframe unwinder proposed for userspace: [2].
> > To maintain common definitions and algorithms for sframe lookup, a
> > substantial portion of this patch series aims to refactor the sframe
> > lookup code to support both kernel and userspace sframe sections.
> >
> > Currently, only GNU Binutils support sframe. This series relies on the
> > Sframe V3 format, which is supported in binutils 2.46.
> >
> > These patches are based on Steven Rostedt's sframe/core branch [3],
> > which is and aggregation of existing work done for x86 sframe userspace
> > unwind, and contains [2]. This branch is, in turn, based on Linux
> > v7.0-rc3. This full series (applied to the sframe/core branch) is
> > available on github: [4].
> >
>
> Not sure if related, but after updating my toolchain
> (aarch64-linux-gnu-gcc (Debian 15.2.0-4) 15.2.0), I hit link errors:
> ld.lld: error: arch/arm64/kernel/vdso/vgettimeofday.o:(.sframe) is being placed in '.sframe'
> ld.lld: error: arch/arm64/kernel/vdso/vgetrandom.o:(.sframe) is being placed in '.sframe`

Previously when developing against the SFrame V2 format, I had fixed
these warnings with the VDSO Makefile change currently in this series:

diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index 7dec05dd33b7..c60ef921956f 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -38,7 +38,7 @@ ccflags-y += -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
 CC_FLAGS_REMOVE_VDSO := $(CC_FLAGS_FTRACE) -Os $(CC_FLAGS_SCS) \
                        $(RANDSTRUCT_CFLAGS) $(KSTACK_ERASE_CFLAGS) \
                        $(GCC_PLUGINS_CFLAGS) \
-                       $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) \
+                       $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) $(CC_FLAGS_SFRAME) \
                        -Wmissing-prototypes -Wmissing-declarations

 CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables

But the warnings seem to have returned after upgrading my toolchain,
possibly due to SFrame V3 or some confounding change in GCC. The
--gsframe in the assembler should be set to 'no' by default, so
perhaps GCC is providing an override --gsframe internally?

>
> I applied this series hoping that fix it, but it doesn't, so far I
> have this hack :
> diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S
> index 52314be29191..53bdf757ee44 100644
> --- a/arch/arm64/kernel/vdso/vdso.lds.S
> +++ b/arch/arm64/kernel/vdso/vdso.lds.S
> @@ -77,7 +77,7 @@ SECTIONS
>         /DISCARD/       : {
>                 *(.data .data.* .gnu.linkonce.d.* .sdata*)
>                 *(.bss .sbss .dynbss .dynsbss)
> -               *(.eh_frame .eh_frame_hdr)
> +               *(.eh_frame .eh_frame_hdr .sframe)
>         }
>  }
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 60c8c22fd3e4..759903acd6fc 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -1064,6 +1064,7 @@
>         /* ld.bfd warns about .gnu.version* even when not emitted */    \
>         *(.gnu.version*)                                                \
>         *(__tracepoint_check)                                           \
> +       *(.sframe)                                                      \
>
>  #define DISCARDS                                                       \
>         /DISCARD/ : {                                                   \

Since this series only handles kernel stacktrace, I believe it's
better to omit the .sframe section entirely in the case where only
ARCH_SUPPORTS_UNWIND_KERNEL_SFRAME is enabled. I think this hack may
work better for this purpose:

diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index c60ef921956f..29f802bfedb1 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -41,7 +41,7 @@ CC_FLAGS_REMOVE_VDSO := $(CC_FLAGS_FTRACE) -Os
$(CC_FLAGS_SCS) \
                        $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) $(CC_FLAGS_SFRAME) \
                        -Wmissing-prototypes -Wmissing-declarations

-CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables
+CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables
-Wa,--gsframe=no

 CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_REMOVE_VDSO)
 CFLAGS_REMOVE_vgetrandom.o = $(CC_FLAGS_REMOVE_VDSO)

Though, I don't understand why it is necessary to provide --gsframe=no
explicitly. If this approach seems ok to other folks/maintainers, I
can fold this into my series.

On the topic of SFrame for VDSO, Jens has a patch adding support for
this as part of a series to support userspace SFrame unwinding for
arm64:

https://lore.kernel.org/lkml/[email protected]/

>
>
> Thanks,
> Mostafa
>

Thanks,
Dylan