[RFC PATCH v3 08/17] s390/vdso: Enable SFrame generation in vDSO

Jens Remus
2025-12-08T17:16:49Z

This replicates Josh's x86 patch "x86/vdso: Enable sframe generation
in VDSO" [1] for s390.

Test whether the assembler supports generating SFrame stack trace
information. Note that it is insufficient to test whether the assembler
supports option --gsframe, as GNU assembler supports that regardless
of whether it is actually capable of generating SFrame stack trace
information for the architecture.

If so enable SFrame stack trace information generation in the vDSO
library so kernel and user space can unwind through it.

[1]: x86/vdso: Enable sframe generation in VDSO,
https://lore.kernel.org/all/20250425024023.173709192@goodmis.org/

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
Changes in RFC v2:
- Introduce config option AS_SFRAME instead of requiring Josh's x86
patch as pre-requisite.
- Reword commit message.

Link to Josh's latest x86 patch:
https://lore.kernel.org/all/20250425024023.173709192@goodmis.org/

arch/Kconfig | 3 +++
arch/s390/include/asm/dwarf.h | 4 ++++
arch/s390/kernel/vdso64/Makefile | 7 ++++++-
arch/s390/kernel/vdso64/vdso64.lds.S | 9 +++++++++
4 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 06c4f909398c..7fa89d70b244 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -482,6 +482,9 @@ config HAVE_HARDLOCKUP_DETECTOR_ARCH
It uses the same command line parameters, and sysctl interface,
as the generic hardlockup detectors.

+config AS_SFRAME
+ def_bool $(as-instr,.cfi_sections .sframe\n.cfi_startproc\n.cfi_endproc)
+
config UNWIND_USER
bool

diff --git a/arch/s390/include/asm/dwarf.h b/arch/s390/include/asm/dwarf.h
index 6bcf37256feb..2f148b15fd7d 100644
--- a/arch/s390/include/asm/dwarf.h
+++ b/arch/s390/include/asm/dwarf.h
@@ -16,7 +16,11 @@
* symbols for the .dbg file.
*/

+#ifdef CONFIG_AS_SFRAME
+ .cfi_sections .eh_frame, .debug_frame, .sframe
+#else
.cfi_sections .eh_frame, .debug_frame
+#endif

#define CFI_STARTPROC .cfi_startproc
#define CFI_ENDPROC .cfi_endproc
diff --git a/arch/s390/kernel/vdso64/Makefile b/arch/s390/kernel/vdso64/Makefile
index 8e78dc3ba025..f597f3b863d7 100644
--- a/arch/s390/kernel/vdso64/Makefile
+++ b/arch/s390/kernel/vdso64/Makefile
@@ -20,7 +20,11 @@ targets := $(obj-vdso64) $(obj-cvdso64) vdso64.so vdso64.so.dbg
obj-vdso64 := $(addprefix $(obj)/, $(obj-vdso64))
obj-cvdso64 := $(addprefix $(obj)/, $(obj-cvdso64))

-KBUILD_AFLAGS += -DBUILD_VDSO
+ifeq ($(CONFIG_AS_SFRAME),y)
+ SFRAME_CFLAGS := -Wa,--gsframe
+endif
+
+KBUILD_AFLAGS += -DBUILD_VDSO $(SFRAME_CFLAGS)
KBUILD_CFLAGS += -DBUILD_VDSO -DDISABLE_BRANCH_PROFILING

KBUILD_AFLAGS_64 := $(filter-out -m64,$(KBUILD_AFLAGS))
@@ -32,6 +36,7 @@ KBUILD_CFLAGS_64 := $(filter-out -mno-pic-data-is-text-relative,$(KBUILD_CFLAGS_
KBUILD_CFLAGS_64 := $(filter-out -munaligned-symbols,$(KBUILD_CFLAGS_64))
KBUILD_CFLAGS_64 := $(filter-out -fno-asynchronous-unwind-tables,$(KBUILD_CFLAGS_64))
KBUILD_CFLAGS_64 += -m64 -fPIC -fno-common -fno-builtin -fasynchronous-unwind-tables
+KBUILD_CFLAGS_64 += $(SFRAME_CFLAGS)
ldflags-y := -shared -soname=linux-vdso64.so.1 \
--hash-style=both --build-id=sha1 -T

diff --git a/arch/s390/kernel/vdso64/vdso64.lds.S b/arch/s390/kernel/vdso64/vdso64.lds.S
index e4f6551ae898..0205d84369ca 100644
--- a/arch/s390/kernel/vdso64/vdso64.lds.S
+++ b/arch/s390/kernel/vdso64/vdso64.lds.S
@@ -50,6 +50,11 @@ SECTIONS

.eh_frame_hdr : { *(.eh_frame_hdr) } :text :eh_frame_hdr
.eh_frame : { KEEP (*(.eh_frame)) } :text
+
+#ifdef CONFIG_AS_SFRAME
+ .sframe : { *(.sframe) } :text :sframe
+#endif
+
.gcc_except_table : { *(.gcc_except_table .gcc_except_table.*) }

.rela.dyn ALIGN(8) : { *(.rela.dyn) }
@@ -114,6 +119,7 @@ SECTIONS
* Very old versions of ld do not recognize this name token; use the constant.
*/
#define PT_GNU_EH_FRAME 0x6474e550
+#define PT_GNU_SFRAME 0x6474e554

/*
* We must supply the ELF program headers explicitly to get just one
@@ -125,6 +131,9 @@ PHDRS
dynamic PT_DYNAMIC FLAGS(4); /* PF_R */
note PT_NOTE FLAGS(4); /* PF_R */
eh_frame_hdr PT_GNU_EH_FRAME;
+#ifdef CONFIG_AS_SFRAME
+ sframe PT_GNU_SFRAME;
+#endif
}

/*
--
2.51.0