[RFC PATCH v1 15/16] PREREQ: x86/vdso: Enable sframe generation in VDSO

[Jens Remus <jremus@linux.ibm.com>]

From: Josh Poimboeuf <jpoimboe@kernel.org>

Enable sframe generation in the VDSO library so kernel and user space
can unwind through it.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---

Notes (jremus):
    Link: https://lore.kernel.org/all/20250425024023.173709192@goodmis.org/
    
    This is a prerequisite for patch "s390/vdso: Enable SFrame generation in
    vDSO" to actually generate SFrame in vDSO, as it introduces config
    option CONFIG_AS_SFRAME.

 arch/Kconfig                          |  3 +++
 arch/x86/entry/vdso/Makefile          | 10 +++++++---
 arch/x86/entry/vdso/vdso-layout.lds.S |  3 +++
 arch/x86/include/asm/dwarf2.h         |  5 ++++-
 4 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 4fe16ad6f053..1c3daccd9072 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -435,6 +435,9 @@ 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/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 54d3e9774d62..2dc0e0ca19a7 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -47,13 +47,17 @@ quiet_cmd_vdso2c = VDSO2C  $@
 $(obj)/vdso-image-%.c: $(obj)/vdso%.so.dbg $(obj)/vdso%.so $(obj)/vdso2c FORCE
 	$(call if_changed,vdso2c)
 
+ifeq ($(CONFIG_AS_SFRAME),y)
+  SFRAME_CFLAGS := -Wa,-gsframe
+endif
+
 #
 # Don't omit frame pointers for ease of userspace debugging, but do
 # optimize sibling calls.
 #
 CFL := $(PROFILING) -mcmodel=small -fPIC -O2 -fasynchronous-unwind-tables -m64 \
        $(filter -g%,$(KBUILD_CFLAGS)) -fno-stack-protector \
-       -fno-omit-frame-pointer -foptimize-sibling-calls \
+       -fno-omit-frame-pointer $(SFRAME_CFLAGS) -foptimize-sibling-calls \
        -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
 
 ifdef CONFIG_MITIGATION_RETPOLINE
@@ -63,7 +67,7 @@ endif
 endif
 
 $(vobjs): KBUILD_CFLAGS := $(filter-out $(PADDING_CFLAGS) $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
-$(vobjs): KBUILD_AFLAGS += -DBUILD_VDSO
+$(vobjs): KBUILD_AFLAGS += -DBUILD_VDSO $(SFRAME_CFLAGS)
 
 #
 # vDSO code runs in userspace and -pg doesn't help with profiling anyway.
@@ -104,7 +108,7 @@ $(obj)/%-x32.o: $(obj)/%.o FORCE
 
 targets += vdsox32.lds $(vobjx32s-y)
 
-$(obj)/%.so: OBJCOPYFLAGS := -S --remove-section __ex_table
+$(obj)/%.so: OBJCOPYFLAGS := -g --remove-section __ex_table
 $(obj)/%.so: $(obj)/%.so.dbg FORCE
 	$(call if_changed,objcopy)
 
diff --git a/arch/x86/entry/vdso/vdso-layout.lds.S b/arch/x86/entry/vdso/vdso-layout.lds.S
index ec1ac191a057..c57cfddfc37d 100644
--- a/arch/x86/entry/vdso/vdso-layout.lds.S
+++ b/arch/x86/entry/vdso/vdso-layout.lds.S
@@ -59,6 +59,7 @@ SECTIONS
 	.eh_frame_hdr	: { *(.eh_frame_hdr) }		:text	:eh_frame_hdr
 	.eh_frame	: { KEEP (*(.eh_frame)) }	:text
 
+	.sframe		: { *(.sframe) }		:text	:sframe
 
 	/*
 	 * Text is well-separated from actual data: there's plenty of
@@ -87,6 +88,7 @@ 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
@@ -98,4 +100,5 @@ PHDRS
 	dynamic		PT_DYNAMIC	FLAGS(4);		/* PF_R */
 	note		PT_NOTE		FLAGS(4);		/* PF_R */
 	eh_frame_hdr	PT_GNU_EH_FRAME;
+	sframe		PT_GNU_SFRAME;
 }
diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h
index 65d958ef1178..ce294e6c9017 100644
--- a/arch/x86/include/asm/dwarf2.h
+++ b/arch/x86/include/asm/dwarf2.h
@@ -12,8 +12,11 @@
 	 * For the vDSO, emit both runtime unwind information and debug
 	 * symbols for the .dbg file.
 	 */
-
+#if defined(__x86_64__) && defined(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
-- 
2.48.1