[RFC PATCH v1 03/16] s390/vdso: Avoid emitting DWARF CFI for non-vDSO

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

This replicates Josh's x86 commit TODO ("x86/asm: Avoid emitting DWARF
CFI for non-VDSO") for s390.  It also aligns asm/dwarf.h to x86
asm/dwarf2.h.

It was decided years ago that .cfi_* annotations aren't maintainable in
the kernel.  For the kernel proper, ensure the CFI_* macros don't do
anything.

On the other hand the vDSO library *does* use them, so user space can
unwind through it.

Make sure these macros only work for vDSO.  They aren't actually being
used outside of vDSO anyway, so there's no functional change.

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

Notes (jremus):
    Link to latest x86 patch:
    https://lore.kernel.org/all/20250425024022.477374378@goodmis.org/

 arch/s390/include/asm/dwarf.h | 45 ++++++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 16 deletions(-)

diff --git a/arch/s390/include/asm/dwarf.h b/arch/s390/include/asm/dwarf.h
index fdf51f66a9ed..5052ee6a40f7 100644
--- a/arch/s390/include/asm/dwarf.h
+++ b/arch/s390/include/asm/dwarf.h
@@ -6,6 +6,18 @@
 #warning "asm/dwarf.h should be only included in pure assembly files"
 #endif
 
+.macro nocfi args:vararg
+.endm
+
+#ifdef BUILD_VDSO
+
+	/*
+	 * For the vDSO, emit both runtime unwind information and debug
+	 * symbols for the .dbg file.
+	 */
+
+	.cfi_sections .eh_frame, .debug_frame
+
 #define CFI_STARTPROC		.cfi_startproc
 #define CFI_ENDPROC		.cfi_endproc
 #define CFI_DEF_CFA_OFFSET	.cfi_def_cfa_offset
@@ -16,23 +28,24 @@
 #ifdef CONFIG_AS_CFI_VAL_OFFSET
 #define CFI_VAL_OFFSET		.cfi_val_offset
 #else
-#define CFI_VAL_OFFSET		#
+#define CFI_VAL_OFFSET		nocfi
 #endif
 
-#ifndef BUILD_VDSO
-	/*
-	 * Emit CFI data in .debug_frame sections and not in .eh_frame
-	 * sections.  The .eh_frame CFI is used for runtime unwind
-	 * information that is not being used.  Hence, vmlinux.lds.S
-	 * can discard the .eh_frame sections.
-	 */
-	.cfi_sections .debug_frame
-#else
-	/*
-	 * For vDSO, emit CFI data in both, .eh_frame and .debug_frame
-	 * sections.
-	 */
-	.cfi_sections .eh_frame, .debug_frame
-#endif
+#else /* !BUILD_VDSO */
+
+/*
+ * On s390, these macros aren't used outside vDSO.  As well they shouldn't be:
+ * they're fragile and very difficult to maintain.
+ */
+
+#define CFI_STARTPROC		nocfi
+#define CFI_ENDPROC		nocfi
+#define CFI_DEF_CFA_OFFSET	nocfi
+#define CFI_ADJUST_CFA_OFFSET	nocfi
+#define CFI_RESTORE		nocfi
+#define CFI_REL_OFFSET		nocfi
+#define CFI_VAL_OFFSET		nocfi
+
+#endif /* !BUILD_VDSO */
 
 #endif	/* _ASM_S390_DWARF_H */
-- 
2.48.1