[PATCH v4 05/39] x86/asm: Avoid emitting DWARF CFI for non-VDSO

Josh Poimboeuf
2025-01-22T02:31:56Z

It was decided years ago that .cfi_* annotations aren't maintainable in
the kernel. They were replaced by objtool unwind hints. 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: Josh Poimboeuf <jpoimboe@kernel.org>
---
arch/x86/include/asm/dwarf2.h | 51 ++++++++++++++++++++++++-----------
1 file changed, 35 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h
index 430fca13bb56..b195b3c8677e 100644
--- a/arch/x86/include/asm/dwarf2.h
+++ b/arch/x86/include/asm/dwarf2.h
@@ -6,6 +6,15 @@
#warning "asm/dwarf2.h should be only included in pure assembly files"
#endif

+#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 .cfi_def_cfa
@@ -21,21 +30,31 @@
#define CFI_UNDEFINED .cfi_undefined
#define CFI_ESCAPE .cfi_escape

-#ifndef BUILD_VDSO
- /*
- * Emit CFI data in .debug_frame sections, not .eh_frame sections.
- * The latter we currently just discard since we don't do DWARF
- * unwinding at runtime. So only the offline DWARF information is
- * useful to anyone. Note we should not use this directive if we
- * ever decide to enable DWARF unwinding at runtime.
- */
- .cfi_sections .debug_frame
-#else
- /*
- * For the vDSO, emit both runtime unwind information and debug
- * symbols for the .dbg file.
- */
- .cfi_sections .eh_frame, .debug_frame
-#endif
+#else /* !BUILD_VDSO */
+
+/*
+ * On x86, these macros aren't used outside VDSO. As well they shouldn't be:
+ * they're fragile and very difficult to maintain.
+ */
+
+.macro nocfi args:vararg
+.endm
+
+#define CFI_STARTPROC nocfi
+#define CFI_ENDPROC nocfi
+#define CFI_DEF_CFA nocfi
+#define CFI_DEF_CFA_REGISTER nocfi
+#define CFI_DEF_CFA_OFFSET nocfi
+#define CFI_ADJUST_CFA_OFFSET nocfi
+#define CFI_OFFSET nocfi
+#define CFI_REL_OFFSET nocfi
+#define CFI_REGISTER nocfi
+#define CFI_RESTORE nocfi
+#define CFI_REMEMBER_STATE nocfi
+#define CFI_RESTORE_STATE nocfi
+#define CFI_UNDEFINED nocfi
+#define CFI_ESCAPE nocfi
+
+#endif /* !BUILD_VDSO */

#endif /* _ASM_X86_DWARF2_H */
--
2.48.1