Re: [PATCH v7 2/6] x86/asm: Avoid emitting DWARF CFI for non-VDSO

["H. Peter Anvin" <hpa@zytor.com>]

On February 3, 2026 9:19:53 AM PST, Jens Remus <jremus@linux.ibm.com> wrote:
>From: Josh Poimboeuf <jpoimboe@kernel.org>
>
>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>
>Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
>Signed-off-by: Jens Remus <jremus@linux.ibm.com>
>---
> 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 09c9684d3ad6..7cc30500c095 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
>@@ -22,21 +31,31 @@
> #define CFI_ESCAPE		.cfi_escape
> #define CFI_SIGNAL_FRAME	.cfi_signal_frame
> 
>-#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 */

Maybe we should move these into x86/entry/vdso/common instead?