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

On 22.01.2025 03:30, Josh Poimboeuf wrote:
> 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>

> diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h

> -#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 */
> +

Did you remove ".cfi_sections .debug_frame" on purpose from the
!BUILD_VDSO path compared to V3? Presumably to not only not emit
DWARF CFI from assembler, but any source?

> +/*
> + * On x86, these macros aren't used outside VDSO.  As well they shouldn't be:
> + * they're fragile and very difficult to maintain.
> + */

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

IBM

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IBM Data Privacy Statement: https://www.ibm.com/privacy/

On Fri, Jan 24, 2025 at 05:08:57PM +0100, Jens Remus wrote:
> On 22.01.2025 03:30, Josh Poimboeuf wrote:
> > -#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 */
> > +
> 
> Did you remove ".cfi_sections .debug_frame" on purpose from the
> !BUILD_VDSO path compared to V3?

Yes, since non-VDSO assembly files won't be emitting any .cfi, there's
no .debug_frame to output anyway.

> Presumably to not only not emit DWARF CFI from assembler, but any
> source?

This only impacts assembly files, notice the __ASSEMBLY__ check at the
top of the file.

-- 
Josh