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

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Message-ID: <F16410E4-43A2-4CC1-8049-AFB803733869@zytor.com>
Date: Tue, 03 Feb 2026 09:35:30 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Jens Remus <jremus@...ux.ibm.com>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        x86@...nel.org, linux-mm@...ck.org,
        Steven Rostedt <rostedt@...nel.org>
CC: Josh Poimboeuf <jpoimboe@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrii Nakryiko <andrii@...nel.org>,
        Indu Bhagat <indu.bhagat@...cle.com>,
        "Jose E. Marchesi" <jemarch@....org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Florian Weimer <fweimer@...hat.com>, Kees Cook <kees@...nel.org>,
        "Carlos O'Donell" <codonell@...hat.com>, Sam James <sam@...too.org>,
        Dylan Hatch <dylanbhatch@...gle.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>
Subject: Re: [PATCH v7 2/6] x86/asm: Avoid emitting DWARF CFI for non-VDSO

On February 3, 2026 9:19:53 AM PST, Jens Remus <jremus@...ux.ibm.com> wrote:
>From: Josh Poimboeuf <jpoimboe@...nel.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@...nel.org>
>Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
>Signed-off-by: Jens Remus <jremus@...ux.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?