From: Josh Poimboeuf <jpoimboe@kernel.org>

Enable sframe generation in the VDSO library so kernel and user space
can unwind through it.

SFrame isn't supported for x32 or x86-32.  Discard .sframe sections for
those VDSOs.

[ Jens Remus: Add support for SFrame V3.  Prevent GNU_SFRAME program
table entry to empty .sframe section. ]

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>
---

Notes (jremus):
    Changes in v8:
    - Discard .sframe for x32 and x86-32 VDSOs. (Josh/Indu)
      Note that the use of KEEP_SFRAME enables to define it for x86-64
      VDSO only.  Unlike CONFIG_AS_SFRAME, which may also be defined
      for x32 and x86-32 VDSO.  In x32 VDSO it would result in superfluous
      .sframe (copied from the x86-64 build - could be removed in X32
      build step).  In x86-32 VDSO it would cause a bogus GNU_SFRAME
      program table entry.
    - Reword commit message (append Josh's private fix).
    - Drop .cfi_sections .sframe in dwarf2.h in favor of the explicitly
      specified more specific assembler option --gsframe-3.  With this
      it is not necessary to undefine CONFIG_AS_SFRAME in
      fake_32bit_build.h.

 arch/Kconfig                                 |  7 +++++++
 arch/x86/entry/vdso/common/vdso-layout.lds.S | 15 +++++++++++++++
 arch/x86/entry/vdso/vdso64/Makefile          |  1 +
 arch/x86/entry/vdso/vdso64/vdso64.lds.S      |  4 ++++
 4 files changed, 27 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 31220f512b16..8170e492a44c 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -479,6 +479,13 @@ config HAVE_HARDLOCKUP_DETECTOR_ARCH
 	  It uses the same command line parameters, and sysctl interface,
 	  as the generic hardlockup detectors.
 
+config AS_SFRAME
+	bool
+
+config AS_SFRAME3
+	def_bool $(as-instr,.cfi_startproc\n.cfi_endproc,-Wa$(comma)--gsframe-3)
+	select AS_SFRAME
+
 config UNWIND_USER
 	bool
 
diff --git a/arch/x86/entry/vdso/common/vdso-layout.lds.S b/arch/x86/entry/vdso/common/vdso-layout.lds.S
index 856b8b9d278c..47e6a6a0560a 100644
--- a/arch/x86/entry/vdso/common/vdso-layout.lds.S
+++ b/arch/x86/entry/vdso/common/vdso-layout.lds.S
@@ -60,6 +60,13 @@ SECTIONS
 		*(.eh_frame.*)
 	}					:text
 
+#ifdef KEEP_SFRAME
+	.sframe		: {
+		KEEP (*(.sframe))
+		*(.sframe.*)
+	}					:text	:sframe
+#endif
+
 	/*
 	 * Text is well-separated from actual data: there's plenty of
 	 * stuff that isn't used at runtime in between.
@@ -80,6 +87,10 @@ SECTIONS
 		*(.discard)
 		*(.discard.*)
 		*(__bug_table)
+#ifndef KEEP_SFRAME
+		*(.sframe)
+		*(.sframe.*)
+#endif
 	}
 }
 
@@ -89,6 +100,7 @@ SECTIONS
 #define PT_GNU_EH_FRAME	0x6474e550
 #define PT_GNU_STACK	0x6474e551
 #define PT_GNU_PROPERTY	0x6474e553
+#define PT_GNU_SFRAME	0x6474e554
 
 /*
  * We must supply the ELF program headers explicitly to get just one
@@ -104,6 +116,9 @@ PHDRS
 	dynamic		PT_DYNAMIC	PF_R;
 	note		PT_NOTE		PF_R;
 	eh_frame_hdr	PT_GNU_EH_FRAME PF_R;
+#ifdef KEEP_SFRAME
+	sframe		PT_GNU_SFRAME	PF_R;
+#endif
 	gnu_stack	PT_GNU_STACK	PF_RW;
 	gnu_property	PT_GNU_PROPERTY	PF_R;
 }
diff --git a/arch/x86/entry/vdso/vdso64/Makefile b/arch/x86/entry/vdso/vdso64/Makefile
index bfffaf1aeecc..459f8026531e 100644
--- a/arch/x86/entry/vdso/vdso64/Makefile
+++ b/arch/x86/entry/vdso/vdso64/Makefile
@@ -14,6 +14,7 @@ vobjs-$(CONFIG_X86_SGX)		+= vsgx.o
 
 # Compilation flags
 flags-y				:= -DBUILD_VDSO64 -m64 -mcmodel=small
+flags-$(CONFIG_AS_SFRAME3)	+= -Wa,--gsframe-3
 
 # The location of this include matters!
 include $(src)/../common/Makefile.include
diff --git a/arch/x86/entry/vdso/vdso64/vdso64.lds.S b/arch/x86/entry/vdso/vdso64/vdso64.lds.S
index 5ce3f2b6373a..13e30d3b4827 100644
--- a/arch/x86/entry/vdso/vdso64/vdso64.lds.S
+++ b/arch/x86/entry/vdso/vdso64/vdso64.lds.S
@@ -9,6 +9,10 @@
 
 #define BUILD_VDSO64
 
+#ifdef CONFIG_AS_SFRAME
+# define KEEP_SFRAME
+#endif
+
 #include "common/vdso-layout.lds.S"
 
 /*
-- 
2.51.0

On 2026-02-06 11:36, Jens Remus wrote:
> From: Josh Poimboeuf <jpoimboe@kernel.org>
> 
> Enable sframe generation in the VDSO library so kernel and user space
> can unwind through it.
> 
> SFrame isn't supported for x32 or x86-32.  Discard .sframe sections for
> those VDSOs.
> 
> [ Jens Remus: Add support for SFrame V3.  Prevent GNU_SFRAME program
> table entry to empty .sframe section. ]
> 

This will not break the x86-32 build if the assembler encounters .sframe?

> Notes (jremus):
>     Changes in v8:
>     - Discard .sframe for x32 and x86-32 VDSOs. (Josh/Indu)
>       Note that the use of KEEP_SFRAME enables to define it for x86-64
>       VDSO only.  Unlike CONFIG_AS_SFRAME, which may also be defined
>       for x32 and x86-32 VDSO.  In x32 VDSO it would result in superfluous
>       .sframe (copied from the x86-64 build - could be removed in X32
>       build step).  In x86-32 VDSO it would cause a bogus GNU_SFRAME
>       program table entry.

For x32, this would be a "valid" sframe, right, even if the tools currently
don't know how to consume it (and potentially never will)? If so, is there
really any reason to explicitly remove it?

>  	/*
>  	 * Text is well-separated from actual data: there's plenty of
>  	 * stuff that isn't used at runtime in between.
> @@ -80,6 +87,10 @@ SECTIONS
>  		*(.discard)
>  		*(.discard.*)
>  		*(__bug_table)
> +#ifndef KEEP_SFRAME
> +		*(.sframe)
> +		*(.sframe.*)
> +#endif

This #ifndef is actually not necessary: if we have already "consumed" the
.sframe* sections they will not be encountered here.

I would prefer to have KEEP_SFRAME always defined (as true or false, and using
#if) instead of using #ifdef. I believe that also means you can do:

#define KEEP_SFRAME IS_ENABLED(CONFIG_AS_SFRAME)

... instead of #ifdef.

	-hpa