[PATCH v4 03/12] s390/vdso: Keep function symbols in vDSO

[Jens Remus <jremus@linux.ibm.com>]

Keep all function symbols in the vDSO .symtab for stack trace purposes.
This enables a stack tracer, such as perf, to lookup these function
symbols in addition to those already exported in vDSO .dynsym.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
    Changes in RFC v2:
    - Use objcopy flag "-g" instead of "-S" with the cumbersome filter
      "-w -K "__arch_*" -K "__cvdso_*" -K "__s390_vdso_*" to keep the
      function symbols, as Josh did in "x86/vdso: Enable sframe
      generation in VDSO":
      https://lore.kernel.org/all/20250425024023.173709192@goodmis.org/
    - Reword commit message.
    
    Note that unlike Josh I did not squash this into the subsequent patch
    "s390/vdso: Enable SFrame generation in vDSO", as this change is
    unrelated to enabling the use of SFrame.  perf report/script do also
    benefit from this change when using perf record --call-graph dwarf.
    
    Note that this change does not cause the vDSO build-id to change.
    perf record may therefore not dump an updated copy of the vDSO to
    ~/.debug/[vdso]/<build-id>/vdso, so that perf report/script may
    use a stale copy without .symtab.  Resolve by deleting ~/.debug/.

 arch/s390/kernel/vdso/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/kernel/vdso/Makefile b/arch/s390/kernel/vdso/Makefile
index 2fa12d4ac106..3a8bde5716f4 100644
--- a/arch/s390/kernel/vdso/Makefile
+++ b/arch/s390/kernel/vdso/Makefile
@@ -50,7 +50,7 @@ $(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) $(obj-cvdso) FORCE
 	$(call if_changed,vdso_and_check)
 
 # strip rule for the .so file
-$(obj)/%.so: OBJCOPYFLAGS := -S
+$(obj)/%.so: OBJCOPYFLAGS := -g
 $(obj)/%.so: $(obj)/%.so.dbg FORCE
 	$(call if_changed,objcopy)
 
-- 
2.51.0