riscv: add Linux note to vdso

The Linux note in the vdso allows glibc to check the running kernel
version without having to issue the uname syscall.
Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/vdso/Makefile

@@ -12,7 +12,7 @@ vdso-syms += getcpu
 vdso-syms += flush_icache
 
 # Files to link into the vdso
-obj-vdso = $(patsubst %, %.o, $(vdso-syms))
+obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o
 
 # Build rules
 targets := $(obj-vdso) vdso.so vdso.so.dbg vdso.lds vdso-dummy.o

arch/riscv/kernel/vdso/note.S

+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * This supplies .note.* sections to go into the PT_NOTE inside the vDSO text.
+ * Here we can supply some information useful to userland.
+ */
+
+#include <linux/elfnote.h>
+#include <linux/version.h>
+
+ELFNOTE_START(Linux, 0, "a")
+	.long LINUX_VERSION_CODE
+ELFNOTE_END