kbuild: remove PROVIDE() for kallsyms symbols

This reimplements commit 951bcae6 ("kallsyms: Avoid weak references
for kallsyms symbols") because I am not a big fan of PROVIDE().

As an alternative solution, this commit prepends one more kallsyms step.

    KSYMS   .tmp_vmlinux.kallsyms0.S          # added
    AS      .tmp_vmlinux.kallsyms0.o          # added
    LD      .tmp_vmlinux.btf
    BTF     .btf.vmlinux.bin.o
    LD      .tmp_vmlinux.kallsyms1
    NM      .tmp_vmlinux.kallsyms1.syms
    KSYMS   .tmp_vmlinux.kallsyms1.S
    AS      .tmp_vmlinux.kallsyms1.o
    LD      .tmp_vmlinux.kallsyms2
    NM      .tmp_vmlinux.kallsyms2.syms
    KSYMS   .tmp_vmlinux.kallsyms2.S
    AS      .tmp_vmlinux.kallsyms2.o
    LD      vmlinux

Step 0 takes /dev/null as input, and generates .tmp_vmlinux.kallsyms0.o,
which has a valid kallsyms format with the empty symbol list, and can be
linked to vmlinux. Since it is really small, the added compile-time cost
is negligible.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

include/asm-generic/vmlinux.lds.h

@@ -451,30 +451,11 @@
 #endif
 #endif
 
-/*
- * Some symbol definitions will not exist yet during the first pass of the
- * link, but are guaranteed to exist in the final link. Provide preliminary
- * definitions that will be superseded in the final link to avoid having to
- * rely on weak external linkage, which requires a GOT when used in position
- * independent code.
- */
-#define PRELIMINARY_SYMBOL_DEFINITIONS					\
-	PROVIDE(kallsyms_addresses = .);				\
-	PROVIDE(kallsyms_offsets = .);					\
-	PROVIDE(kallsyms_names = .);					\
-	PROVIDE(kallsyms_num_syms = .);					\
-	PROVIDE(kallsyms_relative_base = .);				\
-	PROVIDE(kallsyms_token_table = .);				\
-	PROVIDE(kallsyms_token_index = .);				\
-	PROVIDE(kallsyms_markers = .);					\
-	PROVIDE(kallsyms_seqs_of_names = .);
-
 /*
  * Read only Data
  */
 #define RO_DATA(align)							\
 	. = ALIGN((align));						\
-	PRELIMINARY_SYMBOL_DEFINITIONS					\
 	.rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {		\
 		__start_rodata = .;					\
 		*(.rodata) *(.rodata.*)					\

kernel/kallsyms_internal.h

@@ -4,11 +4,6 @@
 
 #include <linux/types.h>
 
-/*
- * These will be re-linked against their real values during the second link
- * stage. Preliminary values must be provided in the linker script using the
- * PROVIDE() directive so that the first link stage can complete successfully.
- */
 extern const unsigned long kallsyms_addresses[];
 extern const int kallsyms_offsets[];
 extern const u8 kallsyms_names[];

scripts/kallsyms.c

@@ -259,12 +259,6 @@ static void shrink_table(void)
 		}
 	}
 	table_cnt = pos;
-
-	/* When valid symbol is not registered, exit to error */
-	if (!table_cnt) {
-		fprintf(stderr, "No valid symbol.\n");
-		exit(1);
-	}
 }
 
 static void read_map(const char *in)

scripts/link-vmlinux.sh

@@ -227,6 +227,10 @@ btf_vmlinux_bin_o=
 kallsymso=
 strip_debug=
 
+if is_enabled CONFIG_KALLSYMS; then
+	kallsyms /dev/null .tmp_vmlinux.kallsyms0
+fi
+
 if is_enabled CONFIG_DEBUG_INFO_BTF; then
 	if ! gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then
 		echo >&2 "Failed to generate BTF for vmlinux"
@@ -239,9 +243,10 @@ if is_enabled CONFIG_KALLSYMS; then
 
 	# kallsyms support
 	# Generate section listing all symbols and add it into vmlinux
-	# It's a three step process:
+	# It's a four step process:
+	# 0)  Generate a dummy __kallsyms with empty symbol list.
 	# 1)  Link .tmp_vmlinux.kallsyms1 so it has all symbols and sections,
-	#     but __kallsyms is empty.
+	#     with a dummy __kallsyms.
 	#     Running kallsyms on that gives us .tmp_kallsyms1.o with
 	#     the right size
 	# 2)  Link .tmp_vmlinux.kallsyms2 so it now has a __kallsyms section of