kallsyms: Get rid of kallsyms relative base

When the kallsyms relative base was introduced, per-CPU variable
references on x86_64 SMP were implemented as offsets into the respective
per-CPU region, rather than offsets relative to the location of the
variable's template in the kernel image, which is how other
architectures implement it.

This required kallsyms to reason about the difference between the two,
and the sign of the value in the kallsyms_offsets[] array was used to
distinguish them. This meant that negative offsets were not permitted
for ordinary variables, and so it was crucial that the relative base was
chosen such that all offsets were positive numbers.

This is no longer needed: instead, the offsets can simply be encoded as
values in the range -/+ 2 GiB, which is precisely what PC32 relocations
provide on most architectures. So it is possible to simplify the logic,
and just use _text as the anchor directly, and let the linker calculate
the final value based on the location of the entry itself.

Some architectures (nios2, extensa) do not support place-relative
relocations at all, but these are all 32-bit and non-relocatable, and so
there is no need for place-relative relocations in the first place, and
the actual symbol values can just be stored directly.

This makes all entries in the kallsyms_offsets[] array visible as
place-relative references in the ELF metadata, which will be important
when implementing ELF-based fg-kaslr.

Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://patch.msgid.link/20260116093359.2442297-6-ardb+git@google.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

kernel/kallsyms.c

@@ -151,8 +151,10 @@ static unsigned int get_symbol_offset(unsigned long pos)
 
 unsigned long kallsyms_sym_address(int idx)
 {
-	/* values are unsigned offsets */
-	return kallsyms_relative_base + (u32)kallsyms_offsets[idx];
+	/* non-relocatable 32-bit kernels just embed the value directly */
+	if (!IS_ENABLED(CONFIG_64BIT) && !IS_ENABLED(CONFIG_RELOCATABLE))
+		return (u32)kallsyms_offsets[idx];
+	return (unsigned long)offset_to_ptr(kallsyms_offsets + idx);
 }
 
 static unsigned int get_symbol_seq(int index)

kernel/kallsyms_internal.h

@@ -8,7 +8,6 @@ extern const int kallsyms_offsets[];
 extern const u8 kallsyms_names[];
 
 extern const unsigned int kallsyms_num_syms;
-extern const unsigned long kallsyms_relative_base;
 
 extern const char kallsyms_token_table[];
 extern const u16 kallsyms_token_index[];

kernel/vmcore_info.c

@@ -238,7 +238,6 @@ static int __init crash_save_vmcoreinfo_init(void)
 	VMCOREINFO_SYMBOL(kallsyms_token_table);
 	VMCOREINFO_SYMBOL(kallsyms_token_index);
 	VMCOREINFO_SYMBOL(kallsyms_offsets);
-	VMCOREINFO_SYMBOL(kallsyms_relative_base);
 #endif /* CONFIG_KALLSYMS */
 
 	arch_crash_save_vmcoreinfo();

scripts/kallsyms.c

@@ -46,7 +46,6 @@ struct addr_range {
 };
 
 static unsigned long long _text;
-static unsigned long long relative_base;
 static struct addr_range text_ranges[] = {
 	{ "_stext",     "_etext"     },
 	{ "_sinittext", "_einittext" },
@@ -57,6 +56,7 @@ static struct addr_range text_ranges[] = {
 static struct sym_entry **table;
 static unsigned int table_size, table_cnt;
 static int all_symbols;
+static int pc_relative;
 
 static int token_profit[0x10000];
 
@@ -280,7 +280,7 @@ static void read_map(const char *in)
 static void output_label(const char *label)
 {
 	printf(".globl %s\n", label);
-	printf("\tALGN\n");
+	printf("\t.balign 4\n");
 	printf("%s:\n", label);
 }
 
@@ -343,15 +343,6 @@ static void write_src(void)
 	unsigned int *markers, markers_cnt;
 	char buf[KSYM_NAME_LEN];
 
-	printf("#include <asm/bitsperlong.h>\n");
-	printf("#if BITS_PER_LONG == 64\n");
-	printf("#define PTR .quad\n");
-	printf("#define ALGN .balign 8\n");
-	printf("#else\n");
-	printf("#define PTR .long\n");
-	printf("#define ALGN .balign 4\n");
-	printf("#endif\n");
-
 	printf("\t.section .rodata, \"a\"\n");
 
 	output_label("kallsyms_num_syms");
@@ -434,34 +425,24 @@ static void write_src(void)
 	output_label("kallsyms_offsets");
 
 	for (i = 0; i < table_cnt; i++) {
-		/*
-		 * Use the offset relative to the lowest value
-		 * encountered of all relative symbols, and emit
-		 * non-relocatable fixed offsets that will be fixed
-		 * up at runtime.
-		 */
+		if (pc_relative) {
+			long long offset = table[i]->addr - _text;
 
-		long long offset;
-
-		offset = table[i]->addr - relative_base;
-		if (offset < 0 || offset > UINT_MAX) {
-			fprintf(stderr, "kallsyms failure: "
-				"relative symbol value %#llx out of range\n",
-				table[i]->addr);
-			exit(EXIT_FAILURE);
+			if (offset < INT_MIN || offset > INT_MAX) {
+				fprintf(stderr, "kallsyms failure: "
+					"relative symbol value %#llx out of range\n",
+					table[i]->addr);
+				exit(EXIT_FAILURE);
+			}
+			printf("\t.long\t_text - . + (%d)\t/* %s */\n",
+			       (int)offset, table[i]->sym);
+		} else {
+			printf("\t.long\t%#x\t/* %s */\n",
+			       (unsigned int)table[i]->addr, table[i]->sym);
 		}
-		printf("\t.long\t%#x\t/* %s */\n", (int)offset, table[i]->sym);
 	}
 	printf("\n");
 
-	output_label("kallsyms_relative_base");
-	/* Provide proper symbols relocatability by their '_text' relativeness. */
-	if (_text <= relative_base)
-		printf("\tPTR\t_text + %#llx\n", relative_base - _text);
-	else
-		printf("\tPTR\t_text - %#llx\n", _text - relative_base);
-	printf("\n");
-
 	sort_symbols_by_name();
 	output_label("kallsyms_seqs_of_names");
 	for (i = 0; i < table_cnt; i++)
@@ -701,22 +682,12 @@ static void sort_symbols(void)
 	qsort(table, table_cnt, sizeof(table[0]), compare_symbols);
 }
 
-/* find the minimum non-absolute symbol address */
-static void record_relative_base(void)
-{
-	/*
-	 * The table is sorted by address.
-	 * Take the first symbol value.
-	 */
-	if (table_cnt)
-		relative_base = table[0]->addr;
-}
-
 int main(int argc, char **argv)
 {
 	while (1) {
 		static const struct option long_options[] = {
 			{"all-symbols",     no_argument, &all_symbols,     1},
+			{"pc-relative",     no_argument, &pc_relative,     1},
 			{},
 		};
 
@@ -734,7 +705,6 @@ int main(int argc, char **argv)
 	read_map(argv[optind]);
 	shrink_table();
 	sort_symbols();
-	record_relative_base();
 	optimize_token_table();
 	write_src();
 

scripts/link-vmlinux.sh

@@ -143,6 +143,10 @@ kallsyms()
 		kallsymopt="${kallsymopt} --all-symbols"
 	fi
 
+	if is_enabled CONFIG_64BIT || is_enabled CONFIG_RELOCATABLE; then
+		kallsymopt="${kallsymopt} --pc-relative"
+	fi
+
 	info KSYMS "${2}.S"
 	scripts/kallsyms ${kallsymopt} "${1}" > "${2}.S"
 

tools/perf/tests/vmlinux-kallsyms.c

@@ -27,7 +27,6 @@ static bool is_ignored_symbol(const char *name, char type)
 		 * stable symbol list.
 		 */
 		"kallsyms_offsets",
-		"kallsyms_relative_base",
 		"kallsyms_num_syms",
 		"kallsyms_names",
 		"kallsyms_markers",