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Commit c6bda7c9 authored by Rusty Russell's avatar Rusty Russell
Browse files

kallsyms: fix percpu vars on x86-64 with relocation. 



x86-64 has a problem: per-cpu variables are actually represented by
their absolute offsets within the per-cpu area, but the symbols are
not emitted as absolute.  Thus kallsyms naively creates them as offsets
from _text, meaning their values change if the kernel is relocated
(especially noticeable with CONFIG_RANDOMIZE_BASE):

 $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.nokaslr
 0000000000000000 D __per_cpu_start
 0000000000004000 D gdt_page
 0000000000014280 D __per_cpu_end
 ffffffff810001c8 T _stext
 ffffffff81ee53c0 D __per_cpu_offset
 $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr1
 000000001f200000 D __per_cpu_start
 000000001f204000 D gdt_page
 000000001f214280 D __per_cpu_end
 ffffffffa02001c8 T _stext
 ffffffffa10e53c0 D __per_cpu_offset

Making them absolute symbols is the Right Thing, but requires fixes to
the relocs tool.  So for the moment, we add a --absolute-percpu option
which makes them absolute from a kallsyms perspective:

 $ egrep ' (gdt_|_(stext|_per_cpu_))' /proc/kallsyms # no KASLR
 0000000000000000 A __per_cpu_start
 000000000000a000 A gdt_page
 0000000000013040 A __per_cpu_end
 ffffffff802001c8 T _stext
 ffffffff8099b180 D __per_cpu_offset
 ffffffff809a3000 D __per_cpu_load
 $ egrep ' (gdt_|_(stext|_per_cpu_))' /proc/kallsyms # With KASLR
 0000000000000000 A __per_cpu_start
 000000000000a000 A gdt_page
 0000000000013040 A __per_cpu_end
 ffffffff89c001c8 T _stext
 ffffffff8a39d180 D __per_cpu_offset
 ffffffff8a3a5000 D __per_cpu_load

Based-on-the-original-screenplay-by: default avatarAndy Honig <ahonig@google.com>
Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
Acked-by: default avatarKees Cook <keescook@chromium.org>
parent 78eb7159

scripts/kallsyms.c

+21 −0
Original line number Diff line number Diff line
@@ -51,9 +51,14 @@ static struct addr_range text_ranges[] = { 
#define text_range_text     (&text_ranges[0])

#define text_range_inittext (&text_ranges[1])



static struct addr_range percpu_range = {

	"__per_cpu_start", "__per_cpu_end", -1ULL, 0

};



static struct sym_entry *table;

static unsigned int table_size, table_cnt;

static int all_symbols = 0;

static int absolute_percpu = 0;

static char symbol_prefix_char = '\0';

static unsigned long long kernel_start_addr = 0;
@@ -166,6 +171,9 @@ static int read_symbol(FILE *in, struct sym_entry *s) 
	strcpy((char *)s->sym + 1, str);

	s->sym[0] = stype;



	/* Record if we've found __per_cpu_start/end. */

	check_symbol_range(sym, s->addr, &percpu_range, 1);



	return 0;

}
@@ -657,6 +665,15 @@ static void sort_symbols(void) 
	qsort(table, table_cnt, sizeof(struct sym_entry), compare_symbols);

}



static void make_percpus_absolute(void)

{

	unsigned int i;



	for (i = 0; i < table_cnt; i++)

		if (symbol_in_range(&table[i], &percpu_range, 1))

			table[i].sym[0] = 'A';

}



int main(int argc, char **argv)

{

	if (argc >= 2) {
@@ -664,6 +681,8 @@ int main(int argc, char **argv) 
		for (i = 1; i < argc; i++) {

			if(strcmp(argv[i], "--all-symbols") == 0)

				all_symbols = 1;

			else if (strcmp(argv[i], "--absolute-percpu") == 0)

				absolute_percpu = 1;

			else if (strncmp(argv[i], "--symbol-prefix=", 16) == 0) {

				char *p = &argv[i][16];

				/* skip quote */
@@ -680,6 +699,8 @@ int main(int argc, char **argv) 
		usage();



	read_map(stdin);

	if (absolute_percpu)

		make_percpus_absolute();

	sort_symbols();

	optimize_token_table();

	write_src();

scripts/link-vmlinux.sh

+4 −0
Original line number Diff line number Diff line
@@ -86,6 +86,10 @@ kallsyms() 
		kallsymopt="${kallsymopt} --page-offset=$CONFIG_PAGE_OFFSET"

	fi



	if [ -n "${CONFIG_X86_64}" ]; then

		kallsymopt="${kallsymopt} --absolute-percpu"

	fi



	local aflags="${KBUILD_AFLAGS} ${KBUILD_AFLAGS_KERNEL}               \

		      ${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS}"