[PATCH 2/2] kallsyms: handle special absolute symbols

[Kees Cook <keescook@chromium.org>]

This forces the entire per_cpu range to be reported as absolute without
losing their linker symbol types. Without this, the variables are
incorrectly shown as relocated under kASLR.

Several kallsyms output in different boot states for comparison of
various symbols:

$ 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
$ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr2
000000000d400000 D __per_cpu_start
000000000d404000 D gdt_page
000000000d414280 D __per_cpu_end
ffffffff8e4001c8 T _stext
ffffffff8f2e53c0 D __per_cpu_offset
$ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr-fixed
0000000000000000 D __per_cpu_start
0000000000004000 D gdt_page
0000000000014280 D __per_cpu_end
ffffffffadc001c8 T _stext
ffffffffaeae53c0 D __per_cpu_offset

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 scripts/kallsyms.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 3c6224728960..4dcda98427cb 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -34,6 +34,7 @@ struct sym_entry {
 	unsigned int len;
 	unsigned int start_pos;
 	unsigned char *sym;
+	int force_absolute;
 };
 
 struct addr_range {
@@ -51,6 +52,10 @@ static struct addr_range text_ranges[] = {
 #define text_range_text     (&text_ranges[0])
 #define text_range_inittext (&text_ranges[1])
 
+static struct addr_range abs_ranges[] = {
+	{ "__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;
@@ -165,6 +170,10 @@ static int read_symbol(FILE *in, struct sym_entry *s)
 	}
 	strcpy((char *)s->sym + 1, str);
 	s->sym[0] = stype;
+	s->force_absolute = 0;
+
+	/* Check if we've found special absolute symbol range. */
+	check_symbol_range(sym, s->addr, abs_ranges, ARRAY_SIZE(abs_ranges));
 
 	return 0;
 }
@@ -210,6 +219,10 @@ static int symbol_valid(struct sym_entry *s)
 	if (s->addr < kernel_start_addr)
 		return 0;
 
+	/* Force special absolute symbols into being absolute. */
+	if (symbol_in_range(s, abs_ranges, ARRAY_SIZE(abs_ranges)) == 1)
+		s->force_absolute = 1;
+
 	/* skip prefix char */
 	if (symbol_prefix_char && *(s->sym + 1) == symbol_prefix_char)
 		offset++;
@@ -306,7 +319,7 @@ static int expand_symbol(unsigned char *data, int len, char *result)
 
 static int symbol_absolute(struct sym_entry *s)
 {
-	return toupper(s->sym[0]) == 'A';
+	return s->force_absolute || toupper(s->sym[0]) == 'A';
 }
 
 static void write_src(void)
-- 
1.7.9.5