[PATCH v4 2/2] objtool: fix .cold functions parent symbols search

[Artem Savkov <asavkov@redhat.com>]

Because find_symbol_by_name() traverses the same lists as read_symbols()
changing sym->name in place without copying it affects the result of
find_symbol_by_name() and, in case when ".cold" function precedes it's
parent in sec->symbol_list, can result in function being considered a
parent of itself. This leads to function length being set to 0 and other
consequent side-effects including a segfault in add_switch_table().
The effects of this bug are only visible when building with
-ffunction-sections in KCFLAGS.

Fix by copying the search string instead of modifying it in place.

Fixes: 13810435b9a7 ("objtool: Support GCC 8's cold subfunctions")
Signed-off-by: Artem Savkov <asavkov@redhat.com>
---
 tools/objtool/elf.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index e7a7ac40e045..b8f3cca8e58b 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -31,6 +31,8 @@
 #include "elf.h"
 #include "warn.h"
 
+#define MAX_NAME_LEN 128
+
 struct section *find_section_by_name(struct elf *elf, const char *name)
 {
 	struct section *sec;
@@ -298,6 +300,8 @@ static int read_symbols(struct elf *elf)
 	/* Create parent/child links for any cold subfunctions */
 	list_for_each_entry(sec, &elf->sections, list) {
 		list_for_each_entry(sym, &sec->symbol_list, list) {
+			char pname[MAX_NAME_LEN + 1];
+			size_t pnamelen;
 			if (sym->type != STT_FUNC)
 				continue;
 			sym->pfunc = sym->cfunc = sym;
@@ -305,9 +309,16 @@ static int read_symbols(struct elf *elf)
 			if (!coldstr)
 				continue;
 
-			coldstr[0] = '\0';
-			pfunc = find_symbol_by_name(elf, sym->name);
-			coldstr[0] = '.';
+			pnamelen = coldstr - sym->name;
+			if (pnamelen > MAX_NAME_LEN) {
+				WARN("%s(): parent function name exceeds maximum length of %d characters",
+				     sym->name, MAX_NAME_LEN);
+				return -1;
+			}
+
+			strncpy(pname, sym->name, pnamelen);
+			pname[pnamelen] = '\0';
+			pfunc = find_symbol_by_name(elf, pname);
 
 			if (!pfunc) {
 				WARN("%s(): can't find parent function",
-- 
2.19.1