linux-kernel - [PATCH v2] objtool: fix .cold. functions parent symbols search

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Message-Id: <20181107214515.9183-1-asavkov@redhat.com>
Date:   Wed,  7 Nov 2018 22:45:15 +0100
From:   Artem Savkov <asavkov@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Artem Savkov <asavkov@...hat.com>
Subject: [PATCH v2] objtool: fix .cold. functions parent symbols search

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.

Signed-off-by: Artem Savkov <asavkov@...hat.com>
---
 tools/objtool/elf.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 6dbb9fae0f9d..781c8afb29b9 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -298,6 +298,7 @@ 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;
 			if (sym->type != STT_FUNC)
 				continue;
 			sym->pfunc = sym->cfunc = sym;
@@ -305,9 +306,9 @@ static int read_symbols(struct elf *elf)
 			if (!coldstr)
 				continue;
 
-			coldstr[0] = '\0';
-			pfunc = find_symbol_by_name(elf, sym->name);
-			coldstr[0] = '.';
+			pname = strndup(sym->name, coldstr - sym->name);
+			pfunc = find_symbol_by_name(elf, pname);
+			free(pname);
 
 			if (!pfunc) {
 				WARN("%s(): can't find parent function",
-- 
2.17.2