[PATCH V3 3/9] objtool: Add support for intra-function calls

Alexandre Chartre
2020-04-14T10:57:57Z

Change objtool to support intra-function calls. On x86, an intra-function
call is represented in objtool as a push onto the stack (of the return
address), and a jump to the destination address. That way the stack
information is correctly updated and the call flow is still accurate.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
include/linux/frame.h | 11 ++
.../Documentation/stack-validation.txt | 8 ++
tools/objtool/arch/x86/decode.c | 6 +
tools/objtool/check.c | 126 ++++++++++++++----
tools/objtool/check.h | 1 +
5 files changed, 128 insertions(+), 24 deletions(-)

diff --git a/include/linux/frame.h b/include/linux/frame.h
index 02d3ca2d9598..c7178e6c9d48 100644
--- a/include/linux/frame.h
+++ b/include/linux/frame.h
@@ -15,9 +15,20 @@
static void __used __section(.discard.func_stack_frame_non_standard) \
*__func_stack_frame_non_standard_##func = func

+/*
+ * This macro indicates that the following intra-function call is valid.
+ * Any non-annotated intra-function call will cause objtool to issue warning.
+ */
+#define ANNOTATE_INTRA_FUNCTION_CALL \
+ 999: \
+ .pushsection .discard.intra_function_call; \
+ .long 999b; \
+ .popsection;
+
#else /* !CONFIG_STACK_VALIDATION */

#define STACK_FRAME_NON_STANDARD(func)
+#define ANNOTATE_INTRA_FUNCTION_CALL

#endif /* CONFIG_STACK_VALIDATION */

diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
index de094670050b..09f863fd32d2 100644
--- a/tools/objtool/Documentation/stack-validation.txt
+++ b/tools/objtool/Documentation/stack-validation.txt
@@ -290,6 +290,14 @@ they mean, and suggestions for how to fix them.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646

+9. file.o: warning: unsupported intra-function call
+
+ This warning means that a direct call is done to a destination which
+ is not at the beginning of a function. If this is a legit call, you
+ can remove this warning by putting the ANNOTATE_INTRA_FUNCTION_CALL
+ directive right before the call.
+
+
If the error doesn't seem to make sense, it could be a bug in objtool.
Feel free to ask the objtool maintainer for help.

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index a62e032863a8..f4d70b8835c4 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -437,6 +437,12 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,

case 0xe8:
*type = INSN_CALL;
+ /*
+ * For the impact on the stack, a call behaves like
+ * a push of an immediate value (the return address).
+ */
+ op->src.type = OP_SRC_CONST;
+ op->dest.type = OP_DEST_PUSH;
break;

case 0xfc:
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 8f0525d5d895..ad362c5de281 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -654,44 +654,58 @@ static int add_jump_destinations(struct objtool_file *file)
return 0;
}

+static int configure_call(struct objtool_file *file, struct instruction *insn)
+{
+ unsigned long dest_off;
+
+ dest_off = insn->offset + insn->len + insn->immediate;
+ insn->call_dest = find_func_by_offset(insn->sec, dest_off);
+ if (!insn->call_dest)
+ insn->call_dest = find_symbol_by_offset(insn->sec, dest_off);
+
+ if (insn->call_dest) {
+ /* regular call */
+ if (insn->func && insn->call_dest->type != STT_FUNC) {
+ WARN_FUNC("unsupported call to non-function",
+ insn->sec, insn->offset);
+ return -1;
+ }
+ return 0;
+ }
+
+ /* intra-function call */
+ if (!insn->intra_function_call)
+ WARN_FUNC("intra-function call", insn->sec, insn->offset);
+
+ dest_off = insn->offset + insn->len + insn->immediate;
+ insn->jump_dest = find_insn(file, insn->sec, dest_off);
+ if (!insn->jump_dest) {
+ WARN_FUNC("can't find call dest at %s+0x%lx",
+ insn->sec, insn->offset,
+ insn->sec->name, dest_off);
+ return -1;
+ }
+
+ return 0;
+}
+
/*
* Find the destination instructions for all calls.
*/
static int add_call_destinations(struct objtool_file *file)
{
struct instruction *insn;
- unsigned long dest_off;
struct rela *rela;

for_each_insn(file, insn) {
- if (insn->type != INSN_CALL)
+ if (insn->type != INSN_CALL || insn->ignore)
continue;

rela = find_rela_by_dest_range(file->elf, insn->sec,
insn->offset, insn->len);
if (!rela) {
- dest_off = insn->offset + insn->len + insn->immediate;
- insn->call_dest = find_func_by_offset(insn->sec, dest_off);
- if (!insn->call_dest)
- insn->call_dest = find_symbol_by_offset(insn->sec, dest_off);
-
- if (insn->ignore)
- continue;
-
- if (!insn->call_dest) {
- WARN_FUNC("unsupported intra-function call",
- insn->sec, insn->offset);
- if (retpoline)
- WARN("If this is a retpoline, please patch it in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE.");
- return -1;
- }
-
- if (insn->func && insn->call_dest->type != STT_FUNC) {
- WARN_FUNC("unsupported call to non-function",
- insn->sec, insn->offset);
+ if (configure_call(file, insn))
return -1;
- }
-
} else if (rela->sym->type == STT_SECTION) {
insn->call_dest = find_func_by_offset(rela->sym->sec,
rela->addend+4);
@@ -1337,6 +1351,42 @@ static int read_retpoline_hints(struct objtool_file *file)
return 0;
}

+static int read_intra_function_call(struct objtool_file *file)
+{
+ struct section *sec;
+ struct instruction *insn;
+ struct rela *rela;
+
+ sec = find_section_by_name(file->elf,
+ ".rela.discard.intra_function_call");
+ if (!sec)
+ return 0;
+
+ list_for_each_entry(rela, &sec->rela_list, list) {
+ if (rela->sym->type != STT_SECTION) {
+ WARN("unexpected relocation symbol type in %s",
+ sec->name);
+ return -1;
+ }
+
+ insn = find_insn(file, rela->sym->sec, rela->addend);
+ if (!insn) {
+ WARN("bad .discard.intra_function_call entry");
+ return -1;
+ }
+
+ if (insn->type != INSN_CALL) {
+ WARN_FUNC("intra_function_call not a direct call",
+ insn->sec, insn->offset);
+ return -1;
+ }
+
+ insn->intra_function_call = true;
+ }
+
+ return 0;
+}
+
static void mark_rodata(struct objtool_file *file)
{
struct section *sec;
@@ -1392,6 +1442,10 @@ static int decode_sections(struct objtool_file *file)
if (ret)
return ret;

+ ret = read_intra_function_call(file);
+ if (ret)
+ return ret;
+
ret = add_call_destinations(file);
if (ret)
return ret;
@@ -2155,7 +2209,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
return ret;

if (!no_fp && func && !is_fentry_call(insn) &&
- !has_valid_stack_frame(&state)) {
+ !has_valid_stack_frame(&state) &&
+ !insn->intra_function_call) {
WARN_FUNC("call without frame pointer save/setup",
sec, insn->offset);
return 1;
@@ -2164,6 +2219,29 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
if (dead_end_function(file, insn->call_dest))
return 0;

+ if (insn->intra_function_call) {
+ /*
+ * The call instruction can update the stack
+ * state. Then make the intra-function call
+ * behaves like and unconditional jump.
+ */
+ ret = update_insn_state(insn, &state);
+ if (ret)
+ return ret;
+
+ ret = validate_branch(file, func, insn,
+ insn->jump_dest, state);
+ if (ret) {
+ if (backtrace) {
+ BT_FUNC("(intra-function call)",
+ insn);
+ }
+ return ret;
+ }
+
+ return 0;
+ }
+
break;

case INSN_JUMP_CONDITIONAL:
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index 953db38dfc35..6a80903fc4aa 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -36,6 +36,7 @@ struct instruction {
int alt_group;
bool dead_end, ignore, ignore_alts;
bool hint, save, restore;
+ bool intra_function_call;
bool retpoline_safe;
u8 visited;
struct symbol *call_dest;
--
2.18.2