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Subject[PATCH V2 1/9] objtool: Introduce HINT_RET_OFFSET
Date
From: "Peter Zijlstra (Intel)" <peterz@infradead.org>

Normally objtool ensures a function keeps the stack layout invariant.
But there is a useful exception, it is possible to stuff the return
stack in order to 'inject' a 'call':

push $fun
ret

In this case the invariant mentioned above is violated.

Add an objtool HINT to annotate this and allow a function exit with a
modified stack frame.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/orc_types.h | 1 +
arch/x86/include/asm/unwind_hints.h | 10 ++++++++++
tools/arch/x86/include/asm/orc_types.h | 1 +
tools/objtool/check.c | 26 ++++++++++++++++++--------
tools/objtool/check.h | 5 ++++-
5 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/orc_types.h b/arch/x86/include/asm/orc_types.h
index 6e060907c163..5f18ca7ac51a 100644
--- a/arch/x86/include/asm/orc_types.h
+++ b/arch/x86/include/asm/orc_types.h
@@ -60,6 +60,7 @@
#define ORC_TYPE_REGS_IRET 2
#define UNWIND_HINT_TYPE_SAVE 3
#define UNWIND_HINT_TYPE_RESTORE 4
+#define UNWIND_HINT_TYPE_RET_OFFSET 5

#ifndef __ASSEMBLY__
/*
diff --git a/arch/x86/include/asm/unwind_hints.h b/arch/x86/include/asm/unwind_hints.h
index f5e2eb12cb71..aabf7ace0476 100644
--- a/arch/x86/include/asm/unwind_hints.h
+++ b/arch/x86/include/asm/unwind_hints.h
@@ -94,6 +94,16 @@
UNWIND_HINT type=UNWIND_HINT_TYPE_RESTORE
.endm

+
+/*
+ * RET_OFFSET: Used on instructions that terminate a function; mostly RETURN
+ * and sibling calls. On these, sp_offset denotes the expected offset from
+ * initial_func_cfi.
+ */
+.macro UNWIND_HINT_RET_OFFSET sp_offset=8
+ UNWIND_HINT type=UNWIND_HINT_TYPE_RET_OFFSET sp_offset=\sp_offset
+.endm
+
#else /* !__ASSEMBLY__ */

#define UNWIND_HINT(sp_reg, sp_offset, type, end) \
diff --git a/tools/arch/x86/include/asm/orc_types.h b/tools/arch/x86/include/asm/orc_types.h
index 6e060907c163..5f18ca7ac51a 100644
--- a/tools/arch/x86/include/asm/orc_types.h
+++ b/tools/arch/x86/include/asm/orc_types.h
@@ -60,6 +60,7 @@
#define ORC_TYPE_REGS_IRET 2
#define UNWIND_HINT_TYPE_SAVE 3
#define UNWIND_HINT_TYPE_RESTORE 4
+#define UNWIND_HINT_TYPE_RET_OFFSET 5

#ifndef __ASSEMBLY__
/*
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 4768d91c6d68..bbee26de92ec 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1209,6 +1209,10 @@ static int read_unwind_hints(struct objtool_file *file)
insn->restore = true;
insn->hint = true;
continue;
+
+ } else if (hint->type == UNWIND_HINT_TYPE_RET_OFFSET) {
+ insn->ret_offset = hint->sp_offset;
+ continue;
}

insn->hint = true;
@@ -1371,20 +1375,26 @@ static bool is_fentry_call(struct instruction *insn)
return false;
}

-static bool has_modified_stack_frame(struct insn_state *state)
+static bool has_modified_stack_frame(struct instruction *insn,
+ struct insn_state *state)
{
+ u8 ret_offset = insn->ret_offset;
int i;

- if (state->cfa.base != initial_func_cfi.cfa.base ||
- state->cfa.offset != initial_func_cfi.cfa.offset ||
- state->stack_size != initial_func_cfi.cfa.offset ||
- state->drap)
+ if (state->cfa.base != initial_func_cfi.cfa.base || state->drap)
+ return true;
+
+ if (state->cfa.offset != initial_func_cfi.cfa.offset + ret_offset)
return true;

- for (i = 0; i < CFI_NUM_REGS; i++)
+ if (state->stack_size != initial_func_cfi.cfa.offset + ret_offset)
+ return true;
+
+ for (i = 0; i < CFI_NUM_REGS; i++) {
if (state->regs[i].base != initial_func_cfi.regs[i].base ||
state->regs[i].offset != initial_func_cfi.regs[i].offset)
return true;
+ }

return false;
}
@@ -1926,7 +1936,7 @@ static int validate_call(struct instruction *insn, struct insn_state *state)

static int validate_sibling_call(struct instruction *insn, struct insn_state *state)
{
- if (has_modified_stack_frame(state)) {
+ if (has_modified_stack_frame(insn, state)) {
WARN_FUNC("sibling call from callable instruction with modified stack frame",
insn->sec, insn->offset);
return 1;
@@ -2065,7 +2075,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
return 1;
}

- if (func && has_modified_stack_frame(&state)) {
+ if (func && has_modified_stack_frame(insn, &state)) {
WARN_FUNC("return with modified stack frame",
sec, insn->offset);
return 1;
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index 6d875ca6fce0..7a91497fee7e 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -33,9 +33,12 @@ struct instruction {
unsigned int len;
enum insn_type type;
unsigned long immediate;
- bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
+ unsigned int alt_group;
+ bool dead_end, ignore, ignore_alts;
+ bool hint, save, restore;
bool retpoline_safe;
u8 visited;
+ u8 ret_offset;
struct symbol *call_dest;
struct instruction *jump_dest;
struct instruction *first_jump_src;
--
2.18.2
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