objtool: Better handle IRET

Teach objtool a little more about IRET so that we can avoid using the
SAVE/RESTORE annotation. In particular, make the weird corner case in
insn->restore go away.

The purpose of that corner case is to deal with the fact that
UNWIND_HINT_RESTORE lands on the instruction after IRET, but that
instruction can end up being outside the basic block, consider:

	if (cond)
		sync_core()
	foo();

Then the hint will land on foo(), and we'll encounter the restore
hint without ever having seen the save hint.

By teaching objtool about the arch specific exception frame size, and
assuming that any IRET in an STT_FUNC symbol is an exception frame
sized POP, we can remove the use of save/restore hints for this code.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200416115118.631224674@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 3bcf27caf6c9f740d3ecfb1f82f797d1e65fa170..3eeaaeb756380b85e89b8ac0e33282ec07ed3fe5 100644 (file)
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -727,7 +727,6 @@ static inline void sync_core(void)
        unsigned int tmp;
 
        asm volatile (
-               UNWIND_HINT_SAVE
                "mov %%ss, %0\n\t"
                "pushq %q0\n\t"
                "pushq %%rsp\n\t"
@@ -737,7 +736,6 @@ static inline void sync_core(void)
                "pushq %q0\n\t"
                "pushq $1f\n\t"
                "iretq\n\t"
-               UNWIND_HINT_RESTORE
                "1:"
                : "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");
 #endif

diff --git a/tools/objtool/arch.h b/tools/objtool/arch.h
index f9883c431949123fab4b8c09b92f7acc1d82d83b..55396dfe0d07b5c78d9f21c97174af73fa947531 100644 (file)
--- a/tools/objtool/arch.h
+++ b/tools/objtool/arch.h
@@ -19,6 +19,7 @@ enum insn_type {
        INSN_CALL,
        INSN_CALL_DYNAMIC,
        INSN_RETURN,
+       INSN_EXCEPTION_RETURN,
        INSN_CONTEXT_SWITCH,
        INSN_STACK,
        INSN_BUG,

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 199b4084a13c2994c63d5f6c8da316641d2db27e..32736383ead116209ebe6183b351e98a161594b9 100644 (file)
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -446,9 +446,19 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
                *type = INSN_RETURN;
                break;
 
+       case 0xcf: /* iret */
+               *type = INSN_EXCEPTION_RETURN;
+
+               /* add $40, %rsp */
+               op->src.type = OP_SRC_ADD;
+               op->src.reg = CFI_SP;
+               op->src.offset = 5*8;
+               op->dest.type = OP_DEST_REG;
+               op->dest.reg = CFI_SP;
+               break;
+
        case 0xca: /* retf */
        case 0xcb: /* retf */
-       case 0xcf: /* iret */
                *type = INSN_CONTEXT_SWITCH;
                break;
 
@@ -494,7 +504,7 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
 
        *immediate = insn.immediate.nbytes ? insn.immediate.value : 0;
 
-       if (*type == INSN_STACK)
+       if (*type == INSN_STACK || *type == INSN_EXCEPTION_RETURN)
                list_add_tail(&op->list, ops_list);
        else
                free(op);

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 9e854fd128d450a7371cf425074cf77d59374a13..781b3a3c2ba6b1b21d57ca4d9099564654ec3f81 100644 (file)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2065,15 +2065,14 @@ static int validate_return(struct symbol *func, struct instruction *insn, struct
  * tools/objtool/Documentation/stack-validation.txt.
  */
 static int validate_branch(struct objtool_file *file, struct symbol *func,
-                          struct instruction *first, struct insn_state state)
+                          struct instruction *insn, struct insn_state state)
 {
        struct alternative *alt;
-       struct instruction *insn, *next_insn;
+       struct instruction *next_insn;
        struct section *sec;
        u8 visited;
        int ret;
 
-       insn = first;
        sec = insn->sec;
 
        if (insn->alt_group && list_empty(&insn->alts)) {
@@ -2126,16 +2125,6 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
                                }
 
                                if (!save_insn->visited) {
-                                       /*
-                                        * Oops, no state to copy yet.
-                                        * Hopefully we can reach this
-                                        * instruction from another branch
-                                        * after the save insn has been
-                                        * visited.
-                                        */
-                                       if (insn == first)
-                                               return 0;
-
                                        WARN_FUNC("objtool isn't smart enough to handle this CFI save/restore combo",
                                                  sec, insn->offset);
                                        return 1;
@@ -2228,6 +2217,20 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
                        break;
 
+               case INSN_EXCEPTION_RETURN:
+                       if (handle_insn_ops(insn, &state))
+                               return 1;
+
+                       /*
+                        * This handles x86's sync_core() case, where we use an
+                        * IRET to self. All 'normal' IRET instructions are in
+                        * STT_NOTYPE entry symbols.
+                        */
+                       if (func)
+                               break;
+
+                       return 0;
+
                case INSN_CONTEXT_SWITCH:
                        if (func && (!next_insn || !next_insn->hint)) {
                                WARN_FUNC("unsupported instruction in callable function",