[PATCH 1/2] objtool/LoongArch: Restrict stack operation instruction

[Tiezhu Yang]

After commit a0f7085f6a63 ("LoongArch: Add RANDOMIZE_KSTACK_OFFSET
support"), the code flow of do_syscall() was changed when compiled
with GCC due to the secondary stack of add_random_kstack_offset(),
something like this:

  addi.d          $sp, $sp, -32
  st.d            $fp, $sp, 16
  st.d            $ra, $sp, 24
  addi.d          $fp, $sp, 32
  ...
  sub.d           $sp, $sp, $t1
  ...
  addi.d          $sp, $fp, -32
  ld.d            $ra, $sp, 24
  ld.d            $fp, $sp, 16
  addi.d          $sp, $sp, 32

fp points to the stack top, it is only used to save and restore the
original sp and is not used as cfa base for arch_callee_saved_reg().
In the case OP_SRC_ADD of update_cfi_state(), the above rare case is
not handled so that lead to a wrong stack size, then there exists a
objtool warning "do_syscall+0x11c: return with modified stack frame".

Because the fp related instructions do not modify the stack frame,
no need to decode them, just restrict stack operation instruction
only with the single case "addi.d sp,sp,si12".

By the way, if fp is used as cfa base for arch_callee_saved_reg()
(there is no this behavior on LoongArch at present), then it needs
to decode the related instructions and modify update_cfi_state().

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202407201336.mW8dj1VB-lkp@xxxxxxxxx/
Fixes: b2d23158e6c8 ("objtool/LoongArch: Implement instruction decoder")
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
 tools/objtool/arch/loongarch/decode.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/arch/loongarch/decode.c b/tools/objtool/arch/loongarch/decode.c
index aee479d2191c..6a34af675cee 100644
--- a/tools/objtool/arch/loongarch/decode.c
+++ b/tools/objtool/arch/loongarch/decode.c
@@ -121,8 +121,8 @@ static bool decode_insn_reg2i12_fomat(union loongarch_instruction inst,
 {
 	switch (inst.reg2i12_format.opcode) {
 	case addid_op:
-		if ((inst.reg2i12_format.rd == CFI_SP) || (inst.reg2i12_format.rj == CFI_SP)) {
-			/* addi.d sp,sp,si12 or addi.d fp,sp,si12 */
+		if ((inst.reg2i12_format.rd == CFI_SP) && (inst.reg2i12_format.rj == CFI_SP)) {
+			/* addi.d sp,sp,si12 */
 			insn->immediate = sign_extend64(inst.reg2i12_format.immediate, 11);
 			ADD_OP(op) {
 				op->src.type = OP_SRC_ADD;
-- 
2.42.0