Messages in this thread Patch in this message | | | From | Alexandre Chartre <> | Subject | [PATCH 4/7] objtool: Add support for return trampoline call | Date | Thu, 2 Apr 2020 10:22:17 +0200 |
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With retpoline, the return instruction is used to branch to an address stored on the stack. So, unlike a regular return instruction, when a retpoline return instruction is reached the stack has been modified compared to what we have when the function was entered.
Provide the mechanism to explicitly call-out such return instruction so that objtool can correctly handle them.
Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com> --- tools/objtool/check.c | 78 +++++++++++++++++++++++++++++++++++++++++-- tools/objtool/check.h | 1 + 2 files changed, 76 insertions(+), 3 deletions(-)
diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 0cec91291d46..ed8e3ea1d8da 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -1344,6 +1344,48 @@ static int read_intra_function_call(struct objtool_file *file) return 0; } +static int read_retpoline_ret(struct objtool_file *file) +{ + struct section *sec; + struct instruction *insn; + struct rela *rela; + + sec = find_section_by_name(file->elf, ".rela.discard.retpoline_ret"); + 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.retpoline_ret entry"); + return -1; + } + + if (insn->type != INSN_RETURN) { + WARN_FUNC("retpoline_ret not a return", + insn->sec, insn->offset); + return -1; + } + + insn->retpoline_ret = true; + /* + * For the impact on the stack, make a return trampoline + * behaves like a pop of the return address. + */ + insn->stack_op.src.type = OP_SRC_POP; + insn->stack_op.dest.type = OP_DEST_REG; + insn->stack_op.dest.reg = CFI_RA; + } + + return 0; +} + static void mark_rodata(struct objtool_file *file) { struct section *sec; @@ -1403,6 +1445,10 @@ static int decode_sections(struct objtool_file *file) if (ret) return ret; + ret = read_retpoline_ret(file); + if (ret) + return ret; + ret = add_call_destinations(file); if (ret) return ret; @@ -1432,7 +1478,8 @@ 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 insn_state *state, + bool check_registers) { int i; @@ -1442,6 +1489,9 @@ static bool has_modified_stack_frame(struct insn_state *state) state->drap) return true; + if (!check_registers) + return false; + 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) @@ -1987,7 +2037,7 @@ 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(state, true)) { WARN_FUNC("sibling call from callable instruction with modified stack frame", insn->sec, insn->offset); return 1; @@ -2009,6 +2059,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, struct alternative *alt; struct instruction *insn, *next_insn; struct section *sec; + bool check_registers; u8 visited; int ret; @@ -2130,7 +2181,28 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, return 1; } - if (func && has_modified_stack_frame(&state)) { + /* + * With retpoline, the return instruction is used + * to branch to an address stored on the stack. + * So when we reach the ret instruction, the stack + * frame has been modified with the address to + * branch to and we need update the stack state. + * + * The retpoline address to branch to is typically + * pushed on the stack from a register, but this + * confuses the logic which checks callee saved + * registers. So we don't check if registers have + * been modified if we have a return trampoline. + */ + if (insn->retpoline_ret) { + update_insn_state(insn, &state); + check_registers = false; + } else { + check_registers = true; + } + + if (func && has_modified_stack_frame(&state, + check_registers)) { 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 2bd6d2f46baa..5ecd16ad71a8 100644 --- a/tools/objtool/check.h +++ b/tools/objtool/check.h @@ -37,6 +37,7 @@ struct instruction { bool dead_end, ignore, hint, save, restore, ignore_alts; bool intra_function_call; bool retpoline_safe; + bool retpoline_ret; u8 visited; struct symbol *call_dest; struct instruction *jump_dest; -- 2.18.2
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