From 7117f16bf460ef8cd132e6e80c989677397b4868 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 28 Apr 2020 19:37:01 +0200 Subject: [PATCH] objtool: Fix ORC vs alternatives Jann reported that (for instance) entry_64.o:general_protection has very odd ORC data: 0000000000000f40 : #######sp:sp+8 bp:(und) type:iret end:0 f40: 90 nop #######sp:(und) bp:(und) type:call end:0 f41: 90 nop f42: 90 nop #######sp:sp+8 bp:(und) type:iret end:0 f43: e8 a8 01 00 00 callq 10f0 #######sp:sp+0 bp:(und) type:regs end:0 f48: f6 84 24 88 00 00 00 testb $0x3,0x88(%rsp) f4f: 03 f50: 74 00 je f52 f52: 48 89 e7 mov %rsp,%rdi f55: 48 8b 74 24 78 mov 0x78(%rsp),%rsi f5a: 48 c7 44 24 78 ff ff movq $0xffffffffffffffff,0x78(%rsp) f61: ff ff f63: e8 00 00 00 00 callq f68 f68: e9 73 02 00 00 jmpq 11e0 #######sp:(und) bp:(und) type:call end:0 f6d: 0f 1f 00 nopl (%rax) Note the entry at 0xf41. Josh found this was the result of commit: 764eef4b109a ("objtool: Rewrite alt->skip_orig") Due to the early return in validate_branch() we no longer set insn->cfi of the original instruction stream (the NOPs at 0xf41 and 0xf42) and we'll end up with the above weirdness. In other discussions we realized alternatives should be ORC invariant; that is, due to there being only a single ORC table, it must be valid for all alternatives. The easiest way to ensure this is to not allow any stack modifications in alternatives. When we enforce this latter observation, we get the property that the whole alternative must have the same CFI, which we can employ to fix the former report. Fixes: 764eef4b109a ("objtool: Rewrite alt->skip_orig") Reported-by: Jann Horn Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Miroslav Benes Acked-by: Josh Poimboeuf Link: https://lkml.kernel.org/r/20200428191659.499074346@infradead.org --- tools/objtool/Documentation/stack-validation.txt | 7 +++++ tools/objtool/check.c | 34 +++++++++++++++++++++++- 2 files changed, 40 insertions(+), 1 deletion(-) diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt index faa47c3..0189039 100644 --- a/tools/objtool/Documentation/stack-validation.txt +++ b/tools/objtool/Documentation/stack-validation.txt @@ -315,6 +315,13 @@ they mean, and suggestions for how to fix them. function tracing inserts additional calls, which is not obvious from the sources). +10. file.o: warning: func()+0x5c: alternative modifies stack + + This means that an alternative includes instructions that modify the + stack. The problem is that there is only one ORC unwind table, this means + that the ORC unwind entries must be valid for each of the alternatives. + The easiest way to enforce this is to ensure alternatives do not contain + any ORC entries, which in turn implies the above constraint. 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/check.c b/tools/objtool/check.c index 4da6bfb..fa9bf36 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -1983,6 +1983,11 @@ static int handle_insn_ops(struct instruction *insn, struct insn_state *state) list_for_each_entry(op, &insn->stack_ops, list) { int res; + if (insn->alt_group) { + WARN_FUNC("alternative modifies stack", insn->sec, insn->offset); + return -1; + } + res = update_cfi_state(insn, &state->cfi, op); if (res) return res; @@ -2150,6 +2155,30 @@ static int validate_return(struct symbol *func, struct instruction *insn, struct } /* + * Alternatives should not contain any ORC entries, this in turn means they + * should not contain any CFI ops, which implies all instructions should have + * the same same CFI state. + * + * It is possible to constuct alternatives that have unreachable holes that go + * unreported (because they're NOPs), such holes would result in CFI_UNDEFINED + * states which then results in ORC entries, which we just said we didn't want. + * + * Avoid them by copying the CFI entry of the first instruction into the whole + * alternative. + */ +static void fill_alternative_cfi(struct objtool_file *file, struct instruction *insn) +{ + struct instruction *first_insn = insn; + int alt_group = insn->alt_group; + + sec_for_each_insn_continue(file, insn) { + if (insn->alt_group != alt_group) + break; + insn->cfi = first_insn->cfi; + } +} + +/* * Follow the branch starting at the given instruction, and recursively follow * any other branches (jumps). Meanwhile, track the frame pointer state at * each instruction and validate all the rules described in @@ -2200,7 +2229,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, insn->visited |= visited; - if (!insn->ignore_alts) { + if (!insn->ignore_alts && !list_empty(&insn->alts)) { bool skip_orig = false; list_for_each_entry(alt, &insn->alts, list) { @@ -2215,6 +2244,9 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, } } + if (insn->alt_group) + fill_alternative_cfi(file, insn); + if (skip_orig) return 0; } -- 2.7.4