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SubjectRe: [PATCH v3 18/26] objtool: Fix !CFI insn_state propagation
On Tue, Mar 24, 2020 at 04:31:31PM +0100, Peter Zijlstra wrote:
> Objtool keeps per instruction CFI state in struct insn_state and will
> save/restore this where required. However, insn_state has grown some
> !CFI state, and this must not be saved/restored (and thus lost).
>
> Fix this by explicitly preserving the !CFI state and clarify by
> restucturing the code and adding a comment.
>
> XXX, the insn==first condition is not handled right.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> tools/objtool/check.c | 95 +++++++++++++++++++++++++++++---------------------
> tools/objtool/check.h | 8 ++++
> 2 files changed, 64 insertions(+), 39 deletions(-)
>
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -2033,6 +2033,59 @@ static int validate_return(struct symbol
> return 0;
> }
>
> +static int apply_insn_hint(struct objtool_file *file, struct section *sec,
> + struct symbol *func, struct instruction *first,
> + struct instruction *insn, struct insn_state *state)
> +{
> + struct insn_state old = *state;
> +
> + if (insn->restore) {
> + struct instruction *save_insn, *i;
> +
> + i = insn;
> + save_insn = NULL;
> + sym_for_each_insn_continue_reverse(file, func, i) {
> + if (i->save) {
> + save_insn = i;
> + break;
> + }
> + }
> +
> + if (!save_insn) {
> + WARN_FUNC("no corresponding CFI save for CFI restore",
> + sec, insn->offset);
> + return 1;
> + }
> +
> + 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; // XXX

Yeah, moving this code out to apply_insn_hint() seems like a nice idea,
but it wouldn't be worth it if it breaks this case. TBH I don't
remember if this check was for a real-world case. Might be worth
looking at... If this case doesn't exist in reality then we could just
remove this check altogether.

> +
> + WARN_FUNC("objtool isn't smart enough to handle this CFI save/restore combo",
> + sec, insn->offset);
> + return 1;
> + }
> +
> + insn->state = save_insn->state;
> + }
> +
> + *state = insn->state;

This would have been easier to review if apply_insn_hint() were added in
a separate patch.

> +
> + /* restore !CFI state */
> + state->df = old.df;
> + state->uaccess = old.uaccess;
> + state->uaccess_stack = old.uaccess_stack;

Maybe we should just move the CFI stuff into a state->cfi substruct.
That would remove the need for these bits and probably also the comment
above the insn_state declaration.

--
Josh

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