Re: [PATCH 2/7] objtool: Allow branches within the same alternative.

[Alexandre Chartre]

On 4/2/20 2:03 PM, Julien Thierry wrote:
> Hi Alexandre,
>
> I ran into the same issue for the arm64 work:
> https://lkml.org/lkml/2020/1/9/656

Thanks for the reference, I didn't notice that change, but I saw a more
recent one where you were just removing the branch to alternative check
(https://lkml.org/lkml/2020/3/25/151).

> Your solution seems nicer however.
>
> On 4/2/20 9:22 AM, Alexandre Chartre wrote:
> > Currently objtool prevents any branch to an alternative. While preventing
> > branching from the outside to the middle of an alternative makes perfect
> > sense, branching within the same alternative should be allowed. To do so,
> > identify each alternative and check that a branch to an alternative comes
> > from the same alternative.
> >
> > Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
> > ---
> >   tools/objtool/check.c | 19 +++++++++++++------
> >   tools/objtool/check.h |  3 ++-
> >   2 files changed, 15 insertions(+), 7 deletions(-)
> >
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index 708562fb89e1..214809ac2776 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -712,7 +712,9 @@ static int handle_group_alt(struct objtool_file *file,
> >                   struct instruction *orig_insn,
> >                   struct instruction **new_insn)
> >   {
> > +    static unsigned int alt_group_next_index = 1;
> >       struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump = NULL;
> > +    unsigned int alt_group = alt_group_next_index++;
> >       unsigned long dest_off;
> >       last_orig_insn = NULL;
> > @@ -721,7 +723,7 @@ static int handle_group_alt(struct objtool_file *file,
> >           if (insn->offset >= special_alt->orig_off + special_alt->orig_len)
> >               break;
> > -        insn->alt_group = true;
> > +        insn->alt_group = alt_group;
> >           last_orig_insn = insn;
> >       }
> > @@ -1942,6 +1944,7 @@ static int validate_sibling_call(struct instruction *insn, struct insn_state *st
> >    * tools/objtool/Documentation/stack-validation.txt.
> >    */
> >   static int validate_branch(struct objtool_file *file, struct symbol *func,
> > +               struct instruction *from,
>
> Maybe instead of passing a new instruction pointer, just the
> alt_group could be passed? 0 Meaning it was not in an alt group> 
> >                  struct instruction *first, struct insn_state state)
> >   {
> >       struct alternative *alt;
> > @@ -1953,7 +1956,9 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
> >       insn = first;
> >       sec = insn->sec;
> > -    if (insn->alt_group && list_empty(&insn->alts)) {
> > +    if (insn->alt_group &&
> > +        (!from || from->alt_group != insn->alt_group) &&
> > +        list_empty(&insn->alts)) {
>
> This would become
>
>      if (insn->alt_group != alt_group && list_empty(&insn->alts))
>
> And the recursive validate_branch() calls would just take
> insn->alt_group as parameter (and the calls in validate_functions()
> and validate_unwind_hints() would take 0).
>
> Any opinions on that?

Yes, that would work too. I choose to pass the instruction pointer because
I was thinking that having the "from" instruction might be useful in the
future if there's a need to do additional check about the origin of the
branch.

alex.


> >           WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
> >                 sec, insn->offset);
> >           return 1;
> > @@ -2035,7 +2040,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
> >                   if (alt->skip_orig)
> >                       skip_orig = true;
> > -                ret = validate_branch(file, func, alt->insn, state);
> > +                ret = validate_branch(file, func,
> > +                              NULL, alt->insn, state);
> >                   if (ret) {
> >                       if (backtrace)
> >                           BT_FUNC("(alt)", insn);
> > @@ -2105,7 +2111,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
> >                       return ret;
> >               } else if (insn->jump_dest) {
> > -                ret = validate_branch(file, func,
> > +                ret = validate_branch(file, func, insn,
> >                                 insn->jump_dest, state);
> >                   if (ret) {
> >                       if (backtrace)
> > @@ -2236,7 +2242,8 @@ static int validate_unwind_hints(struct objtool_file *file)
> >       for_each_insn(file, insn) {
> >           if (insn->hint && !insn->visited) {
> > -            ret = validate_branch(file, insn->func, insn, state);
> > +            ret = validate_branch(file, insn->func,
> > +                          NULL, insn, state);
> >               if (ret && backtrace)
> >                   BT_FUNC("<=== (hint)", insn);
> >               warnings += ret;
> > @@ -2377,7 +2384,7 @@ static int validate_functions(struct objtool_file *file)
> >               state.uaccess = func->uaccess_safe;
> > -            ret = validate_branch(file, func, insn, state);
> > +            ret = validate_branch(file, func, NULL, insn, state);
> >               if (ret && backtrace)
> >                   BT_FUNC("<=== (func)", insn);
> >               warnings += ret;
> > diff --git a/tools/objtool/check.h b/tools/objtool/check.h
> > index 6d875ca6fce0..cffb23d81782 100644
> > --- a/tools/objtool/check.h
> > +++ b/tools/objtool/check.h
> > @@ -33,7 +33,8 @@ struct instruction {
> >       unsigned int len;
> >       enum insn_type type;
> >       unsigned long immediate;
> > -    bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
> > +    unsigned int alt_group;
> > +    bool dead_end, ignore, hint, save, restore, ignore_alts;
> >       bool retpoline_safe;
> >       u8 visited;
> >       struct symbol *call_dest;
>
> Cheers,