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SubjectRe: [PATCH 2/7] objtool: Allow branches within the same alternative.
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Hi Alexandre,

I ran into the same issue for the arm64 work:
https://lkml.org/lkml/2020/1/9/656

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@oracle.com>
> ---
> 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?

> 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,

--
Julien Thierry

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