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Message-ID: <20200428195301.szkrjfvrjahsbuhb@treble>
Date: Tue, 28 Apr 2020 14:53:01 -0500
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: alexandre.chartre@...cle.com, linux-kernel@...r.kernel.org,
jthierry@...hat.com, tglx@...utronix.de, x86@...nel.org,
mbenes@...e.cz
Subject: Re: [PATCH v2 01/14] objtool: Allow branches within the same
alternative.
On Tue, Apr 28, 2020 at 09:11:02PM +0200, Peter Zijlstra wrote:
> From: Alexandre Chartre <alexandre.chartre@...cle.com>
>
> 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@...cle.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20200414103618.12657-3-alexandre.chartre@oracle.com
I still don't like this patch, other than the "alt_group =
alt_group_next_index++" thing as a prerequisite for the next patch.
Instead of all this, let's just get rid of the
"don't know how to handle branch to middle of alternative instruction group"
warning, which I think we decided is useless and not worth the effort
anyway.
> ---
> tools/objtool/check.c | 26 ++++++++++++++++++++------
> tools/objtool/check.h | 3 ++-
> 2 files changed, 22 insertions(+), 7 deletions(-)
>
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -770,7 +770,9 @@ static int handle_group_alt(struct objto
> 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;
> @@ -779,7 +781,7 @@ static int handle_group_alt(struct objto
> 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;
> }
>
> @@ -813,6 +815,7 @@ static int handle_group_alt(struct objto
> }
>
> last_new_insn = NULL;
> + alt_group = alt_group_next_index++;
> insn = *new_insn;
> sec_for_each_insn_from(file, insn) {
> if (insn->offset >= special_alt->new_off + special_alt->new_len)
> @@ -822,6 +825,7 @@ static int handle_group_alt(struct objto
>
> insn->ignore = orig_insn->ignore_alts;
> insn->func = orig_insn->func;
> + insn->alt_group = alt_group;
>
> /*
> * Since alternative replacement code is copy/pasted by the
> @@ -2163,6 +2167,15 @@ static int validate_return(struct symbol
> return 0;
> }
>
> +static bool is_branch_to_alternative(struct instruction *from,
> + struct instruction *to)
> +{
> + if (!from || !to->alt_group || !list_empty(&to->alts))
> + return false;
> +
> + return (from->alt_group != to->alt_group);
> +}
> +
> /*
> * Follow the branch starting at the given instruction, and recursively follow
> * any other branches (jumps). Meanwhile, track the frame pointer state at
> @@ -2170,6 +2183,7 @@ static int validate_return(struct symbol
> * tools/objtool/Documentation/stack-validation.txt.
> */
> static int validate_branch(struct objtool_file *file, struct symbol *func,
> + struct instruction *from,
> struct instruction *insn, struct insn_state state)
> {
> struct alternative *alt;
> @@ -2180,7 +2194,7 @@ static int validate_branch(struct objtoo
>
> sec = insn->sec;
>
> - if (insn->alt_group && list_empty(&insn->alts)) {
> + if (is_branch_to_alternative(from, insn)) {
> WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
> sec, insn->offset);
> return 1;
> @@ -2227,7 +2241,7 @@ static int validate_branch(struct objtoo
> 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);
> @@ -2271,7 +2285,7 @@ static int validate_branch(struct objtoo
>
> } else if (insn->jump_dest) {
> ret = validate_branch(file, func,
> - insn->jump_dest, state);
> + insn, insn->jump_dest, state);
> if (ret) {
> if (backtrace)
> BT_FUNC("(branch)", insn);
> @@ -2402,7 +2416,7 @@ static int validate_unwind_hints(struct
>
> while (&insn->list != &file->insn_list && (!sec || insn->sec == sec)) {
> 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;
> @@ -2543,7 +2557,7 @@ static int validate_symbol(struct objtoo
>
> state->uaccess = sym->uaccess_safe;
>
> - ret = validate_branch(file, insn->func, insn, *state);
> + ret = validate_branch(file, insn->func, NULL, insn, *state);
> if (ret && backtrace)
> BT_FUNC("<=== (sym)", insn);
> return ret;
> --- a/tools/objtool/check.h
> +++ b/tools/objtool/check.h
> @@ -30,12 +30,13 @@ struct instruction {
> unsigned int len;
> enum insn_type type;
> unsigned long immediate;
> - bool alt_group, dead_end, ignore, ignore_alts;
> + bool dead_end, ignore, ignore_alts;
> bool hint;
> bool retpoline_safe;
> s8 instr;
> u8 visited;
> u8 ret_offset;
> + int alt_group;
> struct symbol *call_dest;
> struct instruction *jump_dest;
> struct instruction *first_jump_src;
>
>
--
Josh
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