Re: [PATCH V3 3/9] objtool: Add support for intra-function calls

Julien Thierry
2020-04-14T12:07:57Z

Hi Alexandre,

On 4/14/20 11:36 AM, Alexandre Chartre wrote:
> Change objtool to support intra-function calls. On x86, an intra-function
> call is represented in objtool as a push onto the stack (of the return
> address), and a jump to the destination address. That way the stack
> information is correctly updated and the call flow is still accurate.
>
> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
> ---
> include/linux/frame.h | 11 ++
> .../Documentation/stack-validation.txt | 8 ++
> tools/objtool/arch/x86/decode.c | 6 +
> tools/objtool/check.c | 126 ++++++++++++++----
> tools/objtool/check.h | 1 +
> 5 files changed, 128 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/frame.h b/include/linux/frame.h
> index 02d3ca2d9598..c7178e6c9d48 100644
> --- a/include/linux/frame.h
> +++ b/include/linux/frame.h
> @@ -15,9 +15,20 @@
> static void __used __section(.discard.func_stack_frame_non_standard) \
> *__func_stack_frame_non_standard_##func = func
>
> +/*
> + * This macro indicates that the following intra-function call is valid.
> + * Any non-annotated intra-function call will cause objtool to issue warning.
> + */
> +#define ANNOTATE_INTRA_FUNCTION_CALL \
> + 999: \
> + .pushsection .discard.intra_function_call; \

Very nit-pickish but my brain wants to add an 's' to that section name.
Like for the sections "unwind_hints" or "altinstructions".

> + .long 999b; \
> + .popsection;
> +
> #else /* !CONFIG_STACK_VALIDATION */
>
> #define STACK_FRAME_NON_STANDARD(func)
> +#define ANNOTATE_INTRA_FUNCTION_CALL
>
> #endif /* CONFIG_STACK_VALIDATION */
>
> diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
> index de094670050b..09f863fd32d2 100644
> --- a/tools/objtool/Documentation/stack-validation.txt
> +++ b/tools/objtool/Documentation/stack-validation.txt
> @@ -290,6 +290,14 @@ they mean, and suggestions for how to fix them.
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
>
>
> +9. file.o: warning: unsupported intra-function call
> +
> + This warning means that a direct call is done to a destination which
> + is not at the beginning of a function. If this is a legit call, you
> + can remove this warning by putting the ANNOTATE_INTRA_FUNCTION_CALL
> + directive right before the call.
> +
> +
> 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/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> index a62e032863a8..f4d70b8835c4 100644
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -437,6 +437,12 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
>
> case 0xe8:
> *type = INSN_CALL;
> + /*
> + * For the impact on the stack, a call behaves like
> + * a push of an immediate value (the return address).
> + */
> + op->src.type = OP_SRC_CONST;
> + op->dest.type = OP_DEST_PUSH;
> break;
>
> case 0xfc:
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 8f0525d5d895..ad362c5de281 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -654,44 +654,58 @@ static int add_jump_destinations(struct objtool_file *file)
> return 0;
> }
>
> +static int configure_call(struct objtool_file *file, struct instruction *insn)

The name "configure_call()" is a bit vague. What about naming this
set_local_call_destination() ? (Since it deals with call destinations
that don't need relocations and sets their actual target destination).

> +{
> + unsigned long dest_off;
> +
> + dest_off = insn->offset + insn->len + insn->immediate;
> + insn->call_dest = find_func_by_offset(insn->sec, dest_off);
> + if (!insn->call_dest)
> + insn->call_dest = find_symbol_by_offset(insn->sec, dest_off);
> +
> + if (insn->call_dest) {
> + /* regular call */
> + if (insn->func && insn->call_dest->type != STT_FUNC) {
> + WARN_FUNC("unsupported call to non-function",
> + insn->sec, insn->offset);
> + return -1;
> + }
> + return 0;
> + }
> +
> + /* intra-function call */
> + if (!insn->intra_function_call)
> + WARN_FUNC("intra-function call", insn->sec, insn->offset);
> +
> + dest_off = insn->offset + insn->len + insn->immediate;
> + insn->jump_dest = find_insn(file, insn->sec, dest_off);
> + if (!insn->jump_dest) {
> + WARN_FUNC("can't find call dest at %s+0x%lx",
> + insn->sec, insn->offset,
> + insn->sec->name, dest_off);
> + return -1;
> + }
> +
> + return 0;
> +}
> +
> /*
> * Find the destination instructions for all calls.
> */
> static int add_call_destinations(struct objtool_file *file)
> {
> struct instruction *insn;
> - unsigned long dest_off;
> struct rela *rela;
>
> for_each_insn(file, insn) {
> - if (insn->type != INSN_CALL)
> + if (insn->type != INSN_CALL || insn->ignore)
> continue;
>
> rela = find_rela_by_dest_range(file->elf, insn->sec,
> insn->offset, insn->len);
> if (!rela) {
> - dest_off = insn->offset + insn->len + insn->immediate;
> - insn->call_dest = find_func_by_offset(insn->sec, dest_off);
> - if (!insn->call_dest)
> - insn->call_dest = find_symbol_by_offset(insn->sec, dest_off);
> -
> - if (insn->ignore)
> - continue;
> -
> - if (!insn->call_dest) {
> - WARN_FUNC("unsupported intra-function call",
> - insn->sec, insn->offset);
> - if (retpoline)
> - WARN("If this is a retpoline, please patch it in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE.");
> - return -1;
> - }
> -
> - if (insn->func && insn->call_dest->type != STT_FUNC) {
> - WARN_FUNC("unsupported call to non-function",
> - insn->sec, insn->offset);
> + if (configure_call(file, insn))
> return -1;
> - }
> -
> } else if (rela->sym->type == STT_SECTION) {
> insn->call_dest = find_func_by_offset(rela->sym->sec,
> rela->addend+4);
> @@ -1337,6 +1351,42 @@ static int read_retpoline_hints(struct objtool_file *file)
> return 0;
> }
>
> +static int read_intra_function_call(struct objtool_file *file)
> +{
> + struct section *sec;
> + struct instruction *insn;
> + struct rela *rela;
> +
> + sec = find_section_by_name(file->elf,
> + ".rela.discard.intra_function_call");
> + if (!sec)
> + return 0;
> +
> + list_for_each_entry(rela, &sec->rela_list, list) {
> + if (rela->sym->type != STT_SECTION) {
> + WARN("unexpected relocation symbol type in %s",
> + sec->name);
> + return -1;
> + }
> +
> + insn = find_insn(file, rela->sym->sec, rela->addend);
> + if (!insn) {
> + WARN("bad .discard.intra_function_call entry");
> + return -1;
> + }
> +
> + if (insn->type != INSN_CALL) {
> + WARN_FUNC("intra_function_call not a direct call",
> + insn->sec, insn->offset);
> + return -1;
> + }
> +
> + insn->intra_function_call = true;

Maybe you could directly lookup the target instruction here and set
insn->jump_dest here.

Then you wouldn't need as much changes in add_call_destination(), just
adding the following in the "!rela" branch:

if (insn->intra_function_call)
continue;

> + }
> +
> + return 0;
> +}
> +
> static void mark_rodata(struct objtool_file *file)
> {
> struct section *sec;
> @@ -1392,6 +1442,10 @@ static int decode_sections(struct objtool_file *file)
> if (ret)
> return ret;
>
> + ret = read_intra_function_call(file);
> + if (ret)
> + return ret;
> +
> ret = add_call_destinations(file);
> if (ret)
> return ret;
> @@ -2155,7 +2209,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
> return ret;
>
> if (!no_fp && func && !is_fentry_call(insn) &&
> - !has_valid_stack_frame(&state)) {
> + !has_valid_stack_frame(&state) &&
> + !insn->intra_function_call) {
> WARN_FUNC("call without frame pointer save/setup",
> sec, insn->offset);
> return 1;
> @@ -2164,6 +2219,29 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
> if (dead_end_function(file, insn->call_dest))
> return 0;
>
> + if (insn->intra_function_call) {
> + /*
> + * The call instruction can update the stack
> + * state. Then make the intra-function call
> + * behaves like and unconditional jump.
> + */
> + ret = update_insn_state(insn, &state);
> + if (ret)
> + return ret;
> +
> + ret = validate_branch(file, func, insn,
> + insn->jump_dest, state);
> + if (ret) {
> + if (backtrace) {
> + BT_FUNC("(intra-function call)",
> + insn);
> + }
> + return ret;
> + }
> +
> + return 0;
> + }
> +
> break;
>
> case INSN_JUMP_CONDITIONAL:
> diff --git a/tools/objtool/check.h b/tools/objtool/check.h
> index 953db38dfc35..6a80903fc4aa 100644
> --- a/tools/objtool/check.h
> +++ b/tools/objtool/check.h
> @@ -36,6 +36,7 @@ struct instruction {
> int alt_group;
> bool dead_end, ignore, ignore_alts;
> bool hint, save, restore;
> + bool intra_function_call;
> bool retpoline_safe;
> u8 visited;
> struct symbol *call_dest;
>

Cheers,

--
Julien Thierry