Re: [PATCH v2 35/62] objtool: Refactor add_jump_destinations()

[Joe Lawrence <joe.lawrence@redhat.com>]

On 5/9/25 4:16 PM, Josh Poimboeuf wrote:
> The add_jump_destinations() logic is a bit weird and convoluted after
> being incrementally tweaked over the years.  Refactor it to hopefully be
> more logical and straightforward.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
>  tools/objtool/check.c               | 227 +++++++++++++---------------
>  tools/objtool/include/objtool/elf.h |   4 +-
>  2 files changed, 104 insertions(+), 127 deletions(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 66cbeebd16ea..e4ca5edf73ad 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1439,9 +1439,14 @@ static void add_return_call(struct objtool_file *file, struct instruction *insn,
>  }
>  
>  static bool is_first_func_insn(struct objtool_file *file,
> -			       struct instruction *insn, struct symbol *sym)
> +			       struct instruction *insn)
>  {
> -	if (insn->offset == sym->offset)
> +	struct symbol *func = insn_func(insn);
> +
> +	if (!func)
> +		return false;
> +
> +	if (insn->offset == func->offset)
>  		return true;
>  
>  	/* Allow direct CALL/JMP past ENDBR */
> @@ -1449,52 +1454,32 @@ static bool is_first_func_insn(struct objtool_file *file,
>  		struct instruction *prev = prev_insn_same_sym(file, insn);
>  
>  		if (prev && prev->type == INSN_ENDBR &&
> -		    insn->offset == sym->offset + prev->len)
> +		    insn->offset == func->offset + prev->len)
>  			return true;
>  	}
>  
>  	return false;
>  }
>  
> -/*
> - * A sibling call is a tail-call to another symbol -- to differentiate from a
> - * recursive tail-call which is to the same symbol.
> - */
> -static bool jump_is_sibling_call(struct objtool_file *file,
> -				 struct instruction *from, struct instruction *to)
> -{
> -	struct symbol *fs = from->sym;
> -	struct symbol *ts = to->sym;
> -
> -	/* Not a sibling call if from/to a symbol hole */
> -	if (!fs || !ts)
> -		return false;
> -
> -	/* Not a sibling call if not targeting the start of a symbol. */
> -	if (!is_first_func_insn(file, to, ts))
> -		return false;
> -
> -	/* Disallow sibling calls into STT_NOTYPE */
> -	if (is_notype_sym(ts))
> -		return false;
> -
> -	/* Must not be self to be a sibling */
> -	return fs->pfunc != ts->pfunc;
> -}
> -
>  /*
>   * Find the destination instructions for all jumps.
>   */
>  static int add_jump_destinations(struct objtool_file *file)
>  {
> -	struct instruction *insn, *jump_dest;
> +	struct instruction *insn;
>  	struct reloc *reloc;
> -	struct section *dest_sec;
> -	unsigned long dest_off;
>  	int ret;
>  
>  	for_each_insn(file, insn) {
>  		struct symbol *func = insn_func(insn);
> +		struct instruction *dest_insn;
> +		struct section *dest_sec;
> +		struct symbol *dest_sym;
> +		unsigned long dest_off;
> +		bool dest_undef = false;
> +
> +		if (!is_static_jump(insn))
> +			continue;
>  
>  		if (insn->jump_dest) {
>  			/*
> @@ -1503,129 +1488,121 @@ static int add_jump_destinations(struct objtool_file *file)
>  			 */
>  			continue;
>  		}
> -		if (!is_static_jump(insn))
> -			continue;
>  
>  		reloc = insn_reloc(file, insn);
>  		if (!reloc) {
>  			dest_sec = insn->sec;
>  			dest_off = arch_jump_destination(insn);
> -		} else if (is_sec_sym(reloc->sym)) {
> +		} else if (is_undef_sym(reloc->sym)) {
> +			dest_sym = reloc->sym;
> +			dest_undef = true;
> +		} else {
>  			dest_sec = reloc->sym->sec;
> -			dest_off = arch_insn_adjusted_addend(insn, reloc);
> -		} else if (reloc->sym->retpoline_thunk) {
> +			dest_off = reloc->sym->sym.st_value +
> +				   arch_insn_adjusted_addend(insn, reloc);
> +		}
> +
> +		if (!dest_undef) {
> +			dest_insn = find_insn(file, dest_sec, dest_off);
> +			if (!dest_insn) {
> +				struct symbol *sym = find_symbol_by_offset(dest_sec, dest_off);
> +
> +				/*
> +				 * retbleed_untrain_ret() jumps to
> +				 * __x86_return_thunk(), but objtool can't find
> +				 * the thunk's starting RET instruction,
> +				 * because the RET is also in the middle of
> +				 * another instruction.  Objtool only knows
> +				 * about the outer instruction.
> +				 */
> +				if (sym && sym->embedded_insn) {
> +					add_return_call(file, insn, false);
> +					continue;
> +				}
> +
> +				/*
> +				 * GCOV/KCOV dead code can jump to the end of
> +				 * the function/section.
> +				 */
> +				if (file->ignore_unreachables && func &&
> +				    dest_sec == insn->sec &&
> +				    dest_off == func->offset + func->len)
> +					continue;
> +
> +				ERROR_INSN(insn, "can't find jump dest instruction at %s+0x%lx",
> +					  dest_sec->name, dest_off);
> +				return -1;
> +			}
> +
> +			dest_sym = dest_insn->sym;
> +			if (!dest_sym)
> +				goto set_jump_dest;
> +		}
> +
> +		if (dest_sym->retpoline_thunk) {
>  			ret = add_retpoline_call(file, insn);
>  			if (ret)
>  				return ret;
>  			continue;
> -		} else if (reloc->sym->return_thunk) {
> +		}
> +
> +		if (dest_sym->return_thunk) {
>  			add_return_call(file, insn, true);
>  			continue;
> -		} else if (func) {
> -			/*
> -			 * External sibling call or internal sibling call with
> -			 * STT_FUNC reloc.
> -			 */
> -			ret = add_call_dest(file, insn, reloc->sym, true);
> -			if (ret)
> -				return ret;
> -			continue;
> -		} else if (reloc->sym->sec->idx) {
> -			dest_sec = reloc->sym->sec;
> -			dest_off = reloc->sym->sym.st_value +
> -				   arch_dest_reloc_offset(reloc_addend(reloc));

Way back in ("[PATCH v2 18/62] objtool: Fix x86 addend calculation"),
arch_dest_reloc_offset() was replaced with arch_insn_adjusted_addend(),
so I think that patch missed this callsite and breaks bisectability.

-- 
Joe