Re: [PATCH] objtool: Teach get_alt_entry() about more relocation types

Re: [PATCH] objtool: Teach get_alt_entry() about more relocation types

[Nathan Chancellor]

On Thu, Sep 30, 2021 at 12:43:10PM +0200, Peter Zijlstra wrote:
> 
> Occasionally objtool encounters symbol (as opposed to section)
> relocations in .altinstructions. Typically they are the alternatives
> written by elf_add_alternative() as encountered on a noinstr
> validation run on vmlinux after having already ran objtool on the
> individual .o files.
> 
> Basically this is the counterpart of commit 44f6a7c0755d ("objtool:
> Fix seg fault with Clang non-section symbols"), because when these new
> assemblers (binutils now also does this) strip the section symbols,
> elf_add_reloc_to_insn() is forced to emit symbol based relocations.
> 
> As such, teach get_alt_entry() about different relocation types.
> 
> Fixes: 9bc0bb50727c ("objtool/x86: Rewrite retpoline thunk calls")
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Reported-by: Borislav Petkov <bp@alien8.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

This resolves the instance of the warning that I see with clang-12:

Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  tools/objtool/special.c |   32 +++++++++++++++++++++++++-------
>  1 file changed, 25 insertions(+), 7 deletions(-)
> 
> --- a/tools/objtool/special.c
> +++ b/tools/objtool/special.c
> @@ -58,6 +58,24 @@ void __weak arch_handle_alternative(unsi
>  {
>  }
>  
> +static bool reloc2sec_off(struct reloc *reloc, struct section **sec, unsigned long *off)
> +{
> +	switch (reloc->sym->type) {
> +	case STT_FUNC:
> +		*sec = reloc->sym->sec;
> +		*off = reloc->sym->offset + reloc->addend;
> +		return true;
> +
> +	case STT_SECTION:
> +		*sec = reloc->sym->sec;
> +		*off = reloc->addend;
> +		return true;
> +
> +	default:
> +		return false;
> +	}
> +}
> +
>  static int get_alt_entry(struct elf *elf, struct special_entry *entry,
>  			 struct section *sec, int idx,
>  			 struct special_alt *alt)
> @@ -91,15 +109,12 @@ static int get_alt_entry(struct elf *elf
>  		WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
>  		return -1;
>  	}
> -	if (orig_reloc->sym->type != STT_SECTION) {
> -		WARN_FUNC("don't know how to handle non-section reloc symbol %s",
> +	if (!reloc2sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off)) {
> +		WARN_FUNC("don't know how to handle reloc symbol type: %s",
>  			   sec, offset + entry->orig, orig_reloc->sym->name);
>  		return -1;
>  	}
>  
> -	alt->orig_sec = orig_reloc->sym->sec;
> -	alt->orig_off = orig_reloc->addend;
> -
>  	if (!entry->group || alt->new_len) {
>  		new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
>  		if (!new_reloc) {
> @@ -116,8 +131,11 @@ static int get_alt_entry(struct elf *elf
>  		if (arch_is_retpoline(new_reloc->sym))
>  			return 1;
>  
> -		alt->new_sec = new_reloc->sym->sec;
> -		alt->new_off = (unsigned int)new_reloc->addend;
> +		if (!reloc2sec_off(new_reloc, &alt->new_sec, &alt->new_off)) {
> +			WARN_FUNC("don't know how to handle reloc symbol type: %s",
> +				  sec, offset + entry->new, new_reloc->sym->name);
> +			return -1;
> +		}
>  
>  		/* _ASM_EXTABLE_EX hack */
>  		if (alt->new_off >= 0x7ffffff0)
>