linux-kernel - Re: [PATCH] objtool: include symbol value in check for contiguous objects

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Date:   Mon, 17 May 2021 17:47:58 +0200
From:   Vanessa Hack <vanessa.hack@....de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Jonas Rabenstein <rabenstein@...fau.de>
Subject: Re: [PATCH] objtool: include symbol value in check for contiguous
 objects

Hi Josh,

I think there might have been a problem with my mail client, so I’m sorry
if this has been sent twice.

> I believe add_jump_table() -- in addition to all the other jump table
> code -- assumes that reloc->sym is a section symbol (STT_SECTION),
> rather than a function symbol (STT_FUNC).  Was it an STT_FUNC symbol
> which caused the problem?

Indeed as you proposed it was a symbol with another type than STT_SECTION 
that caused the problem. This resulted of our mistake of not having compiled 
our code using -fno-pic and the fact, that it was i386 code as we are trying to 
adapt objtool for our 32 bit operating system. Consequently, local labels for a 
switch jump table were created, which the existing objtool code seems not to 
be intended to handle. 

> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 5e5388a38e2a..4f30a763a4e3 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1344,6 +1344,10 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
> 		if (prev_offset && reloc->offset != prev_offset + 8)
> 			break;
> 
> +		/* Jump table relocs are always STT_SECTION: */
> +		if (reloc->sym->type != STT_SECTION)
> +			break;
> +
> 		/* Detect function pointers from contiguous objects: */
> 		if (reloc->sym->sec == pfunc->sec &&
> 		    reloc->addend == pfunc->offset)

Your suggested patch would have helped us detecting the problem earlier and 
seems like a good idea to check.

Regards,
Vanessa