linux-kernel - Re: [PATCH] objtool: Don't fail on missing symbol table

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Message-ID: <alpine.LSU.2.21.2101151217000.4375@pobox.suse.cz>
Date:   Fri, 15 Jan 2021 12:31:26 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH] objtool: Don't fail on missing symbol table

On Thu, 14 Jan 2021, Josh Poimboeuf wrote:

> Thanks to a recent binutils change which doesn't generate unused
> symbols, it's now possible for thunk_64.o be completely empty with
> CONFIG_PREEMPTION: no text, no data, no symbols.

"without CONFIG_PREEMPTION", or did I misunderstand?
 
> We could edit the Makefile to only build that file when
> CONFIG_PREEMPTION is enabled, but that will likely create confusion
> if/when the thunks end up getting used by some other code again.
> 
> Just ignore it and move on.
> 
> Reported-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

Reviewed-by: Miroslav Benes <mbenes@...e.cz>

with the note below.

> ---
>  tools/objtool/elf.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index be89c741ba9a..2b0f4f52f7b5 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -380,8 +380,11 @@ static int read_symbols(struct elf *elf)
>  
>  	symtab = find_section_by_name(elf, ".symtab");
>  	if (!symtab) {
> -		WARN("missing symbol table");
> -		return -1;
> +		/*
> +		 * A missing symbol table is actually possible if it's an empty
> +		 * .o file.  This can happen for thunk_64.o.
> +		 */
> +		return 0;
>  	}

We rely on .symtab presence elsewhere in the code. See 
elf_create_{rel,rela}_reloc_section(). However, there should never be a 
problem. If there is a need to create a new reloc section (either for a 
static call site, or ORC), there should always be a symbol to create it 
for (or because of it).

Miroslav