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

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Date:   Fri, 15 Jan 2021 09:32:42 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
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 Fri, Jan 15, 2021 at 12:31:26PM +0100, Miroslav Benes wrote:
> 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?

Fixed.

>  
> > 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>

Thanks!

> 
> 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).

Right, let us hope so :-)

-- 
Josh