linux-kernel - Re: [PATCH v2 11/14] objtool: Add elf_create_undef

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Message-ID: <YFRZOZtZrRdsLSwP@hirez.programming.kicks-ass.net>
Date:   Fri, 19 Mar 2021 08:56:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, jgross@...e.com, mbenes@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 11/14] objtool: Add elf_create_undef_symbol()

On Thu, Mar 18, 2021 at 09:29:23PM -0500, Josh Poimboeuf wrote:
> On Thu, Mar 18, 2021 at 06:11:14PM +0100, Peter Zijlstra wrote:
> > Allow objtool to create undefined symbols; this allows creating
> > relocations to symbols not currently in the symbol table.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  tools/objtool/elf.c                 |   63 ++++++++++++++++++++++++++++++++++++
> >  tools/objtool/include/objtool/elf.h |    1 
> >  2 files changed, 64 insertions(+)
> > 
> > --- a/tools/objtool/elf.c
> > +++ b/tools/objtool/elf.c
> > @@ -724,6 +724,69 @@ static int elf_strtab_concat(struct elf
> >  	return len;
> >  }
> >  
> > +struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
> > +{
> > +	struct section *symtab;
> > +	struct symbol *sym;
> > +	Elf_Data *data;
> > +	Elf_Scn *s;
> > +
> > +	sym = malloc(sizeof(*sym));
> > +	if (!sym) {
> > +		perror("malloc");
> > +		return NULL;
> > +	}
> > +	memset(sym, 0, sizeof(*sym));
> > +
> > +	sym->name = strdup(name);
> > +
> > +	sym->sym.st_name = elf_strtab_concat(elf, sym->name, NULL);
> > +	if (sym->sym.st_name == -1)
> > +		return NULL;
> > +
> > +	sym->sym.st_info = 0x10; /* STB_GLOBAL, STT_NOTYPE */
> 
> There's a generic macro for this:
> 	
> 	sym->sym.st_info = GELF_ST_INFO(STB_GLOBAL, STT_NOTYPE);

Ah, I remember not finding that many moons ago when I wrote that ..

> And sym->bind and sym->type should probably get set.

They are, it's in that elf_add_symbol() thing.