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

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Date:   Thu, 18 Mar 2021 21:29:23 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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);

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

-- 
Josh