linux-kernel - Re: [PATCH v5 4/6] livepatch: reuse module loader code to write relocations

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Date:	Mon, 21 Mar 2016 11:46:51 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Jessica Yu <jeyu@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jiri Kosina <jikos@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Miroslav Benes <mbenes@...e.cz>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 4/6] livepatch: reuse module loader code to write
 relocations

On Mon, Mar 21, 2016 at 05:31:57PM +0100, Petr Mladek wrote:
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index 780f00c..2aa20fa 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > +static int klp_resolve_symbols(Elf_Shdr *relasec, struct module *pmod)
> > +{
> > +	int i, cnt, vmlinux, ret;
> > +	struct klp_buf bufs = {0};
> > +	Elf_Rela *relas;
> > +	Elf_Sym *sym;
> > +	char *symname;
> > +	unsigned long sympos;
> > +
> > +	relas = (Elf_Rela *) relasec->sh_addr;
> > +	/* For each rela in this klp relocation section */
> > +	for (i = 0; i < relasec->sh_size / sizeof(Elf_Rela); i++) {
> > +		sym = pmod->core_kallsyms.symtab + ELF_R_SYM(relas[i].r_info);
> > +		if (sym->st_shndx != SHN_LIVEPATCH)
> > +			return -EINVAL;
> > +
> > +		klp_clear_buf(&bufs);
> > +
> > +		/* Format: .klp.sym.objname.symbol_name,sympos */
> > +		symname = pmod->core_kallsyms.strtab + sym->st_name;
> > +		cnt = sscanf(symname, ".klp.sym.%64[^.].%128[^,],%lu",
> > +			     bufs.objname, bufs.symname, &sympos);
> 
> Note that MODULE_NAME_LEN even is not 64. It is defined by:
> 
> #define MAX_PARAM_PREFIX_LEN (64 - sizeof(unsigned long))
> 
> I strongly suggest to use the proposal from Josh.

Hm, looks like my suggestion to use __stringify(MODULE_NAME_LEN) doesn't
work.  It results in the string "MODULE_NAME_LEN".  Which surprises me:
isn't is supposed to resolve the macro before applying the '#' operation
to it?

I was going to suggest another idea: hard-code it at 63 and then do
something like

  BUILD_BUG_ON(MODULE_NAME_LEN != 64)

But you're right... it's not even 64!

Need to think on this some more...

-- 
Josh