Re: livepatch: reuse module loader code to write relocations

[Jessica Yu]

+++ Josh Poimboeuf [21/03/16 12:36 -0500]:
> On Mon, Mar 21, 2016 at 11:46:51AM -0500, Josh Poimboeuf wrote:
> > 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?
>
> Turns out I hadn't included module.h.  When I do so,
> __stringify(MODULE_NAME_LEN) becomes "(64 - sizeof(unsigned long))".
> Which is still not going to work :-/

Hm, we probably won't be able to make use of preprocessor tricks here,
since I don't think the preprocessor can even evaluate that expression
(esp. with that sizeof there). This might mean building the format
string at runtime, which may be more trouble than it's worth...

> > 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