Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

[Peter Zijlstra]

On Tue, Mar 01, 2022 at 11:49:05AM +0900, Masami Hiramatsu wrote:
> > +static kprobe_opcode_t *
> > +_kprobe_addr(kprobe_opcode_t *addr, const char *symbol_name,
> > +	     unsigned long offset, bool *on_func_entry)
> >  {
> >  	if ((symbol_name && addr) || (!symbol_name && !addr))
> >  		goto invalid;
> >  
> >  	if (symbol_name) {
> > +		/*
> > +		 * Input: @sym + @offset
> > +		 * Output: @addr + @offset
> > +		 *
> > +		 * NOTE: kprobe_lookup_name() does *NOT* fold the offset
> > +		 *       argument into it's output!
> > +		 */
> >  		addr = kprobe_lookup_name(symbol_name, offset);
> 
> Hmm, there are 2 issues.
> 
> - the 'addr' includes the 'offset' here.

AFAICT it doesn't (I ever wrote that in the comment on top). There's two
implementations of kprobe_lookup_name(), the weak version doesn't even
use the offset argument, and the PowerPC implementation only checks for
!offset and doesn't fold it.

> - the 'offset' is NOT limited under the symbol size.
>   (e.g. symbol_name = "_text" and @offset points the offset of target symbol from _text)
> 
> This means we need to call kallsyms_lookup_size_offset() in this case too.

I'm feeling we should error out in that case. Using sym+offset beyond
the limits of sym is just daft.

But if you really want/need to retain that, then yes, we need that
else branch unconditionally :/