Re: [PATCH v5 2/9] fprobe: Add ftrace based probe APIs

[Jiri Olsa]

On Tue, Jan 25, 2022 at 05:41:24PM +0100, Jiri Olsa wrote:
> On Tue, Jan 25, 2022 at 09:11:57PM +0900, Masami Hiramatsu wrote:
> 
> SNIP
> 
> > +
> > +/* Convert ftrace location address from symbols */
> > +static int convert_func_addresses(struct fprobe *fp)
> > +{
> > +	unsigned long addr, size;
> > +	unsigned int i;
> > +
> > +	/* Convert symbols to symbol address */
> > +	if (fp->syms) {
> > +		fp->addrs = kcalloc(fp->nentry, sizeof(*fp->addrs), GFP_KERNEL);
> > +		if (!fp->addrs)
> > +			return -ENOMEM;
> > +
> > +		for (i = 0; i < fp->nentry; i++) {
> > +			fp->addrs[i] = kallsyms_lookup_name(fp->syms[i]);
> > +			if (!fp->addrs[i])	/* Maybe wrong symbol */
> > +				goto error;
> > +		}
> > +	}
> > +
> > +	/* Convert symbol address to ftrace location. */
> > +	for (i = 0; i < fp->nentry; i++) {
> > +		if (!kallsyms_lookup_size_offset(fp->addrs[i], &size, NULL))
> > +			size = MCOUNT_INSN_SIZE;
> > +		addr = ftrace_location_range(fp->addrs[i], fp->addrs[i] + size);
> 
> you need to substract 1 from 'end' in here, as explained in
> __within_notrace_func comment:
> 
>         /*
>          * Since ftrace_location_range() does inclusive range check, we need
>          * to subtract 1 byte from the end address.
>          */
> 
> like in the patch below
> 
> also this convert is for archs where address from kallsyms does not match
> the real attach addresss, like for arm you mentioned earlier, right?
> 
> could we have that arch specific, so we don't have extra heavy search
> loop for archs that do not need it?

one more question..

I'm adding support for user to pass function symbols to bpf fprobe link
and I thought I'd pass symbols array to register_fprobe, but I'd need to
copy the whole array of strings from user space first, which could take
lot of memory considering attachment of 10k+ functions

so I'm thinking better way is to resolve symbols already in bpf fprobe
link code and pass just addresses to register_fprobe

I assume you want to keep symbol interface, right? could we have some
flag ensuring the conversion code is skipped, so we don't go through
it twice?

in any case I need addresses before I call register_fprobe, because
of the bpf cookies setup

thanks,
jirka