Subject: Re: bpf indirect calls

From: Matt Bobrowski <[email-masked]>
Date: Fri, 6 Oct 2023 09:36:05 +0000

On Fri, Sep 29, 2023 at 02:06:10PM -0700, Alexei Starovoitov wrote:
> On Wed, Sep 27, 2023 at 6:27 AM Matt Bobrowski <[email-masked]> wrote:
> > static void testing(void) {
> >   bpf_printk("testing");
> > }
> >
> > struct iter_ctx {
> >   void (*f) (void);
> > };
> > static u64 iter_callback(struct bpf_map *map, u32 *key,
> >                          u64 *value, struct iter_ctx *ctx) {
> >   if (ctx->f) {
> >     ctx->f();
> >   }
> >   return 0;
> > }
> >
> > SEC("lsm.s/file_open")
> > int BPF_PROG(file_open, struct file *file)
> > {
> >   struct iter_ctx iter_ctx = {
> >     .f = testing,
> >   };
> >   bpf_for_each_map_elem(&map, iter_callback, &iter_ctx, 0);
> >   return 0;
> > }
> > ```
> ...
> > The fundamental difference between the two call instructions if I'm
> > not mistaken is that one attempts to perform a call using an immediate
> > value as its source operand, whereas the other attempts to perform a
> > call using a source register as its source operand. AFAIU, the latter
> > is not currently permitted by the BPF verifier. Is that right?
> 
> Correct. Indirect calls via 'callx' instruction are not supported yet.
> Please use bpf_tail_call() as a workaround for now.

Noted.

> Over the years the verifier became progressively smarter and maybe
> now is a good time to support true indirect calls.

This is something that I wouldn't mind exploring myself as a little
research/contribution project. Would you object to me taking this on?
I feel as though this would give me an opportunity to develop a better
understanding when it comes to the internals of the BPF subsystem.

> For certain cases like your example above it's relatively easy to
> add such support, but before we do that please describe the full use
> case that you wanted to implement with indirect calls.

For the specific example I provided above, using indirect calls was an
approach that I considered using within one of our BPF programs in
order to work around this [0] specific BPF verifier shortcoming. For
the workaround, I needed to implement 2 BPF programs that more or less
done the same thing using the same set of routines, but differed ever
so slightly for one particular routine. The way I envisioned
controlling that one small difference between the 2 BPF programs is by
supplying in different function pointers within the iteration context
passed to bpf_for_each_map_elem(), but I quickly figured out that it
wasn't possible and now I'm left with some highly undersirable code
duplication across the 2 BPF programs. Understandably, this was an
incredibly trivial and niche use case where using indirect calls would
nicely solve this kind of problem, but I'm almost certain I could find
other places where using indirect calls would be considered incredibly
useful.

[0] https://lore.kernel.org/bpf/[email-masked]/

/M