Re: [PATCH bpf-next v1 08/11] bpf: Report arena faults to BPF stderr

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-05-09 at 22:01 +0200, Kumar Kartikeya Dwivedi wrote:

[...]

> > >  bool ex_handler_bpf(const struct exception_table_entry *x, struct pt_regs *regs)
> > >  {
> > > -     u32 reg = x->fixup >> 8;
> > > +     u32 arena_reg = (x->fixup >> 8) & 0xff;
> > > +     bool is_arena = !!arena_reg;
> > > +     u32 reg = x->fixup >> 16;
> > > +     unsigned long addr;
> > > +
> > > +     /* Read here, if src_reg is dst_reg for load, we'll write 0 to it. */
> > > +     if (is_arena)
> > > +             addr = *(unsigned long *)((void *)regs + arena_reg);
> > 
> > Is it necessary to also take offset into account when calculating address?
> > 
> 
> Not sure what you mean? "arena_reg" is basically the offset of the
> register holding the arena address within pt_regs.

Arena access is translated as an instruction with three operands, e.g.:

  `movzx <dst>, byte ptr [<src> + r12 + <off>]`

As far as I understand the code, currently `addr` takes into account
`<src>` value, but not the `<off>` value.

[...]