On Mon, 17 Jul 2023 at 23:17, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Mon, Jul 17, 2023 at 9:47 AM Kumar Kartikeya Dwivedi
> <memxor@xxxxxxxxx> wrote:
> >
> > On Sat, 15 Jul 2023 at 04:17, Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > On Thu, Jul 13, 2023 at 08:02:30AM +0530, Kumar Kartikeya Dwivedi wrote:
> > > > By default, the subprog generated by the verifier to handle a thrown
> > > > exception hardcodes a return value of 0. To allow user-defined logic
> > > > and modification of the return value when an exception is thrown,
> > > > introduce the bpf_set_exception_callback kfunc, which installs a
> > > > callback as the default exception handler for the program.
> > > >
> > > > Compared to runtime kfuncs, this kfunc acts a built-in, i.e. it only
> > > > takes semantic effect during verification, and is erased from the
> > > > program at runtime.
> > > >
> > > > This kfunc can only be called once within a program, and always sets the
> > > > global exception handler, regardless of whether it was invoked in all
> > > > paths of the program or not. The kfunc is idempotent, and the default
> > > > exception callback cannot be modified at runtime.
> > > >
> > > > Allowing modification of the callback for the current program execution
> > > > at runtime leads to issues when the programs begin to nest, as any
> > > > per-CPU state maintaing this information will have to be saved and
> > > > restored. We don't want it to stay in bpf_prog_aux as this takes a
> > > > global effect for all programs. An alternative solution is spilling
> > > > the callback pointer at a known location on the program stack on entry,
> > > > and then passing this location to bpf_throw as a parameter.
> > > >
> > > > However, since exceptions are geared more towards a use case where they
> > > > are ideally never invoked, optimizing for this use case and adding to
> > > > the complexity has diminishing returns.
> > >
> > > Right. No run-time changes pls.
> > >
> >
> > +1
> >
> > > Instead of bpf_set_exception_callback() how about adding a
> > > btf_tag("exception_handler") or better name
> > > and check that such global func is a single func in a program and
> > > it's argument is a single u64.
> > >
> >
> > That does seem better. Also, a conditional bpf_set_exception_callback
> > taking effect globally may be confusing for users.
> > I will switch to the BTF tag.
> >
> > Any specific reason it has to be a global func and cannot have static linkage?
>
> The compiler will warn about the unused static function.
> Even if we silence the warn somehow the verifier will not verify that
> static unused subprog.
Ah, right. Makes sense. I will change this in v2.
