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.
