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Date:   Thu, 28 Apr 2022 11:02:35 +0100
From:   Qais Yousef <qais.yousef@arm.com>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Delyan Kratunov <delyank@fb.com>,
        Namhyung Kim <namhyung@kernel.org>,
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Subject: Re: [PATCH] sched/tracing: append prev_state to tp args instead
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On 04/27/22 11:17, Andrii Nakryiko wrote:
> On Wed, Apr 27, 2022 at 3:35 AM Qais Yousef <qais.yousef@arm.com> wrote:
> >
> > On 04/26/22 08:54, Andrii Nakryiko wrote:
> > > On Tue, Apr 26, 2022 at 7:10 AM Qais Yousef <qais.yousef@arm.com> wrote:
> > > >
> > > > On 04/26/22 14:28, Peter Zijlstra wrote:
> > > > > On Fri, Apr 22, 2022 at 11:30:12AM -0700, Alexei Starovoitov wrote:
> > > > > > On Fri, Apr 22, 2022 at 10:22 AM Delyan Kratunov <delyank@fb.com> wrote:
> > > > > > >
> > > > > > > On Fri, 2022-04-22 at 13:09 +0200, Peter Zijlstra wrote:
> > > > > > > > And on the other hand; those users need to be fixed anyway, right?
> > > > > > > > Accessing prev->__state is equally broken.
> > > > > > >
> > > > > > > The users that access prev->__state would most likely have to be fixed, for sure.
> > > > > > >
> > > > > > > However, not all users access prev->__state. `offcputime` for example just takes a
> > > > > > > stack trace and associates it with the switched out task. This kind of user
> > > > > > > would continue working with the proposed patch.
> > > > > > >
> > > > > > > > If bpf wants to ride on them, it needs to suffer the pain of doing so.
> > > > > > >
> > > > > > > Sure, I'm just advocating for a fairly trivial patch to avoid some of the suffering,
> > > > > > > hopefully without being a burden to development. If that's not the case, then it's a
> > > > > > > clear no-go.
> > > > > >
> > > > > >
> > > > > > Namhyung just sent this patch set:
> > > > > > https://patchwork.kernel.org/project/netdevbpf/patch/20220422053401.208207-3-namhyung@kernel.org/
> > > > >
> > > > > That has:
> > > > >
> > > > > + * recently task_struct->state renamed to __state so it made an incompatible
> > > > > + * change.
> > > > >
> > > > > git tells me:
> > > > >
> > > > >   2f064a59a11f ("sched: Change task_struct::state")
> > > > >
> > > > > is almost a year old by now. That don't qualify as recently in my book.
> > > > > That says that 'old kernels used to call this...'.
> > > > >
> > > > > > to add off-cpu profiling to perf.
> > > > > > It also hooks into sched_switch tracepoint.
> > > > > > Notice it deals with state->__state rename just fine.
> > > > >
> > > > > So I don't speak BPF much; it always takes me more time to make bpf work
> > > > > than to just hack up the kernel, which makes it hard to get motivated.
> > > > >
> > > > > However, it was not just a rename, state changed type too, which is why I
> > > > > did the rename, to make sure all users would get a compile fail and
> > > > > could adjust.
> > > > >
> > > > > If you're silently making it work by frobbing the name, you loose that.
> > > > >
> > > > > Specifically, task_struct::state used to be 'volatile long', while
> > > > > task_struct::__state is 'unsigned int'. As such, any user must now be
> > > > > very careful to use READ_ONCE(). I don't see that happening with just
> > > > > frobbing the name.
> > > > >
> > > > > Additinoally, by shrinking the field, I suppose BE systems get to keep
> > > > > the pieces?
> > > > >
> > > > > > But it will have a hard time without this patch
> > > > > > until we add all the extra CO-RE features to detect
> > > > > > and automatically adjust bpf progs when tracepoint
> > > > > > arguments order changed.
> > > > >
> > > > > Could be me, but silently making it work sounds like fail :/ There's a
> > > > > reason code changes, users need to adapt, not silently pretend stuff is
> > > > > as before.
> > > > >
> > > > > How will you know you need to fix your tool?
> > > >
> > > > If libbpf doesn't fail, then yeah it's a big problem. I wonder how users of
> > > > kprobe who I suppose are more prone to this kind of problems have been coping.
> > >
> > > See my reply to Peter. libbpf can't know user's intent to fail this
> > > automatically, in general. In some cases when it can it does
> > > accommodate this automatically. In other cases it provides instruments
> > > for user to handle this (bpf_core_field_size(),
> > > BPF_CORE_READ_BITFIELD(), etc).
> >
> > My naiive thinking is that the function signature has changed (there's 1 extra
> > arg not just a subtle swap of args of the same type) - so I thought that can be
> > detected. But maybe it is harder said than done.
> 
> It is. We don't have number of arguments either:
> 
> struct bpf_raw_tracepoint_args {
>         __u64 args[0];
> };
> 
> What BPF program is getting is just an array of u64s.
> 
> >
> > I am trying to remember as I've used this before; I think you get the arg list
> > as part of ctx when you attach to a function?
> >
> > I wonder if it'd be hard to provide a macro for the user to provide the
> > signature of the function they expect; this macro can try then to verify/assert
> > the number, type and order is the same. Not bullet proof and requires opt-in,
> > but could be useful?
> >
> >
> > // dummy pseudo-code
> >
> >         BPF_CORE_ASSERT_SIG(sched_switch, NR_ARGS, ARG0, ARG1, ...)
> >                 if (ctx->nr_args != NR_ARGS)
> >                         assert()
> >                 if (type_of(ctx->args[0]) != type_of(ARG0))
> >                         assert()
> >                 ...
> >
> > I'm not sure if you have any info about the type though..
> 
> What we have now under discussion is more generic way for user to
> check signature of function prototype, struct/union, etc. But all that
> will take some time to implement and finalize. So this patch is a way
> to stop/prevent the bleeding until we have that available to users.

Okay good to know. Alexei mentioned a plan, but I didn't get that it included
signature verification.

Cheers

--
Qais Yousef