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Date:   Sun, 30 Aug 2020 17:53:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     davem@...emloft.net, daniel@...earbox.net, josef@...icpanda.com,
        netdev@...r.kernel.org, bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH bpf-next] bpf: Fix build without BPF_SYSCALL, but with
 BPF_JIT.

On Sun, Aug 30, 2020 at 03:03:13PM -0700, Paul E. McKenney wrote:
> On Sun, Aug 30, 2020 at 01:43:28PM -0700, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@...nel.org>
> > 
> > When CONFIG_BPF_SYSCALL is not set, but CONFIG_BPF_JIT=y
> > the kernel build fails:
> > In file included from ../kernel/bpf/trampoline.c:11:
> > ../kernel/bpf/trampoline.c: In function ‘bpf_trampoline_update’:
> > ../kernel/bpf/trampoline.c:220:39: error: ‘call_rcu_tasks_trace’ undeclared
> > ../kernel/bpf/trampoline.c: In function ‘__bpf_prog_enter_sleepable’:
> > ../kernel/bpf/trampoline.c:411:2: error: implicit declaration of function ‘rcu_read_lock_trace’
> > ../kernel/bpf/trampoline.c: In function ‘__bpf_prog_exit_sleepable’:
> > ../kernel/bpf/trampoline.c:416:2: error: implicit declaration of function ‘rcu_read_unlock_trace’
> > 
> > Add these functions to rcupdate_trace.h.
> > The JIT won't call them and BPF trampoline logic won't be used without BPF_SYSCALL.
> > 
> > Reported-by: kernel test robot <lkp@...el.com>
> > Fixes: 1e6c62a88215 ("bpf: Introduce sleepable BPF programs")
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> 
> A couple of nits below, but overall:
> 
> Acked-by: Paul E. McKenney <paulmck@...nel.org>
> 
> > ---
> >  include/linux/rcupdate_trace.h | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
> > index d9015aac78c6..334840f4f245 100644
> > --- a/include/linux/rcupdate_trace.h
> > +++ b/include/linux/rcupdate_trace.h
> > @@ -82,7 +82,19 @@ static inline void rcu_read_unlock_trace(void)
> >  void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
> >  void synchronize_rcu_tasks_trace(void);
> >  void rcu_barrier_tasks_trace(void);
> > -
> > +#else
> 
> This formulation is a bit novel for RCU.  Could we therefore please add
> a comment something like this?
> 
> // The BPF JIT forms these addresses even when it doesn't call these
> // functions, so provide definitions that result in runtime errors.

ok. will add.
The root of the problem is:
obj-$(CONFIG_BPF_JIT) += trampoline.o
obj-$(CONFIG_BPF_JIT) += dispatcher.o
There is a number of functions that arch/x86/net/bpf_jit_comp.c is
using from these two files, but none of them will be used when
only cBPF is on (which is the case for BPF_SYSCALL=n BPF_JIT=y).
Don't confuse cBPF with eBPF ;)

This patch is imo the lesser of three evils. The other two:
- some serious refactoring of trampoline.c and dipsatcher.c into
  multiple files
- add 'depends on BPF_SYSCALL' to 'config BPF_JIT' in net/Kconfig

> 
> > +static inline void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func)
> > +{
> > +	BUG();
> > +}
> > +static inline void rcu_read_lock_trace(void)
> > +{
> > +	BUG();
> > +}
> > +static inline void rcu_read_unlock_trace(void)
> > +{
> > +	BUG();
> > +}
> 
> People have been moving towards one-liner for things like these last two:
> 
> static inline void rcu_read_lock_trace(void) { BUG(); }
> static inline void rcu_read_unlock_trace(void) { BUG(); }

sure. will respin.

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