Re: [PATCH bpf-next] bpf: Fix build without BPF_SYSCALL, but with BPF_JIT.

["Paul E. McKenney" <paulmck@kernel.org>]

On Sun, Aug 30, 2020 at 01:43:28PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.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@intel.com>
> Fixes: 1e6c62a88215 ("bpf: Introduce sleepable BPF programs")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

A couple of nits below, but overall:

Acked-by: Paul E. McKenney <paulmck@kernel.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.

> +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(); }

>  #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
>  
>  #endif /* __LINUX_RCUPDATE_TRACE_H */
> -- 
> 2.23.0
>