Re: [RFC PATCH RESEND bpf-next 1/4] bpf: Rollback to text_poke when arch not supported ftrace direct call

[Pu Lehui]

On 2023/1/3 20:05, Björn Töpel wrote:
> Pu Lehui <pulehui@xxxxxxxxxxxxxxx> writes:
>
> > On 2022/12/20 10:32, Xu Kuohai wrote:
> > > On 12/20/2022 10:13 AM, Pu Lehui wrote:
> > > > From: Pu Lehui <pulehui@xxxxxxxxxx>
> > > >
> > > > The current bpf trampoline attach to kernel functions via ftrace direct
> > > > call API, while text_poke is applied for bpf2bpf attach and tail call
> > > > optimization. For architectures that do not support ftrace direct call,
> > > > text_poke is still able to attach bpf trampoline to kernel functions.
> > > > Let's relax it by rollback to text_poke when architecture not supported.
> > > >
> > > > Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
> > > > ---
> > > >    kernel/bpf/trampoline.c | 8 ++------
> > > >    1 file changed, 2 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > > > index d6395215b849..386197a7952c 100644
> > > > --- a/kernel/bpf/trampoline.c
> > > > +++ b/kernel/bpf/trampoline.c
> > > > @@ -228,15 +228,11 @@ static int modify_fentry(struct bpf_trampoline
> > > > *tr, void *old_addr, void *new_ad
> > > >    static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
> > > >    {
> > > >        void *ip = tr->func.addr;
> > > > -    unsigned long faddr;
> > > >        int ret;
> > > > -    faddr = ftrace_location((unsigned long)ip);
> > > > -    if (faddr) {
> > > > -        if (!tr->fops)
> > > > -            return -ENOTSUPP;
> > > > +    if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS) &&
> > > > +        !!ftrace_location((unsigned long)ip))
> > > >            tr->func.ftrace_managed = true;
> > > > -    }
> > >
> > > After this patch, a kernel function with true trace_location will be
> > > patched
> > > by bpf when CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS is disabled, which
> > > means
> > > that a kernel function may be patched by both bpf and ftrace in a mutually
> > > unaware way. This will cause ftrace and bpf_arch_text_poke to fail in a
> > > somewhat random way if the function to be patched was already patched
> > > by the other.
> >
> > Thanks for your review. And yes, this is a backward compatible solution
> > for architectures that not support DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
>
> It's not "backward compatible". Reiterating what Kuohai said; The BPF
> trampoline must be aware of ftrace's state -- with this patch, the
> trampoline can't blindly poke the text managed my ftrace.
>
> I'd recommend to remove this patch from the series.

After deep consideration, Kuohai's catching is much more reasonable. 
Will remove it in the next.

In the meantime, I found that song and guoren have worked on supporting 
riscv ftrace with direct call [0], so we can concentrate on making 
bpf_arch_text_poke specifically for the bpf context.

However, riscv ftrace base framework will change because [0] uses t0 as 
the link register of traced function. We should consider the generality 
of riscv bpf trampoline for kernel function and bpf context. It's not 
clear if [0] will be upstreamed, so maybe we should wait for it?

[0] 
https://lore.kernel.org/linux-riscv/20221208091244.203407-7-guoren@xxxxxxxxxx

Anyway, thanks both of you for the review.

> Björn