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

[Pu Lehui]

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.

> >       if (bpf_trampoline_module_get(tr))
> >           return -ENOENT;