Messages in this thread | | | Date | Fri, 6 Jan 2023 10:35:57 +0800 | Subject | Re: [RFC PATCH RESEND bpf-next 1/4] bpf: Rollback to text_poke when arch not supported ftrace direct call | From | Pu Lehui <> |
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On 2023/1/3 20:05, Björn Töpel wrote: > Pu Lehui <pulehui@huaweicloud.com> writes: > >> On 2022/12/20 10:32, Xu Kuohai wrote: >>> On 12/20/2022 10:13 AM, Pu Lehui wrote: >>>> From: Pu Lehui <pulehui@huawei.com> >>>> >>>> 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@huawei.com> >>>> --- >>>> 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@kernel.org
Anyway, thanks both of you for the review.
> > Björn
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