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

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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;




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