Re: [RFC bpf-next v2 4/6] bpf: Provide bpf_copy_from_user() and bpf_copy_to_user().

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On 8/11/23 16:05, Stanislav Fomichev wrote:
On 08/10, thinker.li@xxxxxxxxx wrote:
From: Kui-Feng Lee <kuifeng@xxxxxxxx>

Provide bpf_copy_from_user() and bpf_copy_to_user() to the BPF programs
attached to cgroup/{set,get}sockopt. bpf_copy_to_user() is a new kfunc to
copy data from an kernel space buffer to a user space buffer. They are only
available for sleepable BPF programs. bpf_copy_to_user() is only available
to the BPF programs attached to cgroup/getsockopt.

Signed-off-by: Kui-Feng Lee <thinker.li@xxxxxxxxx>
---
  kernel/bpf/cgroup.c  |  6 ++++++
  kernel/bpf/helpers.c | 31 +++++++++++++++++++++++++++++++
  2 files changed, 37 insertions(+)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 5bf3115b265c..c15a72860d2a 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -2461,6 +2461,12 @@ cg_sockopt_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
  #endif
  	case BPF_FUNC_perf_event_output:
  		return &bpf_event_output_data_proto;
+
+	case BPF_FUNC_copy_from_user:
+		if (prog->aux->sleepable)
+			return &bpf_copy_from_user_proto;
+		return NULL;

If we just allow copy to/from, I'm not sure I understand how the buffer
sharing between sleepable/non-sleepable works.

Let's assume I have two progs in the chain:
1. non-sleepable - copies the buffer, does some modifications; since
    we don't copy the buffer back after every prog run, the modifications
    stay in the kernel buffer
2. sleepable - runs and just gets the user pointer? does it mean this
   sleepable program doesn't see the changes from (1)?

IOW, do we need some custom sockopt copy_to/from that handle this
potential buffer location transparently or am I missing something?

Assuming we want to support this at all. If we do, might deserve a
selftest.

It is why BPF_SOCKOPT_FLAG_OPTVAL_USER is there.
It helps programs to make a right decision.
However, I am going to remove bpf_copy_from_user()
since we have bpf_so_optval_copy_to() and bpf_so_optval_copy_to_r().
Does it make sense to you?




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