Re: [PATCH RFC] bpf: Fix trampoline for functions with variable arguments

On Tue, May 04, 2021 at 09:11:26PM -0700, Alexei Starovoitov wrote:

SNIP

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actualy looks like we need to disable functions with variable arguments
completely, because we don't know how many arguments to save

I tried to disable them in pahole and it's easy fix, will post new fix

Can we still allow access to fixed arguments for such functions and
just disallow the vararg ones?

the problem is that we should save all the registers for arguments,
which is probably doable.. but if caller uses more than 6 arguments,
we need stack data, which will be wrong because of the extra stack
frame we do in bpf trampoline.. so we could crash

the patch below prevents to attach these functions directly in kernel,
so we could keep these functions in BTF

jirka


---

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 0600ed325fa0..f9709dc08c44 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -5213,6 +5213,13 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
                                tname, i, btf_kind_str[BTF_INFO_KIND(t->info)]);
                        return -EINVAL;
                }
+               if (ret == 0) {
+                       bpf_log(log,
+                               "The function %s has variable args, it's unsupported.\n",
+                               tname);
+                       return -EINVAL;
+
+               }

this will work, but the explicit check for vararg should be `i ==
nargs - 1 && args[i].type == 0`. Everything else (if it happens) is
probably a bad BTF data.

Jiri,
could you please resubmit with the check like Andrii suggested?
Thanks!

yes, will send it later today

jirka