cilium, build: Use -ftrap-function=__undefined_trap

We use __builtin_trap() in various places to trigger a compilation
error when code is reachable where it is not expected:

   # define __bpf_unreachable() __builtin_trap()

Recently, in LLVM [0] the latter is being mapped into __bpf_trap()
kfunc which also means that we do not get the desired behavior for
__builtin_trap() in Cilium anymore once we upgrade LLVM.

To work around this, use -ftrap-function=__undefined_trap to remap:

  $ cat test_trap.c
  int buz(int p) { __builtin_trap(); return 0; }
  $ clang --target=bpf -O2 -g -c test_trap.c
  $ clang --target=bpf -O2 -ftrap-function=__undefined_trap -g -c test_trap.c
  test_trap.c:1:18: error: A call to built-in function '__undefined_trap' is not supported.
      1 | int buz(int p) { __builtin_trap(); return 0; }
        |                  ^
  1 error generated.
  $

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: llvm/llvm-project#131731 [0]

Author: borkmann

bpf/Makefile.bpf

@@ -6,6 +6,7 @@ FLAGS := -I$(ROOT_DIR)/bpf -I$(ROOT_DIR)/bpf/include -O2 -g
 CLANG_FLAGS := ${FLAGS} --target=bpf -std=gnu89 -nostdinc
 # eBPF verifier enforces unaligned access checks where necessary, so don't
 # let clang complain too early.
+CLANG_FLAGS += -ftrap-function=__undefined_trap
 CLANG_FLAGS += -Wall -Wextra -Werror -Wshadow
 CLANG_FLAGS += -Wno-address-of-packed-member
 CLANG_FLAGS += -Wno-unknown-warning-option

pkg/datapath/loader/compile.go

@@ -99,6 +99,7 @@ type directoryInfo struct {
 var (
 	StandardCFlags = []string{"-O2", "--target=bpf", "-std=gnu99",
 		"-nostdinc",
+		"-ftrap-function=__undefined_trap",
 		"-Wall", "-Wextra", "-Werror", "-Wshadow",
 		"-Wno-address-of-packed-member",
 		"-Wno-unknown-warning-option",