[PATCH v1] LoongArch: BPF: Make arch_protect_bpf_trampoline() return 0

[Tiezhu Yang]

Occasionally, there exists "text_copy_cb: operation failed" when executing
bpf selftests, the reason is copy_to_kernel_nofault() failed and the ecode
of estat register is 0x4 (PME: Page Modification Exception) due to the pte
is not writeable, the root cause is that there is another place to set pte
readonly which is in the generic weak arch_protect_bpf_trampoline().

There are two ways to fix this race condition issue, the direct way is to
modify the generic weak arch_protect_bpf_trampoline() to add a mutex lock
for set_memory_rox(), but the other simple and proper way is to just make
arch_protect_bpf_trampoline() return 0 in the arch-specific code because
LoongArch already uses BPF prog pack allocator for trampoline.

Here are the trimmed kernel log messages:

  copy_to_kernel_nofault: memory access failed, ecode is 0x4
  copy_to_kernel_nofault: caller is text_copy_cb+0x50/0xa0
  text_copy_cb: operation failed
  ------------[ cut here ]------------
  bpf_prog_pack bug: missing bpf_arch_text_invalidate?
  WARNING: kernel/bpf/core.c:1008 at bpf_prog_pack_free+0x200/0x228
  ...
  Call Trace:
  [<9000000000248914>] show_stack+0x64/0x188
  [<9000000000241308>] dump_stack_lvl+0x6c/0x9c
  [<90000000002705bc>] __warn+0x9c/0x200
  [<9000000001c428c0>] __report_bug+0xa8/0x1c0
  [<9000000001c42b5c>] report_bug+0x64/0x120
  [<9000000001c7dcd0>] do_bp+0x270/0x3c0
  [<9000000000246f40>] handle_bp+0x120/0x1c0
  [<900000000047b030>] bpf_prog_pack_free+0x200/0x228
  [<900000000047b2ec>] bpf_jit_binary_pack_free+0x24/0x60
  [<900000000026989c>] bpf_jit_free+0x54/0xb0
  [<900000000029e10c>] process_one_work+0x184/0x610
  [<900000000029ef8c>] worker_thread+0x24c/0x388
  [<90000000002a902c>] kthread+0x13c/0x170
  [<9000000001c7dfe8>] ret_from_kernel_thread+0x28/0x1c0
  [<9000000000246624>] ret_from_kernel_thread_asm+0xc/0x88

  ---[ end trace 0000000000000000 ]---

Here is a simple shell script to reproduce:

  #!/bin/bash

  for ((i=1; i<=1000; i++))
  do
    echo "under testing $i ..."
    dmesg -c > /dev/null
    ./test_progs -t fentry_attach_stress > /dev/null
    dmesg -t | grep "text_copy_cb: operation failed"
    if [ $? -eq 0 ]; then
      break
    fi
  done

Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
 arch/loongarch/net/bpf_jit.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 3bd89f55960d..4f1af3b7a363 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -1568,6 +1568,11 @@ void arch_free_bpf_trampoline(void *image, unsigned int size)
 	bpf_prog_pack_free(image, size);
 }
 
+int arch_protect_bpf_trampoline(void *image, unsigned int size)
+{
+	return 0;
+}
+
 /*
  * Sign-extend the register if necessary
  */
-- 
2.42.0