On 09/14/2021 03:30 PM, Daniel Borkmann wrote:

On 9/11/21 3:56 AM, Tiezhu Yang wrote:

quoted

[...]

quoted

With this patch, it does not change the current limit 33, 
MAX_TAIL_CALL_CNT
can reflect the actual max tail call count, the tailcall selftests 
can work
well, and also the above failed testcase in test_bpf can be fixed for 
the
interpreter (all archs) and the JIT (all archs except for x86).

  # uname -m
  x86_64
  # echo 1 > /proc/sys/net/core/bpf_jit_enable
  # modprobe test_bpf
  # dmesg | grep -w FAIL
  Tail call error path, max count reached jited:1 ret 33 != 34 FAIL

Could you also state in here which archs you have tested with this 
change? I
presume /every/ arch which has a JIT?

OK, will do it in v3.
I have tested on x86 and mips.

quoted

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---

v2:
   -- fix the typos in the commit message and update the commit message.
   -- fix the failed tailcall selftests for x86 jit.
      I am not quite sure the change on x86 is proper, with this change,
      tailcall selftests passed, but tailcall limit test in test_bpf.ko
      failed, I do not know the reason now, I think this is another 
issue,
      maybe someone more versed in x86 jit could take a look.

There should be a series from Johan coming today with regards to 
test_bpf.ko
that will fix the "tail call error path, max count reached" test which 
had an
assumption in that R0 would always be valid for the fall-through and 
could be
passed to the bpf_exit insn whereas it is not guaranteed and verifier, 
for
example, forbids a subsequent access to R0 w/o reinit. For your 
testing, I
would suggested to recheck once this series is out.

I will test the following patch on x86 and mips:

[PATCH bpf v4 13/14] bpf/tests: Fix error in tail call limit tests

[...]

quoted

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 0fe6aac..74a9e61 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c

@@ -402,7 +402,7 @@ static int get_pop_bytes(bool *callee_regs_used)
   * ... bpf_tail_call(void *ctx, struct bpf_array *array, u64 index) 

...
   *   if (index >= array->map.max_entries)
   *     goto out;
- *   if (++tail_call_cnt > MAX_TAIL_CALL_CNT)
+ *   if (tail_call_cnt++ == MAX_TAIL_CALL_CNT)

Why such inconsistency to e.g. above with arm64 case but also compared to
x86 32 bit which uses JAE? If so, we should cleanly follow the reference
implementation (== interpreter) _everywhere_ and _not_ introduce 
additional
variants/implementations across JITs.

In order tokeep consistencyand make as few changes as possible,
<javascript:void(0);>I will modify the check condition as follows:

#define MAX_TAIL_CALL_CNT 33
(1) for x86, arm64, ... (0 ~ 32)
tcc = 0;
if (tcc == MAX_TAIL_CALL_CNT)
     goto out;
tcc++;

(2) for mips, riscv (33 ~ 1)
tcc = MAX_TAIL_CALL_CNT;
if (tcc == 0)
     goto out;
tcc--;

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