Re: BPF CO-RE and array fields in context struct

[Yonghong Song <yhs@fb.com>]

On 11/22/21 12:44 PM, YiFei Zhu wrote:
> On Mon, Nov 22, 2021 at 8:19 AM YiFei Zhu <zhuyifei@google.com> wrote:
>>
>> Hi
>>
>> I've been investigating the use of BPF CO-RE. I discovered that if I
>> include vmlinux.h and have all structures annotated with
>> __attribute__((preserve_access_index)), including the context struct,
>> then a prog that accesses an array field in the context struct, in
>> some particular way, cannot pass the verifier.
>>
>> A bunch of manual reduction plus creduce gives me this output:
>>
>>    struct bpf_sock_ops {
>>      int family;
>>      int remote_ip6[];
>>    } __attribute__((preserve_access_index));
>>    __attribute__((section("sockops"))) int b(struct bpf_sock_ops *d) {
>>      int a = d->family;
>>      int *c = d->remote_ip6;
>>      c[2] = a;
>>      return 0;
>>    }
>>
>> With Debian clang version 11.1.0-4+build1, this compiles to
>>
>>    0000000000000000 <b>:
>>           0: b7 02 00 00 04 00 00 00 r2 = 4
>>           1: bf 13 00 00 00 00 00 00 r3 = r1
>>           2: 0f 23 00 00 00 00 00 00 r3 += r2
>>           3: 61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
>>           4: 63 13 08 00 00 00 00 00 *(u32 *)(r3 + 8) = r1
>>           5: b7 00 00 00 00 00 00 00 r0 = 0
>>           6: 95 00 00 00 00 00 00 00 exit
>>
>> And the prog will be rejected with this verifier log:
>>
>>    ; __attribute__((section("sockops"))) int b(struct bpf_sock_ops *d) {
>>    0: (b7) r2 = 32
>>    1: (bf) r3 = r1
>>    2: (0f) r3 += r2
>>    last_idx 2 first_idx 0
>>    regs=4 stack=0 before 1: (bf) r3 = r1
>>    regs=4 stack=0 before 0: (b7) r2 = 32
>>    ; int a = d->family;
>>    3: (61) r1 = *(u32 *)(r1 +20)
>>    ; c[2] = a;
>>    4: (63) *(u32 *)(r3 +8) = r1
>>    dereference of modified ctx ptr R3 off=32 disallowed
>>    processed 5 insns (limit 1000000) max_states_per_insn 0 total_states
>> 0 peak_states 0 mark_read 0
>>
>> Looking at check_ctx_reg() and its callsite at check_mem_access() in
>> verifier.c, it seems that the verifier really does not like when the
>> context pointer has an offset, in this case the field offset of
>> d->remote_ip6.
>>
>> I thought this is just an issue with array fields, that field offset
>> relocations may have trouble expressing two field accesses (one struct
>> member, one array memory). However, further testing reveals that this
>> is not the case, because if I simplify out the local variables, the
>> error is gone:
>>
>>    struct bpf_sock_ops {
>>      int family;
>>      int remote_ip6[];
>>    } __attribute__((preserve_access_index));
>>    __attribute__((section("sockops"))) int b(struct bpf_sock_ops *d) {
>>      d->remote_ip6[2] = d->family;
>>      return 0;
>>    }
>>
>> is compiled to:
>>
>>    0000000000000000 <b>:
>>           0: 61 12 00 00 00 00 00 00 r2 = *(u32 *)(r1 + 0)
>>           1: 63 21 0c 00 00 00 00 00 *(u32 *)(r1 + 12) = r2
>>           2: b7 00 00 00 00 00 00 00 r0 = 0
>>           3: 95 00 00 00 00 00 00 00 exit
>>
>> and is loaded as:
>>
>>    ; d->remote_ip6[2] = d->family;
>>    0: (61) r2 = *(u32 *)(r1 +20)
>>    ; d->remote_ip6[2] = d->family;
>>    1: (63) *(u32 *)(r1 +40) = r2
>>    invalid bpf_context access off=40 size=4
>>
>> I believe this error is because d->remote_ip6 is read-only, that this
>> modification might be more of a product of creduce, but we can see
>> that the CO-RE adjected offset of the array element from the context
>> pointer is correct: 32 to remote_ip6, 8 array index, so total offset
>> is 40.
>>
>> Also note that removal of __attribute__((preserve_access_index)) from
>> the first (miscompiled) program produces exactly the same bytecode as
>> this new program (with no locals).
>>
>> What is going on here? Why does the access of an array in context in
>> this particular way cause it to generate code that would not pass the
>> verifier? Is it a bug in Clang/LLVM, or is it the verifier being too
>> strict?
> 
> Additionally, testing the latest LLVM main branch, this test case,
> which does not touch array fields at all, fails but succeeded with
> clang version 11.1.0:
> 
>    struct bpf_sock_ops {
>      int op;
>      int bpf_sock_ops_cb_flags;
>    } __attribute__((preserve_access_index));
>    enum { a, b } static (*c)() = (void *)9;
>    enum d { e } f;
>    enum d g;
>    __attribute__((section("sockops"))) int h(struct bpf_sock_ops *i) {
>      switch (i->op) {
>      case a:
>        f = g = c(i, i->bpf_sock_ops_cb_flags);
>        break;
>      case b:
>        f = g = c(i, i->bpf_sock_ops_cb_flags);
>      }
>      return 0;
>    }

This is another issue which actually appears (even in bpf mailing list)
multiple times.

The following change should fix the issue:

  $ diff t1.c t1-good.c
--- t1.c        2021-11-22 16:00:13.915921544 -0800
+++ t1-good.c   2021-11-22 16:12:32.823710102 -0800
@@ -5,13 +5,14 @@
    enum { a, b } static (*c)() = (void *)9;
    enum d { e } f;
    enum d g;
+  #define __barrier asm volatile("" ::: "memory")
    __attribute__((section("sockops"))) int h(struct bpf_sock_ops *i) {
      switch (i->op) {
      case a:
-      f = g = c(i, i->bpf_sock_ops_cb_flags);
+      f = g = c(i, i->bpf_sock_ops_cb_flags); __barrier;
        break;
      case b:
-      f = g = c(i, i->bpf_sock_ops_cb_flags);
+      f = g = c(i, i->bpf_sock_ops_cb_flags); __barrier;
      }
      return 0;
    }

Basically add a compiler barrier at the end of case statements
to prevent common code sinking.

In the above case, for the original code, latest compiler did an 
optimization like
      case a:
          tmp = reloc_offset(i->bpf_sock_ops_cb_flags);
      case b:
          tmp = reloc_offset(i->bpf_sock_ops_cb_flags);
    common:
      val = load r1, tmp
      ...

Note that reloc_offset is not sinked to the common code
due to its special internal representation.

To avoid such a code generation, add compiler barrier to
the end of case statement to prevent load sinking in which case
we will have
     val = load r1, reloc_offset(...)
and verifier will be happy about this.

The commit you listed below had a big change which may enable
the above compiler optimization and llvm11 may just not do
the code sinking optimization in this particular instance.

I guess the compiler could still enforce this. But since it does
not know whether the memory access is for context or not, doing
so might hurt performance. But any way, this has appeared multiple
times internally and also in the mailing list. We will take a further
look.

> 
> The bad code generation of latest LLVM:
> 
>    0000000000000000 <h>:
>           0: 61 12 00 00 00 00 00 00 r2 = *(u32 *)(r1 + 0)
>           1: 15 02 01 00 01 00 00 00 if r2 == 1 goto +1 <LBB0_2>
>           2: 55 02 0b 00 00 00 00 00 if r2 != 0 goto +11 <LBB0_3>
> 
>    0000000000000018 <LBB0_2>:
>           3: b7 03 00 00 04 00 00 00 r3 = 4
>           4: bf 12 00 00 00 00 00 00 r2 = r1
>           5: 0f 32 00 00 00 00 00 00 r2 += r3
>           6: 61 22 00 00 00 00 00 00 r2 = *(u32 *)(r2 + 0)
>           7: 85 00 00 00 09 00 00 00 call 9
>           8: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
>          10: 63 01 00 00 00 00 00 00 *(u32 *)(r1 + 0) = r0
>          11: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
>          13: 63 01 00 00 00 00 00 00 *(u32 *)(r1 + 0) = r0
> 
>    0000000000000070 <LBB0_3>:
>          14: b7 00 00 00 00 00 00 00 r0 = 0
>          15: 95 00 00 00 00 00 00 00 exit
> 
> The good code generation of LLVM 11.1.0:
> 
>    0000000000000000 <h>:
>           0: 61 12 00 00 00 00 00 00 r2 = *(u32 *)(r1 + 0)
>           1: 25 02 08 00 01 00 00 00 if r2 > 1 goto +8 <LBB0_2>
>           2: 61 12 04 00 00 00 00 00 r2 = *(u32 *)(r1 + 4)
>           3: 85 00 00 00 09 00 00 00 call 9
>           4: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
>           6: 63 01 00 00 00 00 00 00 *(u32 *)(r1 + 0) = r0
>           7: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
>           9: 63 01 00 00 00 00 00 00 *(u32 *)(r1 + 0) = r0
> 
>    0000000000000050 <LBB0_2>:
>          10: b7 00 00 00 00 00 00 00 r0 = 0
>          11: 95 00 00 00 00 00 00 00 exit
> 
> A bisect points me to this commit in LLVM as the first bad commit:
> 
>    commit 54d9f743c8b0f501288119123cf1828bf7ade69c
>    Author: Yonghong Song <yhs@fb.com>
>    Date:   Wed Sep 2 22:56:41 2020 -0700
> 
>        BPF: move AbstractMemberAccess and PreserveDIType passes to
> EP_EarlyAsPossible
> 
>        Move abstractMemberAccess and PreserveDIType passes as early as
>        possible, right after clang code generation.
> 
>    [...]
> 
>        Differential Revision: https://reviews.llvm.org/D87153
> 
> YiFei Zhu
>