Re: [bpf-next 1/4] selftests/bpf: map_kptr: force BPF_STX for the scalar store to kptr

[Yonghong Song <yonghong.song@linux.dev>]

On 8/5/26 12:25 PM, Vineet Gupta wrote:
> On 8/5/26 10:47 AM, Yonghong Song wrote:
>>
>> On 8/3/26 10:02 AM, Vineet Gupta wrote:
>>> reject_scalar_store_to_kptr stores a scalar constant to a kptr field:
>>>
>>>           *(volatile u64 *)&v->unref_ptr = 0xBADC0DE;
>>>
>>> Compilers generate one of two encodings for that:
>>>
>>>    1. Materialize the constant into a register and emit BPF_STX:
>>>
>>>           r1 = 0xbadc0de
>>>           *(u64 *)(r0 + 0x8) = r1
>>>
>>>    2. Or fold it into a single BPF_ST (store immediate):
>>>
>>>           *(u64 *)(r0 + 0x8) = 0xbadc0de
>>>
>>> check_map_kptr_access() rejects both, but through very different 
>>> checks.
>>> BPF_STX goes through map_kptr_match_type(), whose first test is
>>> base_type(reg->type) != PTR_TO_BTF_ID - the scalar rejection this 
>>> test is
>>> named for - and which prints "invalid kptr access, R...". BPF_ST 
>>> only gets
>>> the trivial "BPF_ST imm must be 0 when storing to kptr" immediate 
>>> check and
>>> never reaches map_kptr_match_type() at all.
>>>
>>> So on a compiler that folds the constant - bpf-gcc, and clang from
>>> -mcpu=v4, which enabled BPF_ST around v4 support due to historical
>>> verifier limitations - the test fails against its expected message.
>>>
>>> Widening the __msg to accept either message would make it pass 
>>> again, but
>>> on those toolchains it would then only re-test the imm != 0 path, which
>>> verifier/map_kptr.c ("map_kptr: BPF_ST imm != 0") already covers, 
>>> and the
>>> scalar-vs-PTR_TO_BTF_ID check would lose its only test in the tree.
>>>
>>> Route the value through barrier_var() instead, so the store stays a
>>> BPF_STX everywhere and the test keeps asserting what it was written to
>>> assert. clang -mcpu=v1..v4 and bpf-gcc 16.1 all emit the register form
>>> afterwards.
>>>
>>>     bpf-gcc, before: #229/20 map_kptr/reject_scalar_store_to_kptr:FAIL
>>>     bpf-gcc, after : #229/20 map_kptr/reject_scalar_store_to_kptr:OK
>>>
>>> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
>>> ---
>>>    tools/testing/selftests/bpf/progs/map_kptr_fail.c | 11 ++++++++++-
>>>    1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/testing/selftests/bpf/progs/map_kptr_fail.c 
>>> b/tools/testing/selftests/bpf/progs/map_kptr_fail.c
>>> index f11848dfa78f..cb84e23b83c0 100644
>>> --- a/tools/testing/selftests/bpf/progs/map_kptr_fail.c
>>> +++ b/tools/testing/selftests/bpf/progs/map_kptr_fail.c
>>> @@ -390,13 +390,22 @@ __failure __msg("invalid kptr access, R")
>>>    int reject_scalar_store_to_kptr(struct __sk_buff *ctx)
>>>    {
>>>        struct map_value *v;
>>> +    u64 val = 0xBADC0DE;
>>>        int key = 0;
>>>           v = bpf_map_lookup_elem(&array_map, &key);
>>>        if (!v)
>>>            return 0;
>>>    -    *(volatile u64 *)&v->unref_ptr = 0xBADC0DE;
>>> +    /*
>>> +     * Keep the value in a register so this stays a BPF_STX and keeps
>>> +     * exercising map_kptr_match_type(). Compilers that fold the 
>>> constant
>>> +     * into a BPF_ST (store immediate) instead - bpf-gcc, and clang 
>>> from
>>> +     * -mcpu=v4 - would be rejected by the far weaker "BPF_ST imm 
>>> must be
>>> +     * 0" check, which verifier/map_kptr.c already covers.
>>> +     */
>>> +    barrier_var(val);
>>> +    *(volatile u64 *)&v->unref_ptr = val;
>> Could you explain in details why gcc16 (with cpuv4) won't work with 
>> BPF_ST?
>> What code gcc16 (with cpuv4) generates?
>
> GCC generates BPF_ST (and clang -mcpu=v4 does as well)
>
>    7: (7a) *(u64 *)(r0 +8) = 195936478
>    BPF_ST imm must be 0 when storing to kptr at off=8
>    processed 7 insns (limit 1000000) max_states_per_insn 0 total_states
>    0 peak_states 0 mark_read 0
>    =============
>    EXPECTED   SUBSTR: 'invalid kptr access, R'
>
>
> The issue is not codegen or BPF_ST vs. BPF_STX, its a deliberate bad 
> write to a pointer.
> The current __msg in test only matches v3 (STX) form. v4's 
> store-immediate hits a different verifier check with a different message.
> My first approach was to support both in the __msg, but then Claude 
> suggested to not do it that way because it would reduce coverage as 
> mentioned in the changelog above.

For clang side, we can do:

diff --git a/tools/testing/selftests/bpf/progs/map_kptr_fail.c b/tools/testing/selftests/bpf/progs/map_kptr_fail.c
index f11848dfa78f..b9f6d6764431 100644
--- a/tools/testing/selftests/bpf/progs/map_kptr_fail.c
+++ b/tools/testing/selftests/bpf/progs/map_kptr_fail.c
@@ -386,7 +386,11 @@ int kptr_xchg_possibly_null(struct __sk_buff *ctx)
  }
  
  SEC("?tc")
+#ifdef __BPF_FEATURE_ST
+__failure __msg("BPF_ST imm must be 0 when storing to kptr at off=8")
+#else
  __failure __msg("invalid kptr access, R")
+#endif
  int reject_scalar_store_to_kptr(struct __sk_buff *ctx)
  {
         struct map_value *v;
...

Note that __BPF_FEATURE_ST is only enabled for cpuv4 in clang.
Does gcc has similar macro like above __BPF_FEATURE_ST?

>
> Thx
> -Vineet