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

[Vineet Gupta]

On 8/6/26 9:37 AM, Yonghong Song wrote:
> 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@xxxxxxxxx>
> > > > ---
> > > >     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?

Yes it does and also similarly enabled for cpu=v4.
The suggestion does look cleaner, but feels fragile.
Both forms of codegen are valid from selftest passing perspective.
BPF_FEATURE_ST implies compiler supports the feature but the actual 
generation is subject to optimization level etc.

Your call.

Thx,
-Vineet