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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
These go through different rejection paths and output different
messages.
- BPF_STX goes through map_kptr_match_type(), which prints
"invalid kptr access, R...".
- BPF_ST only gets the immediate check printing
"BPF_ST imm must be 0 when storing to kptr"
The test only expects the BPF_STX message, so it fails on a toolchain
that folds the constant - bpf-gcc, and clang -mcpu=v4:
7: (7a) *(u64 *)(r0 +8) = 195936478
BPF_ST imm must be 0 when storing to kptr at off=8
...
EXPECTED SUBSTR: 'invalid kptr access, R'
Pick the expected message with __BPF_FEATURE_ST, which clang and bpf-gcc
both define exactly when BPF_ST codegen is available - cpuv4 for clang,
and by default for bpf-gcc, whose default cpu is v4.
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
Two caveats worth noting:
- On a BPF_ST toolchain the test now only exercises the imm != 0 check
and never reaches map_kptr_match_type(), so the scalar-vs-PTR_TO_BTF_ID
rejection the test is named for is only covered by the non-ST builds.
The imm path itself is already covered compiler-independently by
verifier/map_kptr.c ("map_kptr: BPF_ST imm != 0").
- __BPF_FEATURE_ST says the compiler *can* emit BPF_ST, not that it will.
The encoding also depends on the optimization level: clang -mcpu=v4 -O0
still emits BPF_STX, which would send the #ifdef down the wrong branch
and fail the test. Selftests always build BPF objects at -O2 so this
does not bite today, but it is a latent failure mode if that changes.
Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20260807204434.1036279-2-vineet.gupta@linux.dev
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