From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F038737F30E for ; Thu, 6 Aug 2026 16:51:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786035098; cv=none; b=K1CVGlc3eGTHiQRjR9BQ40CUpefu7B3nBuMR6+qJ2Clu2rp+ZiDORvfEY7BYEETd+cWw3bGiMSHJ7TQlUOWlaa+pP/PkWH8rcGYakVwxaKN8O7dbyhe8qhPFHv4FIldUCUwvfCsoEPLFuT556FkuM9dD19YtVEHRFYLdbMQagPE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786035098; c=relaxed/simple; bh=KThKQta56dMpvBu0TEFOgMfwqr0+qOm6N1vzNkcAPLc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=L6/PPuQ8/6+xbEG8drmrv9aKXaa/0cVT+iVFh/9OjQs7lBozBo4kTyPSDw+6hvODWyDinbBjgYwYqNoCCGKm5qJFgtC9l4BYE9af39d3u6WeAG6vAUz+CQN1QUCV9PlpnyPfzkV1HYiz9Sp2piizTFaWZV1CvXjZEcIRCJJGeLg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=giIUdzs4; arc=none smtp.client-ip=91.218.175.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="giIUdzs4" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1786035082; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=TryxzgWqZQI2DT9mmhsWxctZDISmjHPgtJQJ++MzMOk=; b=giIUdzs4m7dIlg6x1lIVqFRLAdwaVaOkJZ9NWp+DDwNbUy6Mf/vkkHz29cJPC7H8wcJLzG 5ikDMWfYoWCTDqkD/dBEZpnHWpp5XeN4zPPoB7W33g+DOBwdgCOANZF30O5V7ZvlCLRsgX BIBp/nRd7VP2YrU8id3BiVKYZEPyU0A= Date: Thu, 6 Aug 2026 09:50:59 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [bpf-next 1/4] selftests/bpf: map_kptr: force BPF_STX for the scalar store to kptr To: Yonghong Song , bpf@vger.kernel.org, ast@kernel.org, Eduard Zingerman , Andrii Nakryiko , Ihor Solodrai Cc: linux-kernel@vger.kernel.org References: <20260803170251.1898102-1-vineet.gupta@linux.dev> <20260803170251.1898102-2-vineet.gupta@linux.dev> <5a47b073-e1d5-499c-89bd-44500a88bf27@linux.dev> <66f7ffe9-b8f3-4369-996a-967636b60401@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Vineet Gupta Content-Language: en-US In-Reply-To: <66f7ffe9-b8f3-4369-996a-967636b60401@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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 >>>> --- >>>>    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