From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta1.migadu.com (mta1.migadu.com [37.59.57.117]) (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 470D23CF05D for ; Wed, 5 Aug 2026 17:48:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=37.59.57.117 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785952099; cv=none; b=XX1Ir2Qh9ZdO687m4J3NukAZwghV2fZQOhrqObPc+NxPzdDvT/gmboJhT9HvJyByWlV3zG5Tylel717fUyESQZjj76nY181OOGhnajYMDTuh94eikzlxpCedS2zAFQjzhvonBEmwY/vwUl2yiJRQ02JoCP5iYlRmKKV4R95qBwo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785952099; c=relaxed/simple; bh=wFePT5+Yd7Tp3foLToQuH9d38IUyxkn5QQmpAY2+fEo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=NXM+Bx3d4YN3woBNQH+kPrpOI+mvxKW18zl4ts32e85+VmgNMmlEpL+2J+Fl7FzG5+WAuxCgqAWEnZj34aIXZHcRqZXWZQqrx34sMHeAN5pbHw6B+kzre4N+MFonlk3tOvMCta+v7rBptNkBm2e5kgI487EhOxvT8iOFHIYrZEg= 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=DviHr3Tg; arc=none smtp.client-ip=37.59.57.117 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="DviHr3Tg" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785952095; 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=My2Outn5Pkhz5i5qO6fw/vJqKE6oWiD3uq/q9fPYqqw=; b=DviHr3TgZ6EzbwCCc0xzaWFKEzzRU0KpOI1fgVtnVEb8Mvu1TFaVOlOBxaxPGBr/xK0k/J pJWSsbmyXNSdSwE7jVgAKWzEjcm1KrcVgsKZSL+pyjq9ol92bMdfwncWPyvLRSCriFRG5c Or46taz2Dey9X9CXGpQl8arrWPHFMDE= Date: Wed, 5 Aug 2026 10:47:47 -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 Content-Language: en-GB To: Vineet Gupta , 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> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <20260803170251.1898102-2-vineet.gupta@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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? > return 0; > } >