From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta0.migadu.com (out-179.mta0.migadu.com [91.218.175.179]) (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 EB2E33DF002 for ; Wed, 5 Aug 2026 19:26:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785957968; cv=none; b=LtRHR07nWnkX5GLARlqzJ4CRSdZ9xZv9IDuxXHfGMfbfdoBoOcsEy5mioNNRfpChzJuNo8HF5UX3S33Xn7s18F7Y3JmhDMWfdUJ5MkL1plWyyBk3xcaUpmlVaTcxfN4GYcdLrelVqX7sjUXPnXR9Tpj710drPZ26G10lowdcBGA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785957968; c=relaxed/simple; bh=OpF20Har8RMO2prIXkdvhaJDmaTlkBoO/pB/CclzjSI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jhMp4a+wlCiAhCBxx5JV6NDmSNwDrpqnSw6+Tgn9tTVBLGjAwKrKtCPOLTgIriCyhyBUhtqFkeoaxAKWXyqsIkEaeD8KHewcvBGy8pBum79nc41X22hM0CdN0/K9Yd3KzlqyekkGBXR4RFlBAnVnpg3g151dXF6lZ1uJe1UdiM4= 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=vXrQzT6G; arc=none smtp.client-ip=91.218.175.179 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="vXrQzT6G" Message-ID: <5a47b073-e1d5-499c-89bd-44500a88bf27@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785957949; 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=GMNFF23M2mDpZsFVIBmMK7jwW4BARwxeY3eW7N4r6k4=; b=vXrQzT6G1ZN9r8MXV0XNMEf3UNCOXkTw9Jd8lUHGX6WHqbiOPgqv23Oh0vJ8iS3KFlgjL4 i6Nm9icPClPtBgjkM0fu8ZsfeCm/5th1HloCakQ+15qOkwz6egTRf8yA9CSMLvHtPYWUJI nFZ5npCkWAce08YaAQd5AKNlXnN9fGA= Date: Wed, 5 Aug 2026 12:25:36 -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> 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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. Thx -Vineet