From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-186.mta0.migadu.com (out-186.mta0.migadu.com [91.218.175.186]) (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 AC7BC3624CF for ; Thu, 6 Aug 2026 16:37:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.186 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786034278; cv=none; b=gXqPIlqj7akujrmV/BjGIQ824vyXYJNCOlXhtVUSVf6jSdqkaBaALWpbs+aibjdxfc0fA58w/fV5bZi4aFimX0yzsdG5i5ImYMJWwwdQ3bebMETBAZHMFQ7O6eqYKWiRylcRTmLroU1rmEwE/ChUshVrlnMw2QjB5Hp3+Moh9ek= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786034278; c=relaxed/simple; bh=Gdf90uai8UNLUYccETYocbfdf4J9IKR52Id7sJSaZdE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=j3C0CUH1SIpXBtxTjfbC26UB3GWozfUNKJ8a7C3y3CbUDKmSucouChJl+FMMyutddXiZvRZ7KLHb8S/c4ZNOPLOJdQ0dLR2kQ2bz6ECaV/SklytZ4zg3P2mRS8DNK2TI3jjCnnu6Cu6IerpsBDr9bZH8VjNhTERebn6wcPQQZFU= 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=CF/1tVh9; arc=none smtp.client-ip=91.218.175.186 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="CF/1tVh9" Message-ID: <66f7ffe9-b8f3-4369-996a-967636b60401@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1786034264; 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=en/NbZmtc4s/0RzdihM4A9GFcVk7yx+CiNiS9pOQgCU=; b=CF/1tVh9BDJ1wHG3r5UsYfl9s5b2BK+Yirws21nezOka3YYVN3H3sD9gAIpmJG1lW+SKqf j1VyHRAM/9NR4oOiX3pfJ44icQKji+OqAq1ma5LVXpHWI3PUBHsg0b3LseUFlpbZGIR/dH n9pdf1OoVRBaYNIEAU5POILXuzGfHf0= Date: Thu, 6 Aug 2026 09:37:38 -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> <5a47b073-e1d5-499c-89bd-44500a88bf27@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <5a47b073-e1d5-499c-89bd-44500a88bf27@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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? > > Thx > -Vineet