From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f202.google.com (mail-pf1-f202.google.com [209.85.210.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EA02A19F13F for ; Mon, 29 Sep 2025 18:10:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759169450; cv=none; b=U12fzPmn/3vhvTXeKdNftdhmMg7QHInUanQCdcFo0sqa6HYxjsXlHxjmYwpXIDslDRiDKDfJ+7QgX7tk4F6J2bNU2DIpXvjkogyoOhc8iJBO3hDa9dLqoTN6hVn5Gbm87/w+ZM5BI93YK632PoLLT491mgMsTYJvhuI+EbP2Rwg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759169450; c=relaxed/simple; bh=hTi/1LDjxXLqAzqWGP9Nw1rFwML0IskqpYBN+kRb1b4=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=cPyPcAFzehYz13/9jrfxY37tIHBy40cQvKQcJwRkGsEzcfQQ7w2fYq498SkjcOZ2U2XXzcx5ggvkblyDkK7q9RPncmNoK3nD8MyuCd07TkDWFDWBUKQP8zm9czX8QRr1biBB/313Ge4NlT46TVFjox49vT98cP2RoqpD2Nbr20k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=LyXZcHJ3; arc=none smtp.client-ip=209.85.210.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="LyXZcHJ3" Received: by mail-pf1-f202.google.com with SMTP id d2e1a72fcca58-781253de15aso5385845b3a.2 for ; Mon, 29 Sep 2025 11:10:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1759169448; x=1759774248; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=c1YZoUF94m19t8ktsyL6eUQtbJ+D8OzjtqR3WKoUgkk=; b=LyXZcHJ3cvXtONE1XhxZ4RCBBnb23NjxvDxuZfn+N0yynEL9pRXLQPPkVp0KXVUW2K Om3BfnEaKH5g6QaDUii8sNkNBAgamJzxNgAdlN8gvZBewBG1Rs8IPdiZG9EYmZz5n/fe F3G8Ot+eDSkAjiXSQHfSI+V5ELQDfaiSgA6+74PMOo/dGPcZmeaK9549revxQW+ktIjv 5mkM45IGLUid3+kqyGtwI6WXN0wSEd+SpWQinGMlmrPFX5QOp8Kn7cRsyNeE8wXJi93l 7K9RfjTA5AQZrVxT0WNAc4pbdpnWAtcYcQIVkSpGDjBmjoa8da4e2HzrO7Mph9yn3/Vu yCYQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1759169448; x=1759774248; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=c1YZoUF94m19t8ktsyL6eUQtbJ+D8OzjtqR3WKoUgkk=; b=fWGg3cAWQphTOLgPfu9YMZF3Fht3G/twNZwZgxJ06E7ldR4uLLkq9gVIKWgc9MB+XE PXneda/Ckb33lBjGz5rmO8b3vkuXYwrCHq9jOrIPIUs4UafBS6sXtQsqBokUoMoyrcDW NdIJF+ywr96IzxKmUF8n/9cUcPGjz9PdDbihPBm458MlR35yvJ8T84cKJ4SuyOTcGiQM TW49SowrLaZmEkQgkKtYToUC8frNupZvkJe8VNbIhaGqLSDs/75o4uV3edj7L9pOUDYp q3YWgRYGOyThGFGQdE1diwGiAg7RfC+QgXYFdUrODulfdgS5gA/H7/qYxAwY3uz6+K2D dO3Q== X-Forwarded-Encrypted: i=1; AJvYcCWkF5Cu+e/+tH8iaFrkvvs8f3bQrySfCz0zdEeN/qL9YqpwY/wz66fiKoJEno+01o/2wbdfPyCemV/I9ck=@vger.kernel.org X-Gm-Message-State: AOJu0Yzt+XFXGmKAPgFFMegHLNYvVvVlJmgY8kGHJcaLJUTcQmDMc1Bd EJCNgN0Qd2B6fVBfQvpB6MIbwdYH8Q+WffF/NO5dKPw1SfaF53vZvpgcaVdAzMUgtv8QgYOepus gjKe9Mg== X-Google-Smtp-Source: AGHT+IE78WkBOFZpjBW8uPN1lnIxmEpEFLNS96Wf2/GXDz81pjOZfJGsUaoO1CMv0KsumeZoy8UT9eytn3s= X-Received: from pfbhw13.prod.google.com ([2002:a05:6a00:890d:b0:771:ea87:e37d]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:1953:b0:783:cb49:c67b with SMTP id d2e1a72fcca58-783cb49c7cdmr7979337b3a.32.1759169448174; Mon, 29 Sep 2025 11:10:48 -0700 (PDT) Date: Mon, 29 Sep 2025 11:10:46 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250926163114.2626257-1-seanjc@google.com> <20250926163114.2626257-7-seanjc@google.com> Message-ID: Subject: Re: [PATCH 6/6] KVM: selftests: Verify that faulting in private guest_memfd memory fails From: Sean Christopherson To: Ackerley Tng Cc: Paolo Bonzini , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, David Hildenbrand , Fuad Tabba Content-Type: text/plain; charset="us-ascii" On Mon, Sep 29, 2025, Ackerley Tng wrote: > Sean Christopherson writes: > > > Add a guest_memfd testcase to verify that faulting in private memory gets > > a SIGBUS. For now, test only the case where memory is private by default > > since KVM doesn't yet support in-place conversion. > > > > Cc: Ackerley Tng > > Signed-off-by: Sean Christopherson > > --- > > .../testing/selftests/kvm/guest_memfd_test.c | 62 ++++++++++++++----- > > 1 file changed, 46 insertions(+), 16 deletions(-) > > > > diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c > > index 5dd40b77dc07..b5a631aca933 100644 > > --- a/tools/testing/selftests/kvm/guest_memfd_test.c > > +++ b/tools/testing/selftests/kvm/guest_memfd_test.c > > @@ -40,17 +40,26 @@ static void test_file_read_write(int fd, size_t total_size) > > "pwrite on a guest_mem fd should fail"); > > } > > > > I feel that the tests should be grouped by concepts being tested > > + test_cow_not_supported() > + mmap() should fail > + test_mmap_supported() > + kvm_mmap() > + regular, successful accesses to offsets within the size of the fd > + kvm_munmap() > + test_fault_overflow() > + kvm_mmap() > + a helper (perhaps "assert_fault_sigbus(char *mem)"?) that purely > tries to access beyond the size of the fd and catches SIGBUS > + regular, successful accesses to offsets within the size of the fd > + kvm_munmap() > + test_fault_private() > + kvm_mmap() > + a helper (perhaps "assert_fault_sigbus(char *mem)"?) that purely > tries to access within the size of the fd and catches SIGBUS > + kvm_munmap() > > I think some code duplication in tests is okay if it makes the test flow > more obvious. Yeah, depends on what is being duplicated, and how much. > > -static void test_mmap_supported(int fd, size_t total_size) > > +static void *test_mmap_common(int fd, size_t size) > > { > > - const char val = 0xaa; > > - char *mem; > > - size_t i; > > - int ret; > > + void *mem; > > > > - mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0); > > + mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0); > > TEST_ASSERT(mem == MAP_FAILED, "Copy-on-write not allowed by guest_memfd."); > > > > When grouped this way, test_mmap_common() tests that MAP_PRIVATE or COW > is not allowed twice, once in test_mmap_supported() and once in > test_fault_sigbus(). Is that intentional? Hmm, no? I suspect I just lost track of what was being tested. > > - mem = kvm_mmap(total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd); > > + mem = kvm_mmap(size, PROT_READ | PROT_WRITE, MAP_SHARED, fd); > > + > > + return mem; > > I feel that returning (and using) the userspace address from a test > (test_mmap_common()) is a little hard to follow. Agreed. Should be easy enough to eliminate this helper. > > -static void test_fault_overflow(int fd, size_t total_size) > > +static void *test_fault_sigbus(int fd, size_t size) > > { > > struct sigaction sa_old, sa_new = { > > .sa_handler = fault_sigbus_handler, > > }; > > - size_t map_size = total_size * 4; > > - const char val = 0xaa; > > - char *mem; > > - size_t i; > > + void *mem; > > > > - mem = kvm_mmap(map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd); > > + mem = test_mmap_common(fd, size); > > > > sigaction(SIGBUS, &sa_new, &sa_old); > > if (sigsetjmp(jmpbuf, 1) == 0) { > > - memset(mem, 0xaa, map_size); > > + memset(mem, 0xaa, size); > > TEST_ASSERT(false, "memset() should have triggered SIGBUS."); > > } > > sigaction(SIGBUS, &sa_old, NULL); > > > > + return mem; > > I think returning the userspace address from a test is a little hard to > follow. This one feels even more unexpected because a valid address is > being returned (and used) from a test that has sigbus in its name. Yeah, and it's fugly all around. If we pass in the "accessible" size, then we can reduce the amount of copy+paste, eliminate the weird return and split mmap() versus munmap(), and get bonus coverage that reads SIGBUS as well. How's this look? static void test_fault_sigbus(int fd, size_t accessible_size, size_t mmap_size) { struct sigaction sa_old, sa_new = { .sa_handler = fault_sigbus_handler, }; const uint8_t val = 0xaa; uint8_t *mem; size_t i; mem = kvm_mmap(mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd); sigaction(SIGBUS, &sa_new, &sa_old); if (sigsetjmp(jmpbuf, 1) == 0) { memset(mem, val, mmap_size); TEST_FAIL("memset() should have triggered SIGBUS"); } if (sigsetjmp(jmpbuf, 1) == 0) { (void)READ_ONCE(mem[accessible_size]); TEST_FAIL("load at first unaccessible byte should have triggered SIGBUS"); } sigaction(SIGBUS, &sa_old, NULL); for (i = 0; i < accessible_size; i++) TEST_ASSERT_EQ(READ_ONCE(mem[i]), val); kvm_munmap(mem, mmap_size); } static void test_fault_overflow(int fd, size_t total_size) { test_fault_sigbus(fd, total_size, total_size * 4); } static void test_fault_private(int fd, size_t total_size) { test_fault_sigbus(fd, 0, total_size); } > > +static void test_fault_private(int fd, size_t total_size) > > +{ > > + void *mem = test_fault_sigbus(fd, total_size); > > + > > + kvm_munmap(mem, total_size); > > +} > > + > > Testing that faults fail when GUEST_MEMFD_FLAG_DEFAULT_SHARED is not set > is a good idea. Perhaps it could be even clearer if further split up: > > + test_mmap_supported() > + kvm_mmap() > + kvm_munmap() > + test_mmap_supported_fault_supported() > + kvm_mmap() > + successful accesses to offsets within the size of the fd > + kvm_munmap() > + test_mmap_supported_fault_sigbus() > + kvm_mmap() > + expect SIGBUS from accesses to offsets within the size of the fd > + kvm_munmap() > > > static void test_mmap_not_supported(int fd, size_t total_size) > > { > > char *mem; > > @@ -274,9 +299,12 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags) > > > > gmem_test(file_read_write, vm, flags); > > > > - if (flags & GUEST_MEMFD_FLAG_MMAP) { > > + if (flags & GUEST_MEMFD_FLAG_MMAP && > > + flags & GUEST_MEMFD_FLAG_DEFAULT_SHARED) { > > gmem_test(mmap_supported, vm, flags); > > gmem_test(fault_overflow, vm, flags); > > + } else if (flags & GUEST_MEMFD_FLAG_MMAP) { > > + gmem_test(fault_private, vm, flags); > > test_fault_private() makes me think the test is testing for private > faults, but there's nothing private about this fault, It's a user fault on private memory, not sure how else to describe that :-) The CoCo shared vs. private and MAP_{SHARED,PRIVATE} collision is unfortunate, but I think we should prioritize standardizing on CoCo shared vs. private since that is what KVM will care about 99.9% of the time, i.e. in literally everything except kvm_gmem_mmap(). > and the fault doesn't even come from the guest. Sure, but I don't see what that has to do with anything, e.g. fault_overflow() isn't a fault from the guest either. > > } else { > > gmem_test(mmap_not_supported, vm, flags); > > } > > If split up as described above, this could be > > if (flags & GUEST_MEMFD_FLAG_MMAP && > flags & GUEST_MEMFD_FLAG_DEFAULT_SHARED) { > gmem_test(mmap_supported_fault_supported, vm, flags); > gmem_test(fault_overflow, vm, flags); > } else if (flags & GUEST_MEMFD_FLAG_MMAP) { > gmem_test(mmap_supported_fault_sigbus, vm, flags); I find these unintuitive, e.g. is this one "mmap() supported, test fault sigbus", or is it "mmap(), test supported fault sigbus". I also don't like that some of the test names describe the _result_ (SIBGUS), where as others describe _what_ is being tested. In general, I don't like test names that describe the result, because IMO what is being tested is far more interesting. E.g. from a test coverage persective, I don't care if attempting to fault in (CoCO) private memory gets SIGBUS versus SIGSEGV, but I most definitely care that we have test coverage for the "what". Looking at everything, I think the only that doesn't fit well is the CoW scenario. What if we extract that to its own helper? That would eliminate the ugly test_mmap_common(), So my vote would be to keep things largely the same: if (flags & GUEST_MEMFD_FLAG_MMAP && flags & GUEST_MEMFD_FLAG_DEFAULT_SHARED) { gmem_test(mmap_supported, vm, flags); gmem_test(mmap_cow, vm, flags); gmem_test(fault_overflow, vm, flags); gmem_test(mbind, vm, flags); gmem_test(numa_allocation, vm, flags); } else if (flags & GUEST_MEMFD_FLAG_MMAP) { gmem_test(fault_private, vm, flags); } else { gmem_test(mmap_not_supported, vm, flags); }