Try to avoid UB in `thrust::reference` #2813

miscco · 2024-11-14T13:34:50Z

We were dereferencing a nullptr because we wanted to avoid default constructing a system.

This is obvious UB and prevents us from running the tests with sanitizers, so just default construct the systems.

Addresses #1645

miscco · 2024-11-14T13:35:17Z

@Artem-B can you check whether that fixes some of the UB for you?

We were dereferencing a nullptr because we wanted to avoid default constructing a system. This is obvious UB and prevents us from running the tests with sanitizers, so just default construct the systems. Addresses NVIDIA#1645

thrust/thrust/detail/reference.h

miscco · 2024-11-14T18:11:10Z

@Artem-B

Is that something on your side?

ptxas fatal : Unresolved extern function '_ZN6thrust35THRUST_200800___CUDA_ARCH_LIST___NS2mr19get_global_resourceINS0_26device_ptr_memory_resourceINS0_6system4cuda6detail20cuda_memory_resourceIXadL_ZN3c2h6detail19checked_cuda_mallocEPPvmEEXadL_ZL8cudaFreeSA_EENS0_7pointerIvNS0_8cuda_cub3tagENS0_16tagged_referenceIvSE_EENS0_11use_defaultEEEEEEEEEPT_v'
clang++: error: ptxas command failed with exit code 255 (use -v to see invocation)
Ubuntu clang version 18.1.8 (++20240731024944+3b5b5c1ec4a3-1~~exp1~~20240731145000.144)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
clang++: note: diagnostic msg:

Artem-B · 2024-11-14T20:08:39Z

This could be ptxas bug (see NVIDIA's bug 4924626). It will be supposedly fixed in cuda-12.8, but I do not have the details what exactly triggers it -- there was nothing obvious in the minimized reproducer, but it did seem to be related to setmaxnreg.inc.sync.aligned. Can you check if it's present in the PTX produced by clang in this case? If it has the instruction, it's probably the same issue.

Artem-B · 2024-11-14T20:37:03Z

The patch unbreaks ~500 out of 680 tests that were failing in ubsan-enabled builds. I do not run into the ptxas crash, but I'm using a very recent clang build, so it likely generates different PTX than your builds.

Remaining failing tests report ubsan failures in these places:

third_party/gpus/cccl/v2_6_0/thrust/thrust/iterator/iterator_adaptor.h:197:12: runtime error: reference binding to null pointer of type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior third_party/gpus/cccl/v2_6_0/thrust/thrust/iterator/iterator_adaptor.h:197:12

A handful of tests failed to compile PTX with missing symbol:

ptxas fatal   : Unresolved extern function '_ZN6thrust35THRUST_200601___CUDA_ARCH_LIST___NS2mr19get_global_resourceINS0_26device_ptr_memory_resourceINS0_6system4cuda6detail20cuda_memory_resourceIXadL_ZN3c2h6detail19checked_cuda_mallocEPPvmEEXadL_ZL8cudaFreeSA_EENS0_7pointerIvNS0_8cuda_cub3tagENS0_16tagged_referenceIvSE_EENS0_11use_defaultEEEEEEEEEPT_v'
clang: error: ptxas command failed with exit code 255 (use -v to see invocation)

Reproducible on catch2_test_device_merge_sort, and catch2_test_device_three_way_partition.

This is the same issue miscco mentioned above. I'll take a closer look tomorrow.

jrhemstad · 2024-11-14T21:29:35Z

thrust/thrust/detail/reference.h

-    // for dispatch. This assumes that `get_value` will not access any system
-    // state.
-    typename thrust::iterator_system<pointer>::type* system = nullptr;
+    typename thrust::iterator_system<pointer>::type system;


Can't we just pass the type explicitly instead of materializing a value to pass? That way we can avoid both needing to default construct or using a nullptr.

Suggested change

typename thrust::iterator_system<pointer>::type system;

typename thrust::iterator_system<pointer>::type system;

return convert_to_value_type<decltype(system)>();

I honestly would also prefer if we did not need to construct systems. From the recent CI failures and the generally open design (users can bring their own systems) we have to assume that users could rely on Thrust not trying to default construct a system. Given the current code "worked" and no data of the system was actually accessed, we can assume that only the type information should be needed. I assume/hope there is a better solution.

I wondered how thrust::device_vector produces a system when it inits the elements during construction: It tries to get the system from the allocator. But in case the allocator has no system, it default constructs a system (thrust knows the type of the system for an allocator):

cccl/thrust/thrust/detail/allocator/allocator_traits.inl

Lines 270 to 284 in 87f7246

template <typename Alloc>

_CCCL_HOST_DEVICE ::cuda::std::enable_if_t<has_member_system<Alloc>::value, typename allocator_system<Alloc>::type&>

system(Alloc& a)

{

// return the allocator's system

return a.system();

}

template <typename Alloc>

_CCCL_HOST_DEVICE ::cuda::std::enable_if_t<!has_member_system<Alloc>::value, typename allocator_system<Alloc>::type>

system(Alloc&)

{

// return a copy of a value-initialized system

return typename allocator_system<Alloc>::type();

}

The default allocator for a thrust::device_vector does not seem to contain a system. So it seems we have precedence for default constructing systems when needed.

github-actions · 2024-11-14T22:22:53Z

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Artem-B · 2024-11-16T00:21:19Z

Is that something on your side?

ptxas fatal : Unresolved extern function '_ZN6thrust35THRUST_200800___CUDA_ARCH_LIST___NS2mr19get_global_resourceINS0_26device_ptr_memory_resourceINS0_6system4cuda6detail20cuda_memory_resourceIXadL_ZN3c2h6detail19checked_cuda_mallocEPPvmEEXadL_ZL8cudaFreeSA_EENS0_7pointerIvNS0_8cuda_cub3tagENS0_16tagged_referenceIvSE_EENS0_11use_defaultEEEEEEEEEPT_v'

It appears that allocator.h calls get_global_resource() from a host/device function here:

cccl/thrust/thrust/mr/allocator.h

Line 205 in f4d358a

: base(get_global_resource<Upstream>())

But the function itself is defined as a host only function:

cccl/thrust/thrust/mr/memory_resource.h

Line 207 in f4d358a

_CCCL_HOST MR* get_global_resource()

That does not work for clang. Clang was supposed to diagnose such use of a host function from a host_device function, but it didn't happen (probably because constructors are special, or we've just missed this corner case).
Making the function host/device gets us past this particular compilation issue, but exposes or triggers other issues:

fatal error: error in backend: PTX does not support "atomic" for orderings different than"NotAtomic" or "Monotonic" for sm_60 or older, but order is: "acquire".
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version google3-trunk (f58ce1152703ca753794b8cef36da30bd2668d0f)
Target: x86_64-grtev4-linux-gnu
Thread model: posix
InstalledDir: third_party/crosstool/v18/stable/toolchain/bin

ptxas fatal   : Unresolved extern function '__cxa_guard_acquire'
clang: error: ptxas command failed with exit code 255 (use -v to see invocation)
clang version google3-trunk (f58ce1152703ca753794b8cef36da30bd2668d0f)
Target: x86_64-grtev4-linux-gnu
Thread model: posix
InstalledDir: third_party/crosstool/v18/stable/toolchain/bin

Artem-B · 2024-11-20T00:14:16Z

ptxas fatal : Unresolved extern function '__cxa_guard_acquire'

This is caused by a local static initializer here:

cccl/thrust/thrust/mr/memory_resource.h

Line 209 in 8994dc4

static MR resource;

Looks like it's an odd corner case, which may be something for clang to fix.
Static init is generally not allowed. So, local static variables are supposed to work with an "empty" initializer. That part works fine for both nvcc and clang -- both allow it here.

However, when clang sees an existing, but empty constructor, it allows it, but then that constructor is handled by the usual initializer machinery, and, if the call is not eliminated, ends up generating the guard calls. If we're using the default constructor, everything is fine.

A work-around is to pass -Xarch_device -fno-threadsafe-statics to clang and disable thread-safe guard generation on the GPU.

Passing that option to clang also appears to fix the other error about atomic -- it was apparently a side effect of the thread-safe initializer use.

Artem-B · 2024-11-22T19:54:18Z

@miscco Any thoughts/suggestions on the UB in the thrust_iterator.h ?

third_party/gpus/cccl/v2_6_0/thrust/thrust/iterator/iterator_adaptor.h:197:12: runtime error: reference binding to null pointer of type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior third_party/gpus/cccl/v2_6_0/thrust/thrust/iterator/iterator_adaptor.h:197:12

As for the Unresolved extern function '__cxa_guard_acquire' issue, it's been fixed in upstream clang, but one would need to use -Xarch_device -fno-threadsafe-statics until clang-20 is released.

miscco · 2024-11-25T10:42:44Z

I believe we should be able to just suppress the warning prior to Clang20 and be done with it.

miscco requested review from a team as code owners November 14, 2024 13:34

miscco requested review from elstehle and griwes November 14, 2024 13:34

Try to avoid UB in thrust::reference

9763535

We were dereferencing a nullptr because we wanted to avoid default constructing a system. This is obvious UB and prevents us from running the tests with sanitizers, so just default construct the systems. Addresses NVIDIA#1645

miscco force-pushed the avoid_ub_thrust_reference branch from ffe0ddb to 9763535 Compare November 14, 2024 13:44

bernhardmgruber reviewed Nov 14, 2024

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Make execute_with_allocator default constructible

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Try to avoid UB in `thrust::reference` #2813

Try to avoid UB in `thrust::reference` #2813

miscco commented Nov 14, 2024

miscco commented Nov 14, 2024

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Artem-B commented Nov 14, 2024

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github-actions bot commented Nov 14, 2024

🟨 cub: Pass: 96%/110 | Total: 4d 01h | Avg: 53m 04s | Max: 1h 39m | Hits: 33%/2964

🟨 thrust: Pass: 99%/109 | Total: 3d 10h | Avg: 45m 19s | Max: 6h 01m | Hits: 48%/10544

🟩 cccl_c_parallel: Pass: 100%/2 | Total: 9m 54s | Avg: 4m 57s | Max: 7m 34s

🟩 python: Pass: 100%/1 | Total: 17m 15s | Avg: 17m 15s | Max: 17m 15s

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🏃‍ Runner counts (total jobs: 222)

Artem-B commented Nov 16, 2024

Artem-B commented Nov 20, 2024 •

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Artem-B commented Nov 22, 2024

miscco commented Nov 25, 2024

	typename thrust::iterator_system<pointer>::type system;
	typename thrust::iterator_system<pointer>::type system;
	return convert_to_value_type<decltype(system)>();

	template <typename Alloc>
	_CCCL_HOST_DEVICE ::cuda::std::enable_if_t<has_member_system<Alloc>::value, typename allocator_system<Alloc>::type&>
	system(Alloc& a)
	{
	// return the allocator's system
	return a.system();
	}

	template <typename Alloc>
	_CCCL_HOST_DEVICE ::cuda::std::enable_if_t<!has_member_system<Alloc>::value, typename allocator_system<Alloc>::type>
	system(Alloc&)
	{
	// return a copy of a value-initialized system
	return typename allocator_system<Alloc>::type();
	}

Try to avoid UB in thrust::reference #2813

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miscco commented Nov 14, 2024

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github-actions bot commented Nov 14, 2024

🟨 cub: Pass: 96%/110 | Total: 4d 01h | Avg: 53m 04s | Max: 1h 39m | Hits: 33%/2964

🟨 thrust: Pass: 99%/109 | Total: 3d 10h | Avg: 45m 19s | Max: 6h 01m | Hits: 48%/10544

🟩 cccl_c_parallel: Pass: 100%/2 | Total: 9m 54s | Avg: 4m 57s | Max: 7m 34s

🟩 python: Pass: 100%/1 | Total: 17m 15s | Avg: 17m 15s | Max: 17m 15s

👃 Inspect Changes

Modifications in project?

Modifications in project or dependencies?

🏃‍ Runner counts (total jobs: 222)

Artem-B commented Nov 16, 2024

Artem-B commented Nov 20, 2024 • edited Loading

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miscco commented Nov 25, 2024

Try to avoid UB in `thrust::reference` #2813

Try to avoid UB in `thrust::reference` #2813

Artem-B commented Nov 14, 2024 •

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jrhemstad Nov 14, 2024 •

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bernhardmgruber Nov 15, 2024 •

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