[jira] [Commented] (ARROW-18141) [C++] Alignment not enforced; undefined behavior

["Antoine Pitrou (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-18141?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17623251#comment-17623251 ] 

Antoine Pitrou commented on ARROW-18141:
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Hmm, this is unfortunate. It's true that we mostly don't test for unaligned buffers.

> [C++] Alignment not enforced; undefined behavior
> ------------------------------------------------
>
>                 Key: ARROW-18141
>                 URL: https://issues.apache.org/jira/browse/ARROW-18141
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C++, Python
>            Reporter: Jochen Ott
>            Priority: Major
>             Fix For: 11.0.0
>
>         Attachments: test1.py
>
>
> It is possible to create arrays using unaligned memory addresses (e.g. for int64). This seems to be in line with the arrow specification which as far as I understand does not require alignment [1].
> However, the C++ standard requires alignment, e.g. 8 byte alignment for int64. It is undefined behavior (UB) to create an unaligned pointer / accessing data via an unaligned pointer.
> Typically, this is not an issue in practice on x86, since gcc and other compilers mostly emit instructions that can deal with unaligned data. However, for gcc 6.3.0 (and probably up to including gcc versions 7.X), this code:
> [https://github.com/apache/arrow/blob/4591d76fce2846a29dac33bf01e9ba0337b118e9/cpp/src/parquet/statistics.cc#L355]
> creates an aligned move instruction (movdqa) for the expression {{{}values[i]{}}}. This, in turn, triggers a SIGSEGV in case {{values}} is called via an unaligned buffer. Later compiler versions (in particular gcc 9.X used to build the wheels published on pypi) will emit instructions that can deal with unaligned data (movdqu instead of movdqa).
> The python script "test1.py" reproduces this issue on python-level; note that it will only trigger a SIGSEGV if compiling arrow with a compiler that emits movdqa for the code linked above, e.g. by using gcc 6.3.0 to compile arrow.
> In the wild, unaligned buffers are rare, but can appear, e.g. as a result of deserializing pandas dataframes / numpy arrays using pickle protocol 5 that allows out-of-band byte buffers that are re-used as arrow array buffers.
> I think the line to first enter the UB regime is this reinterpret_cast:
> https://github.com/apache/arrow/blob/33f2c0ec8e281fc4fe8c03b07ed2d32e343d9b0e/cpp/src/parquet/column_writer.cc#L1592
> [1][https://arrow.apache.org/docs/format/Columnar.html#buffer-alignment-and-padding] merely "recommends" that buffers are aligned, but does not require it.



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