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Bignums are artifically restricted in size. Arithmetic and logical
operations check the sizes of resulting bignums, and turn oversize
results into system_limit exceptions.
However, this check is not performed when bignums are constructed by
binary matching. The consequence is that such matchings can construct
oversize bignums that satisfy is_integer/1 yet don't work. Performing
arithmetic such as Term - 0 fails with a system_limit exception. Worse,
performing a logical operation such as Term band Term results in [].
The latter occurs because the size checking (e.g. in erts_band()) is
a simple ASSERT(is_not_nil(...)) on the result of the bignum operation,
which internally is [] (NIL) in the case of oversize results. However,
ASSERT is a no-op in release builds, so the error goes unnoticed and []
is returned as the result of the band/2.
This patch addresses this by preventing oversize bignums from entering
the VM via binary matching:
- the internal bytes_to_big() procedure is augmented to return NIL for
oversize results, just like big_norm()
- callers of bytes_to_big() are augmented to check for NIL returns and
signal errors in those cases
- erts_bs_get_integer_2() can only fail with badmatch, so that is the
Erlang-level result of oversize bignums from binary matches
- big_SUITE.erl is extended with a test case that fails without this
fix (no error signalled) and passes with it (badmatch occurs)
Credit goes to Nico Kruber for the initial bug report.
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