Pushed to trunk now.
On Mon, 8 Apr 2024 at 17:53, Jonathan Wakely <jwak...@redhat.com> wrote:
>
> Patch v2.
>
> I realised that it's not only negative delim values that cause the
> problem, but also ones greater than CHAR_MAX. Calling ignore(n, 'a'+256)
> will cause traits_type::find to match 'a' but then the eq_int_type
> comparison will fail because (int)'a' != (int)('a' + 256).
>
> This version of the patch calls to_int_type on the delim and if that
> alters the value, it's never going to match so skip the loop that tries
> to find it and just ignore up to n chars instead.
>
> Tested x86_64linux and aarch64-linux.
>
> -- >8 --
>
> A negative delim value passed to std::istream::ignore can never match
> any character in the stream, because the comparison is done using
> traits_type::eq_int_type(sb->sgetc(), delim) and sgetc() never returns
> negative values (except at EOF). The optimized version of ignore for the
> std::istream specialization uses traits_type::find to locate the delim
> character in the streambuf, which _can_ match a negative delim on
> platforms where char is signed, but then we do another comparison using
> eq_int_type which fails. The code then keeps looping forever, with
> traits_type::find locating the character and traits_type::eq_int_type
> saying it's not a match, so traits_type::find is used again and finds
> the same character again.
>
> A possible fix would be to check with eq_int_type after a successful
> find, to see whether we really have a match. However, that would be
> suboptimal since we know that a negative delimiter will never match
> using eq_int_type. So a better fix is to adjust the check at the top of
> the function that handles delim==eof(), so that we treat all negative
> delim values as equivalent to EOF. That way we don't bother using find
> to search for something that will never match with eq_int_type.
>
> The version of ignore in the primary template doesn't need a change,
> because it doesn't use traits_type::find, instead characters are
> extracted one-by-one and always matched using eq_int_type. That avoids
> the inconsistency between find and eq_int_type. The specialization for
> std::wistream does use traits_type::find, but traits_type::to_int_type
> is equivalent to an implicit conversion from wchar_t to wint_t, so
> passing a wchar_t directly to ignore without using to_int_type works.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/93672
> * src/c++98/istream.cc (istream::ignore(streamsize, int_type)):
> Treat all negative delimiter values as eof().
> * testsuite/27_io/basic_istream/ignore/char/93672.cc: New test.
> * testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc: New
> test.
> ---
> libstdc++-v3/src/c++98/istream.cc | 13 ++-
> .../27_io/basic_istream/ignore/char/93672.cc | 101 ++++++++++++++++++
> .../basic_istream/ignore/wchar_t/93672.cc | 34 ++++++
> 3 files changed, 146 insertions(+), 2 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc
> create mode 100644
> libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc
>
> diff --git a/libstdc++-v3/src/c++98/istream.cc
> b/libstdc++-v3/src/c++98/istream.cc
> index 07ac739c26a..d1b4444ff2b 100644
> --- a/libstdc++-v3/src/c++98/istream.cc
> +++ b/libstdc++-v3/src/c++98/istream.cc
> @@ -112,8 +112,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> basic_istream<char>::
> ignore(streamsize __n, int_type __delim)
> {
> - if (traits_type::eq_int_type(__delim, traits_type::eof()))
> - return ignore(__n);
> + {
> + // If conversion to int_type changes the value then __delim does not
> + // correspond to a value of type char_type, and so will never match
> + // a character extracted from the input sequence. Just use ignore(n).
> + const int_type chk_delim = traits_type::to_int_type(__delim);
> + const bool matchable = traits_type::eq_int_type(chk_delim, __delim);
> + if (__builtin_expect(!matchable, 0))
> + return ignore(__n);
> + // Now we know that __delim is a valid char_type value, so it's safe
> + // for the code below to use traits_type::find to search for it.
> + }
>
> _M_gcount = 0;
> sentry __cerb(*this, true);
> diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc
> b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc
> new file mode 100644
> index 00000000000..96737485b83
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc
> @@ -0,0 +1,101 @@
> +// { dg-do run }
> +
> +#include <sstream>
> +#include <limits>
> +#include <testsuite_hooks.h>
> +
> +void
> +test_pr93672() // std::basic_istream::ignore hangs if delim MSB is set
> +{
> + std::istringstream in(".\xfc..\xfd...\xfe.");
> +
> + // This should find '\xfd' even on platforms where char is signed,
> + // because the delimiter is correctly converted to the stream's int_type.
> + in.ignore(100, std::char_traits<char>::to_int_type('\xfc'));
> + VERIFY( in.gcount() == 2 );
> + VERIFY( ! in.eof() );
> +
> + // This should work equivalently to traits_type::to_int_type
> + in.ignore(100, (unsigned char)'\xfd');
> + VERIFY( in.gcount() == 3 );
> + VERIFY( ! in.eof() );
> +
> + // This only works if char is unsigned.
> + in.ignore(100, '\xfe');
> + if (std::numeric_limits<char>::is_signed)
> + {
> + // When char is signed, '\xfe' != traits_type::to_int_type('\xfe')
> + // so the delimiter does not match the character in the input sequence,
> + // and ignore consumes all input until EOF.
> + VERIFY( in.gcount() == 5 );
> + VERIFY( in.eof() );
> + }
> + else
> + {
> + // When char is unsigned, '\xfe' == to_int_type('\xfe') so the delimiter
> + // matches the character in the input sequence, and doesn't reach EOF.
> + VERIFY( in.gcount() == 4 );
> + VERIFY( ! in.eof() );
> + }
> +
> + in.clear();
> + in.str(".a.");
> + in.ignore(100, 'a' + 256); // Should not match 'a'
> + VERIFY( in.gcount() == 3 );
> + VERIFY( in.eof() );
> +}
> +
> +// Custom traits type that inherits all behaviour from
> std::char_traits<char>.
> +struct traits : std::char_traits<char> { };
> +
> +void
> +test_primary_template()
> +{
> + // Check that the primary template for std::basic_istream::ignore
> + // works the same as the std::istream::ignore specialization.
> + // The infinite loop bug was never present in the primary template,
> + // because it doesn't use traits_type::find to search the input sequence.
> +
> + std::basic_istringstream<char, traits> in(".\xfc..\xfd...\xfe.");
> +
> + // This should find '\xfd' even on platforms where char is signed,
> + // because the delimiter is correctly converted to the stream's int_type.
> + in.ignore(100, std::char_traits<char>::to_int_type('\xfc'));
> + VERIFY( in.gcount() == 2 );
> + VERIFY( ! in.eof() );
> +
> + // This should work equivalently to traits_type::to_int_type
> + in.ignore(100, (unsigned char)'\xfd');
> + VERIFY( in.gcount() == 3 );
> + VERIFY( ! in.eof() );
> +
> + // This only works if char is unsigned.
> + in.ignore(100, '\xfe');
> + if (std::numeric_limits<char>::is_signed)
> + {
> + // When char is signed, '\xfe' != traits_type::to_int_type('\xfe')
> + // so the delimiter does not match the character in the input sequence,
> + // and ignore consumes all input until EOF.
> + VERIFY( in.gcount() == 5 );
> + VERIFY( in.eof() );
> + }
> + else
> + {
> + // When char is unsigned, '\xfe' == to_int_type('\xfe') so the delimiter
> + // matches the character in the input sequence, and doesn't reach EOF.
> + VERIFY( in.gcount() == 4 );
> + VERIFY( ! in.eof() );
> + }
> +
> + in.clear();
> + in.str(".a.");
> + in.ignore(100, 'a' + 256); // Should not match 'a'
> + VERIFY( in.gcount() == 3 );
> + VERIFY( in.eof() );
> +}
> +
> +int main()
> +{
> + test_pr93672();
> + test_primary_template();
> +}
> diff --git
> a/libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc
> b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc
> new file mode 100644
> index 00000000000..5ce9155e02c
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc
> @@ -0,0 +1,34 @@
> +// { dg-do run }
> +
> +#include <sstream>
> +#include <limits>
> +#include <climits>
> +#include <testsuite_hooks.h>
> +
> +// PR 93672 was a bug in std::istream that never affected std::wistream.
> +// This test ensures that the bug doesn't get introduced to std::wistream.
> +void
> +test_pr93672()
> +{
> + std::wstring str = L".x..x.";
> + str[1] = (wchar_t)-2;
> + str[4] = (wchar_t)-3;
> + std::wistringstream in(str);
> +
> + // This should find the character even on platforms where wchar_t is
> signed,
> + // because the delimiter is correctly converted to the stream's int_type.
> + in.ignore(100, std::char_traits<wchar_t>::to_int_type((wchar_t)-2));
> + VERIFY( in.gcount() == 2 );
> + VERIFY( ! in.eof() );
> +
> + // This also works, because std::char_traits<wchar_t>::to_int_type(wc) is
> + // equivalent to (int_type)wc so using to_int_type isn't needed.
> + in.ignore(100, (wchar_t)-3);
> + VERIFY( in.gcount() == 3 );
> + VERIFY( ! in.eof() );
> +}
> +
> +int main()
> +{
> + test_pr93672();
> +}
> --
> 2.44.0
>
