[v2] libstdc++: Fix infinite loop in std::istream::ignore(n, delim) [PR93672]

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

Pushed to trunk now.


On Mon, 8 Apr 2024 at 17:53, Jonathan Wakely <jwakely@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
>