Re: [PATCH] libstdc++-v3: check for openat

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 23 Jun 2022 at 12:08, Alexandre Oliva <oliva@adacore.com> wrote:
>
> On Jun 22, 2022, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> > There are other interactions between AT_CDCWD and ::openat not covered
> > by this patch. I this this also needs to check HAVE_OPENAT:
>
> Here's an updated version, tested with this additional change.

Did this improve your test results for directory iterators?

In the unlikely even that the target has ::unlinkat but not ::openat
this will cause a behaviour regression, because now _Dir::do_unlink
will use {AT_FDCWD, "full/path/to/file"} instead of
{dirfd(dirp),"file"} but I doubt there are targets that have unlinkat
without openat. I'll try to improve this code later as discussed
anyway, so this is fine for now.

OK for trunk, thanks.



>
>
> libstdc++: check for openat
>
> From: Alexandre Oliva <oliva@adacore.com>
>
> rtems6.0 has fdopendir, and fcntl.h defines AT_FDCWD and declares
> openat, but there's no openat in libc.  Adjust dir-common.h to not
> assume ::openat just because of AT_FDCWD.
>
>
> for  libstdc++-v3/ChangeLog
>
>         * acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Check for
>         openat.
>         * aclocal.m4, configure, config.h.in: Rebuilt.
>         * src/filesystem/dir-common.h (openat): Use ::openat if
>         _GLIBCXX_HAVE_OPENAT.
>         * src/filesystem/dir.cc (dir_and_pathname): Use dirfd if
>         _GLIBCXX_HAVE_OPENAT.
> ---
>  libstdc++-v3/acinclude.m4                |   12 +++++++
>  libstdc++-v3/config.h.in                 |    3 ++
>  libstdc++-v3/configure                   |   55 ++++++++++++++++++++++++++++++
>  libstdc++-v3/src/filesystem/dir-common.h |    2 +
>  libstdc++-v3/src/filesystem/dir.cc       |    2 +
>  5 files changed, 72 insertions(+), 2 deletions(-)
>
> diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
> index 138bd58d86cb9..e3cc3a8e867d3 100644
> --- a/libstdc++-v3/acinclude.m4
> +++ b/libstdc++-v3/acinclude.m4
> @@ -4772,6 +4772,18 @@ dnl
>    if test $glibcxx_cv_dirfd = yes; then
>      AC_DEFINE(HAVE_DIRFD, 1, [Define if dirfd is available in <dirent.h>.])
>    fi
> +dnl
> +  AC_CACHE_CHECK([for openat],
> +    glibcxx_cv_openat, [dnl
> +    GCC_TRY_COMPILE_OR_LINK(
> +      [#include <fcntl.h>],
> +      [int fd = ::openat(AT_FDCWD, "", 0);],
> +      [glibcxx_cv_openat=yes],
> +      [glibcxx_cv_openat=no])
> +  ])
> +  if test $glibcxx_cv_openat = yes; then
> +    AC_DEFINE(HAVE_OPENAT, 1, [Define if openat is available in <fcntl.h>.])
> +  fi
>  dnl
>    AC_CACHE_CHECK([for unlinkat],
>      glibcxx_cv_unlinkat, [dnl
> diff --git a/libstdc++-v3/config.h.in b/libstdc++-v3/config.h.in
> index f30a8c51c458c..2a3972eef5412 100644
> --- a/libstdc++-v3/config.h.in
> +++ b/libstdc++-v3/config.h.in
> @@ -292,6 +292,9 @@
>  /* Define if <math.h> defines obsolete isnan function. */
>  #undef HAVE_OBSOLETE_ISNAN
>
> +/* Define if openat is available in <fcntl.h>. */
> +#undef HAVE_OPENAT
> +
>  /* Define if poll is available in <poll.h>. */
>  #undef HAVE_POLL
>
> diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
> index 9b94fd71e4248..eac6039212168 100755
> --- a/libstdc++-v3/configure
> +++ b/libstdc++-v3/configure
> @@ -77177,6 +77177,61 @@ $as_echo "$glibcxx_cv_dirfd" >&6; }
>
>  $as_echo "#define HAVE_DIRFD 1" >>confdefs.h
>
> +  fi
> +  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for openat" >&5
> +$as_echo_n "checking for openat... " >&6; }
> +if ${glibcxx_cv_openat+:} false; then :
> +  $as_echo_n "(cached) " >&6
> +else
> +      if test x$gcc_no_link = xyes; then
> +  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
> +/* end confdefs.h.  */
> +#include <fcntl.h>
> +int
> +main ()
> +{
> +int fd = ::openat(AT_FDCWD, "", 0);
> +  ;
> +  return 0;
> +}
> +_ACEOF
> +if ac_fn_cxx_try_compile "$LINENO"; then :
> +  glibcxx_cv_openat=yes
> +else
> +  glibcxx_cv_openat=no
> +fi
> +rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
> +else
> +  if test x$gcc_no_link = xyes; then
> +  as_fn_error $? "Link tests are not allowed after GCC_NO_EXECUTABLES." "$LINENO" 5
> +fi
> +cat confdefs.h - <<_ACEOF >conftest.$ac_ext
> +/* end confdefs.h.  */
> +#include <fcntl.h>
> +int
> +main ()
> +{
> +int fd = ::openat(AT_FDCWD, "", 0);
> +  ;
> +  return 0;
> +}
> +_ACEOF
> +if ac_fn_cxx_try_link "$LINENO"; then :
> +  glibcxx_cv_openat=yes
> +else
> +  glibcxx_cv_openat=no
> +fi
> +rm -f core conftest.err conftest.$ac_objext \
> +    conftest$ac_exeext conftest.$ac_ext
> +fi
> +
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $glibcxx_cv_openat" >&5
> +$as_echo "$glibcxx_cv_openat" >&6; }
> +  if test $glibcxx_cv_openat = yes; then
> +
> +$as_echo "#define HAVE_OPENAT 1" >>confdefs.h
> +
>    fi
>    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for unlinkat" >&5
>  $as_echo_n "checking for unlinkat... " >&6; }
> diff --git a/libstdc++-v3/src/filesystem/dir-common.h b/libstdc++-v3/src/filesystem/dir-common.h
> index 365fd527f4d68..669780ea23fe5 100644
> --- a/libstdc++-v3/src/filesystem/dir-common.h
> +++ b/libstdc++-v3/src/filesystem/dir-common.h
> @@ -199,7 +199,7 @@ struct _Dir_base
>  #endif
>
>
> -#ifdef AT_FDCWD
> +#if _GLIBCXX_HAVE_OPENAT && defined AT_FDCWD
>      fd = ::openat(fd, pathname, flags);
>  #else
>      // If we cannot use openat, there's no benefit to using posix::open unless
> diff --git a/libstdc++-v3/src/filesystem/dir.cc b/libstdc++-v3/src/filesystem/dir.cc
> index b451902c4a1b1..e64489162e5ff 100644
> --- a/libstdc++-v3/src/filesystem/dir.cc
> +++ b/libstdc++-v3/src/filesystem/dir.cc
> @@ -120,7 +120,7 @@ struct fs::_Dir : std::filesystem::_Dir_base
>    dir_and_pathname() const noexcept
>    {
>      const fs::path& p = entry.path();
> -#if _GLIBCXX_HAVE_DIRFD
> +#if _GLIBCXX_HAVE_DIRFD && _GLIBCXX_HAVE_OPENAT
>      return {::dirfd(this->dirp), std::prev(p.end())->c_str()};
>  #endif
>      return {this->fdcwd(), p.c_str()};
>
>
> --
> Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
>    Free Software Activist                       GNU Toolchain Engineer
> Disinformation flourishes because many people care deeply about injustice
> but very few check the facts.  Ask me about <https://stallmansupport.org>
>