diff mbox series

[1/4] Sync getcwd with gnulib

Message ID 20200826210246.2830973-1-adhemerval.zanella@linaro.org
State New
Headers show
Series [1/4] Sync getcwd with gnulib | expand

Commit Message

Adhemerval Zanella Aug. 26, 2020, 9:02 p.m. UTC
It sync with gnulib version d4c637e57.  The main changes are:

  - It defines HAVE_OPENAT to 1, D_INO_IN_DIRENT to 1,
    HAVE_MSVC_INVALID_PARAMETER_HANDLER to 0, and
    HAVE_MINIMALLY_WORKING_GETCWD to 0 for _LIBC.

  - It does not include pathmax.h header (glibc does not provide it).

  - It defines and used __close_nocancel_nostatus, stat64, __fstat64,
   __fstatat64, __lstat64, __fdopendir, __openat, __rewinddir,
   __openat64 for !LIBC to enable internal LFS calls.

This requires glibc changes should integrate seamlessly on gnulib.

Checked on x86_64-linux-gnu and i686-linux-gnu.
---
 sysdeps/posix/getcwd.c | 782 +++++++++++++++++++----------------------
 1 file changed, 369 insertions(+), 413 deletions(-)

Comments

Paul Eggert Aug. 26, 2020, 10:39 p.m. UTC | #1
Thanks for looking into this. To help move this along from the Gnulib point of 
view, I merged the Gnulib-relevant parts of those proposed glibc patches into 
Gnulib, here:

https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=615b43e1f914687b6ccd40c79a05626098a361d7

but this Gnulib patch has the following further changes to get this to work, and 
these further changes should be in the glibc version too:

>    - It does not include pathmax.h header (glibc does not provide it).

That #include can be moved into a previous "#if !_LIBC" section.

> +# define __getcwd rpl_getcwd

This should define __getcwd_generic, not __getcwd, since the function name changed.



Also, here are some minor refactorings to the resulting getcwd-generic.c that 
would improve readability. If you add these I plan to merge them into Gnulib too.

* Prefer "#if _LIBC" to "#if !_LIBC", interchanging 'then' and 'else' parts.

* Put those !_LIBC #defines in alphabetic order.
Florian Weimer Aug. 27, 2020, 8:14 a.m. UTC | #2
* Adhemerval Zanella via Libc-alpha:

> +#if HAVE_MINIMALLY_WORKING_GETCWD
> +  /* If AT_FDCWD is not defined, the algorithm below is O(N**2) and
> +     this is much slower than the system getcwd (at least on
> +     GNU/Linux).  So trust the system getcwd's results unless they
> +     look suspicious.
> +
> +     Use the system getcwd even if we have openat support, since the
> +     system getcwd works even when a parent is unreadable, while the
> +     openat-based approach does not.
> +
> +     But on AIX 5.1..7.1, the system getcwd is not even minimally
> +     working: If the current directory name is slightly longer than
> +     PATH_MAX, it omits the first directory component and returns
> +     this wrong result with errno = 0.  */
> +
> +# undef getcwd
> +  dir = getcwd_system (buf, size);
> +  if (dir || (size && errno == ERANGE))
> +    return dir;

This conflicts with the getcwd_system implementation does not set errno.

> +  /* Solaris getcwd (NULL, 0) fails with errno == EINVAL, but it has
> +     internal magic that lets it work even if an ancestor directory is
> +     inaccessible, which is better in many cases.  So in this case try
> +     again with a buffer that's almost always big enough.  */
> +  if (errno == EINVAL && buf == NULL && size == 0)
> +    {
> +      char big_buffer[BIG_FILE_NAME_LENGTH + 1];
> +      dir = getcwd_system (big_buffer, sizeof big_buffer);
> +      if (dir)
> +        return strdup (dir);
> +    }

> +          /* When we've iterated through all directory entries without finding
> +             one with a matching d_ino, rewind the stream and consider each
> +             name again, but this time, using lstat.  This is necessary in a
> +             chroot on at least one system (glibc-2.3.6 + linux 2.6.12), where
> +             .., ../.., ../../.., etc. all had the same device number, yet the
> +             d_ino values for entries in / did not match those obtained
> +             via lstat.  */
> +          if (d == NULL && errno == 0 && use_d_ino)
> +            {
> +              use_d_ino = false;
> +              __rewinddir (dirstream);
> +              d = __readdir (dirstream);
> +            }

I'm not sure if it's worthwhile to have such code in glibc.

The generic getcwd isn't used by Hurd, right?  Would it make sense to
have a trimmed-down Linux implementation as well?

Thanks,
Florian
Adhemerval Zanella Aug. 27, 2020, 10:53 a.m. UTC | #3
On Thu, Aug 27, 2020 at 5:14 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Adhemerval Zanella via Libc-alpha:
>
> > +#if HAVE_MINIMALLY_WORKING_GETCWD
> > +  /* If AT_FDCWD is not defined, the algorithm below is O(N**2) and
> > +     this is much slower than the system getcwd (at least on
> > +     GNU/Linux).  So trust the system getcwd's results unless they
> > +     look suspicious.
> > +
> > +     Use the system getcwd even if we have openat support, since the
> > +     system getcwd works even when a parent is unreadable, while the
> > +     openat-based approach does not.
> > +
> > +     But on AIX 5.1..7.1, the system getcwd is not even minimally
> > +     working: If the current directory name is slightly longer than
> > +     PATH_MAX, it omits the first directory component and returns
> > +     this wrong result with errno = 0.  */
> > +
> > +# undef getcwd
> > +  dir = getcwd_system (buf, size);
> > +  if (dir || (size && errno == ERANGE))
> > +    return dir;
>
> This conflicts with the getcwd_system implementation does not set errno.
>
> > +  /* Solaris getcwd (NULL, 0) fails with errno == EINVAL, but it has
> > +     internal magic that lets it work even if an ancestor directory is
> > +     inaccessible, which is better in many cases.  So in this case try
> > +     again with a buffer that's almost always big enough.  */
> > +  if (errno == EINVAL && buf == NULL && size == 0)
> > +    {
> > +      char big_buffer[BIG_FILE_NAME_LENGTH + 1];
> > +      dir = getcwd_system (big_buffer, sizeof big_buffer);
> > +      if (dir)
> > +        return strdup (dir);
> > +    }
>
> > +          /* When we've iterated through all directory entries without finding
> > +             one with a matching d_ino, rewind the stream and consider each
> > +             name again, but this time, using lstat.  This is necessary in a
> > +             chroot on at least one system (glibc-2.3.6 + linux 2.6.12), where
> > +             .., ../.., ../../.., etc. all had the same device number, yet the
> > +             d_ino values for entries in / did not match those obtained
> > +             via lstat.  */
> > +          if (d == NULL && errno == 0 && use_d_ino)
> > +            {
> > +              use_d_ino = false;
> > +              __rewinddir (dirstream);
> > +              d = __readdir (dirstream);
> > +            }
>
> I'm not sure if it's worthwhile to have such code in glibc.
>
> The generic getcwd isn't used by Hurd, right?  Would it make sense to
> have a trimmed-down Linux implementation as well?

This specific part is not used by glibc, for LIBC my patch sets
HAVE_MINIMALLY_WORKING_GETCWD
explicit to 0. I just added to sync with gnulib implementation and
make future changes less complex
(and it also checks if gnulib code to disable the LIBC part is working
correctly).
Florian Weimer Aug. 27, 2020, 10:58 a.m. UTC | #4
* Adhemerval Zanella:

> This specific part is not used by glibc, for LIBC my patch sets
> HAVE_MINIMALLY_WORKING_GETCWD
> explicit to 0. I just added to sync with gnulib implementation and
> make future changes less complex
> (and it also checks if gnulib code to disable the LIBC part is working
> correctly).

I meant: Is anything of this used on Hurd at all?  I don't think so.

Thanks,
Florian
Adhemerval Zanella Aug. 27, 2020, 11:06 a.m. UTC | #5
On Thu, Aug 27, 2020 at 7:58 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Adhemerval Zanella:
>
> > This specific part is not used by glibc, for LIBC my patch sets
> > HAVE_MINIMALLY_WORKING_GETCWD
> > explicit to 0. I just added to sync with gnulib implementation and
> > make future changes less complex
> > (and it also checks if gnulib code to disable the LIBC part is working
> > correctly).
>
> I meant: Is anything of this used on Hurd at all?  I don't think so.

No, Hurd re implements getcwd.c.  I personally would prefer to get
streamline version without all
the clutter required to build on all the platforms gnulib support, but
my impression from other
gnulib code sync was that we should avoid it.
Adhemerval Zanella Aug. 27, 2020, 11:07 a.m. UTC | #6
On Wed, Aug 26, 2020 at 7:39 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
>
> Thanks for looking into this. To help move this along from the Gnulib point of
> view, I merged the Gnulib-relevant parts of those proposed glibc patches into
> Gnulib, here:
>
> https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=615b43e1f914687b6ccd40c79a05626098a361d7
>
> but this Gnulib patch has the following further changes to get this to work, and
> these further changes should be in the glibc version too:
>
> >    - It does not include pathmax.h header (glibc does not provide it).
>
> That #include can be moved into a previous "#if !_LIBC" section.
>
> > +# define __getcwd rpl_getcwd
>
> This should define __getcwd_generic, not __getcwd, since the function name changed.
>
>
>
> Also, here are some minor refactorings to the resulting getcwd-generic.c that
> would improve readability. If you add these I plan to merge them into Gnulib too.
>
> * Prefer "#if _LIBC" to "#if !_LIBC", interchanging 'then' and 'else' parts.
>
> * Put those !_LIBC #defines in alphabetic order.

Thanks, I will check the modifications you did on gnulib and add back
on my patch.
Florian Weimer Aug. 27, 2020, 11:10 a.m. UTC | #7
* Adhemerval Zanella:

> On Thu, Aug 27, 2020 at 7:58 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Adhemerval Zanella:
>>
>> > This specific part is not used by glibc, for LIBC my patch sets
>> > HAVE_MINIMALLY_WORKING_GETCWD
>> > explicit to 0. I just added to sync with gnulib implementation and
>> > make future changes less complex
>> > (and it also checks if gnulib code to disable the LIBC part is working
>> > correctly).
>>
>> I meant: Is anything of this used on Hurd at all?  I don't think so.
>
> No, Hurd re implements getcwd.c.  I personally would prefer to get
> streamline version without all the clutter required to build on all
> the platforms gnulib support, but my impression from other gnulib code
> sync was that we should avoid it.

Hmm.  Maybe I will give this a try and see how much code is left after
the cleanup.  This shouldn't block the current sync if you want to go
ahead with it.

Thanks,
Florian
Adhemerval Zanella Aug. 27, 2020, 11:33 a.m. UTC | #8
On 27/08/2020 08:10, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> On Thu, Aug 27, 2020 at 7:58 AM Florian Weimer <fweimer@redhat.com> wrote:
>>>
>>> * Adhemerval Zanella:
>>>
>>>> This specific part is not used by glibc, for LIBC my patch sets
>>>> HAVE_MINIMALLY_WORKING_GETCWD
>>>> explicit to 0. I just added to sync with gnulib implementation and
>>>> make future changes less complex
>>>> (and it also checks if gnulib code to disable the LIBC part is working
>>>> correctly).
>>>
>>> I meant: Is anything of this used on Hurd at all?  I don't think so.
>>
>> No, Hurd re implements getcwd.c.  I personally would prefer to get
>> streamline version without all the clutter required to build on all
>> the platforms gnulib support, but my impression from other gnulib code
>> sync was that we should avoid it.
> 
> Hmm.  Maybe I will give this a try and see how much code is left after
> the cleanup.  This shouldn't block the current sync if you want to go
> ahead with it.

The io/getcwd-system.os text size from the last patch in this set is 1439
on x86_64.  I think resulting size reduction possible on ld.so would 
something like that.
diff mbox series

Patch

diff --git a/sysdeps/posix/getcwd.c b/sysdeps/posix/getcwd.c
index f00b337a13..3876e1a641 100644
--- a/sysdeps/posix/getcwd.c
+++ b/sysdeps/posix/getcwd.c
@@ -15,518 +15,474 @@ 
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-/* Wants:
-   AC_STDC_HEADERS
-   AC_DIR_HEADER
-   AC_UNISTD_H
-   AC_MEMORY_H
-   AC_CONST
-   AC_ALLOCA
- */
-
-/* AIX requires this to be the first thing in the file.  */
-#if defined _AIX && !defined __GNUC__
- #pragma alloca
-#endif
-
-#ifdef	HAVE_CONFIG_H
-# include "config.h"
+#if !_LIBC
+# include <config.h>
+# include <unistd.h>
+#else
+# define HAVE_OPENAT 1
+# define D_INO_IN_DIRENT 1
+# define HAVE_MSVC_INVALID_PARAMETER_HANDLER 0
+# define HAVE_MINIMALLY_WORKING_GETCWD 0
 #endif
 
 #include <errno.h>
-#include <fcntl.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <stdbool.h>
+#include <stddef.h>
 
-#ifdef	STDC_HEADERS
-# include <stddef.h>
-#endif
+#include <fcntl.h> /* For AT_FDCWD on Solaris 9.  */
 
-#if !defined __GNU_LIBRARY__ && !defined STDC_HEADERS
-extern int errno;
+/* If this host provides the openat function or if we're using the
+   gnulib replacement function with a native fdopendir, then enable
+   code below to make getcwd more efficient and robust.  */
+#if defined HAVE_OPENAT || (defined GNULIB_OPENAT && defined HAVE_FDOPENDIR)
+# define HAVE_OPENAT_SUPPORT 1
+#else
+# define HAVE_OPENAT_SUPPORT 0
 #endif
+
 #ifndef __set_errno
-# define __set_errno(val) errno = (val)
+# define __set_errno(val) (errno = (val))
 #endif
 
-#ifndef	NULL
-# define NULL	0
+#include <dirent.h>
+#ifndef _D_EXACT_NAMLEN
+# define _D_EXACT_NAMLEN(d) strlen ((d)->d_name)
 #endif
-
-#if defined USGr3 && !defined DIRENT
-# define DIRENT
-#endif /* USGr3 */
-#if defined Xenix && !defined SYSNDIR
-# define SYSNDIR
-#endif /* Xenix */
-
-#if defined POSIX || defined DIRENT || defined __GNU_LIBRARY__
-# include <dirent.h>
-# ifndef __GNU_LIBRARY__
-#  define D_NAMLEN(d) strlen((d)->d_name)
-# else
-#  define HAVE_D_NAMLEN
-#  define D_NAMLEN(d) ((d)->d_namlen)
-# endif
-#else /* not POSIX or DIRENT */
-# define dirent		direct
-# define D_NAMLEN(d)	((d)->d_namlen)
-# define HAVE_D_NAMLEN
-# if defined USG && !defined sgi
-#  if defined SYSNDIR
-#   include <sys/ndir.h>
-#  else /* Not SYSNDIR */
-#   include "ndir.h"
-#  endif /* SYSNDIR */
-# else /* not USG */
-#  include <sys/dir.h>
-# endif /* USG */
-#endif /* POSIX or DIRENT or __GNU_LIBRARY__ */
-
-#if defined HAVE_UNISTD_H || defined __GNU_LIBRARY__
-# include <unistd.h>
+#ifndef _D_ALLOC_NAMLEN
+# define _D_ALLOC_NAMLEN(d) (_D_EXACT_NAMLEN (d) + 1)
 #endif
 
-#if defined STDC_HEADERS || defined __GNU_LIBRARY__ || defined POSIX
-# include <stdlib.h>
-# include <string.h>
-# define ANSI_STRING
-#else	/* No standard headers.  */
-
-# ifdef	USG
-
-#  include <string.h>
-#  ifdef NEED_MEMORY_H
-#   include <memory.h>
-#  endif
-#  define	ANSI_STRING
-
-# else	/* Not USG.  */
-
-#  ifdef NeXT
-
-#   include <string.h>
-
-#  else	/* Not NeXT.  */
-
-#   include <strings.h>
-
-#   ifndef bcmp
-extern int bcmp ();
-#   endif
-#   ifndef bzero
-extern void bzero ();
-#   endif
-#   ifndef bcopy
-extern void bcopy ();
-#   endif
-
-#  endif /* NeXT. */
-
-# endif	/* USG.  */
-
-extern char *malloc (), *realloc ();
-extern void free ();
-
-#endif /* Standard headers.  */
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
 
-#ifndef	ANSI_STRING
-# define memcpy(d, s, n)	bcopy((s), (d), (n))
-# define memmove memcpy
-#endif	/* Not ANSI_STRING.  */
-
-#ifndef MAX
-# define MAX(a, b) ((a) < (b) ? (b) : (a))
-#endif
-
-#ifdef _LIBC
+#if _LIBC
 # ifndef mempcpy
 #  define mempcpy __mempcpy
 # endif
-# define HAVE_MEMPCPY	1
 #endif
 
-#if !defined __alloca && !defined __GNU_LIBRARY__
-
-# ifdef	__GNUC__
-#  undef alloca
-#  define alloca(n)	__builtin_alloca (n)
-# else	/* Not GCC.  */
-#  if	defined sparc || defined HAVE_ALLOCA_H
-#   include <alloca.h>
-#  else	/* Not sparc or HAVE_ALLOCA_H.  */
-#   ifndef _AIX
-extern char *alloca ();
-#   endif /* Not _AIX.  */
-#  endif /* sparc or HAVE_ALLOCA_H.  */
-# endif	/* GCC.  */
-
-# define __alloca	alloca
-
+#ifndef MAX
+# define MAX(a, b) ((a) < (b) ? (b) : (a))
+#endif
+#ifndef MIN
+# define MIN(a, b) ((a) < (b) ? (a) : (b))
 #endif
 
-#if defined HAVE_LIMITS_H || defined STDC_HEADERS || defined __GNU_LIBRARY__
-# include <limits.h>
-#else
-# include <sys/param.h>
+/* In this file, PATH_MAX only serves as a threshold for choosing among two
+   algorithms.  */
+#ifndef PATH_MAX
+# define PATH_MAX 8192
 #endif
 
-#if defined _LIBC
-# include <not-cancel.h>
-# include <kernel-features.h>
+#if D_INO_IN_DIRENT
+# define MATCHING_INO(dp, ino) ((dp)->d_ino == (ino))
 #else
-# define __openat64_nocancel(dfd, name, mode) openat64 (dfd, name, mode)
-# define __close_nocancel_nostatus(fd) close (fd)
+# define MATCHING_INO(dp, ino) true
 #endif
 
-#ifndef PATH_MAX
-# ifdef	MAXPATHLEN
-#  define PATH_MAX MAXPATHLEN
-# else
-#  define PATH_MAX 1024
-# endif
+#if HAVE_MSVC_INVALID_PARAMETER_HANDLER
+# include "msvc-inval.h"
 #endif
 
-#if !defined STDC_HEADERS && !defined __GNU_LIBRARY__
-# undef	size_t
-# define size_t	unsigned int
+#if !_LIBC
+# define __close_nocancel_nostatus close
+# define __getcwd rpl_getcwd
+# define stat64    stat
+# define __fstat64 fstat
+# define __fstatat64 fstatat
+# define __lstat64 lstat
+# define __closedir closedir
+# define __opendir opendir
+# define __readdir readdir
+# define __fdopendir fdopendir
+# define __openat openat
+# define __rewinddir rewinddir
+# define __openat64 openat
+#else
+# include <not-cancel.h>
 #endif
 
-#ifndef __GNU_LIBRARY__
-# define __lstat64	stat64
+/* The results of opendir() in this file are not used with dirfd and fchdir,
+   and we do not leak fds to any single-threaded code that could use stdio,
+   therefore save some unnecessary recursion in fchdir.c.
+   FIXME - if the kernel ever adds support for multi-thread safety for
+   avoiding standard fds, then we should use opendir_safer and
+   openat_safer.  */
+#ifdef GNULIB_defined_opendir
+# undef opendir
 #endif
-
-#ifndef _LIBC
-# define __rewinddir	rewinddir
+#ifdef GNULIB_defined_closedir
+# undef closedir
 #endif
 
-#ifndef _LIBC
-# define __getcwd getcwd
-#endif
+#if defined _WIN32 && !defined __CYGWIN__
+# if HAVE_MSVC_INVALID_PARAMETER_HANDLER
+static char *
+getcwd_nothrow (char *buf, size_t size)
+{
+  char *result;
 
-#ifndef GETCWD_RETURN_TYPE
-# define GETCWD_RETURN_TYPE char *
-#endif
+  TRY_MSVC_INVAL
+    {
+      result = _getcwd (buf, size);
+    }
+  CATCH_MSVC_INVAL
+    {
+      result = NULL;
+      errno = ERANGE;
+    }
+  DONE_MSVC_INVAL;
 
-#ifdef __ASSUME_ATFCTS
-# define __have_atfcts 1
-#elif IS_IN (rtld)
-static int __rtld_have_atfcts;
-# define __have_atfcts __rtld_have_atfcts
+  return result;
+}
+# else
+#  define getcwd_nothrow _getcwd
+# endif
+# define getcwd_system getcwd_nothrow
+#else
+# define getcwd_system getcwd
 #endif
 
-/* Get the pathname of the current working directory, and put it in SIZE
-   bytes of BUF.  Returns NULL if the directory couldn't be determined or
-   SIZE was too small.  If successful, returns BUF.  In GNU, if BUF is
-   NULL, an array is allocated with `malloc'; the array is SIZE bytes long,
-   unless SIZE == 0, in which case it is as big as necessary.  */
+/* Get the name of the current working directory, and put it in SIZE
+   bytes of BUF.  Returns NULL with errno set if the directory couldn't be
+   determined or SIZE was too small.  If successful, returns BUF.  In GNU,
+   if BUF is NULL, an array is allocated with 'malloc'; the array is SIZE
+   bytes long, unless SIZE == 0, in which case it is as big as necessary.  */
 
-GETCWD_RETURN_TYPE
+char *
 __getcwd (char *buf, size_t size)
 {
-#ifndef __ASSUME_ATFCTS
-  static const char dots[]
-    = "../../../../../../../../../../../../../../../../../../../../../../../\
-../../../../../../../../../../../../../../../../../../../../../../../../../../\
-../../../../../../../../../../../../../../../../../../../../../../../../../..";
-  const char *dotp = &dots[sizeof (dots)];
-  const char *dotlist = dots;
-  size_t dotsize = sizeof (dots) - 1;
+  /* Lengths of big file name components and entire file names, and a
+     deep level of file name nesting.  These numbers are not upper
+     bounds; they are merely large values suitable for initial
+     allocations, designed to be large enough for most real-world
+     uses.  */
+  enum
+    {
+      BIG_FILE_NAME_COMPONENT_LENGTH = 255,
+      BIG_FILE_NAME_LENGTH = MIN (4095, PATH_MAX - 1),
+      DEEP_NESTING = 100
+    };
+
+#if HAVE_OPENAT_SUPPORT
+  int fd = AT_FDCWD;
+  bool fd_needs_closing = false;
+#else
+  char dots[DEEP_NESTING * sizeof ".." + BIG_FILE_NAME_COMPONENT_LENGTH + 1];
+  char *dotlist = dots;
+  size_t dotsize = sizeof dots;
+  size_t dotlen = 0;
 #endif
-  int prev_errno = errno;
   DIR *dirstream = NULL;
-  bool fd_needs_closing = false;
-  int fd = AT_FDCWD;
-
-  char *path;
-#ifndef NO_ALLOCATION
+  dev_t rootdev, thisdev;
+  ino_t rootino, thisino;
+  char *dir;
+  register char *dirp;
+  struct stat64 st;
   size_t allocated = size;
+  size_t used;
+
+#if HAVE_MINIMALLY_WORKING_GETCWD
+  /* If AT_FDCWD is not defined, the algorithm below is O(N**2) and
+     this is much slower than the system getcwd (at least on
+     GNU/Linux).  So trust the system getcwd's results unless they
+     look suspicious.
+
+     Use the system getcwd even if we have openat support, since the
+     system getcwd works even when a parent is unreadable, while the
+     openat-based approach does not.
+
+     But on AIX 5.1..7.1, the system getcwd is not even minimally
+     working: If the current directory name is slightly longer than
+     PATH_MAX, it omits the first directory component and returns
+     this wrong result with errno = 0.  */
+
+# undef getcwd
+  dir = getcwd_system (buf, size);
+  if (dir || (size && errno == ERANGE))
+    return dir;
+
+  /* Solaris getcwd (NULL, 0) fails with errno == EINVAL, but it has
+     internal magic that lets it work even if an ancestor directory is
+     inaccessible, which is better in many cases.  So in this case try
+     again with a buffer that's almost always big enough.  */
+  if (errno == EINVAL && buf == NULL && size == 0)
+    {
+      char big_buffer[BIG_FILE_NAME_LENGTH + 1];
+      dir = getcwd_system (big_buffer, sizeof big_buffer);
+      if (dir)
+        return strdup (dir);
+    }
+
+# if HAVE_PARTLY_WORKING_GETCWD
+  /* The system getcwd works, except it sometimes fails when it
+     shouldn't, setting errno to ERANGE, ENAMETOOLONG, or ENOENT.    */
+  if (errno != ERANGE && errno != ENAMETOOLONG && errno != ENOENT)
+    return NULL;
+# endif
+#endif
   if (size == 0)
     {
       if (buf != NULL)
-	{
-	  __set_errno (EINVAL);
-	  return NULL;
-	}
+        {
+          __set_errno (EINVAL);
+          return NULL;
+        }
 
-      allocated = PATH_MAX + 1;
+      allocated = BIG_FILE_NAME_LENGTH + 1;
     }
 
   if (buf == NULL)
     {
-      path = malloc (allocated);
-      if (path == NULL)
-	return NULL;
+      dir = malloc (allocated);
+      if (dir == NULL)
+        return NULL;
     }
   else
-#else
-# define allocated size
-#endif
-    path = buf;
+    dir = buf;
 
-  char *pathp = path + allocated;
-  *--pathp = '\0';
+  dirp = dir + allocated;
+  *--dirp = '\0';
 
-  struct stat64 st;
   if (__lstat64 (".", &st) < 0)
     goto lose;
-  dev_t thisdev = st.st_dev;
-  ino_t thisino = st.st_ino;
+  thisdev = st.st_dev;
+  thisino = st.st_ino;
 
   if (__lstat64 ("/", &st) < 0)
     goto lose;
-  dev_t rootdev = st.st_dev;
-  ino_t rootino = st.st_ino;
+  rootdev = st.st_dev;
+  rootino = st.st_ino;
 
   while (!(thisdev == rootdev && thisino == rootino))
     {
-      if (__have_atfcts >= 0)
-	  fd = __openat64_nocancel (fd, "..", O_RDONLY | O_CLOEXEC);
-      else
-	fd = -1;
-      if (fd >= 0)
-	{
-	  fd_needs_closing = true;
-	  if (__fstat64 (fd, &st) < 0)
-	    goto lose;
-	}
-#ifndef __ASSUME_ATFCTS
-      else if (errno == ENOSYS)
-	{
-	  __have_atfcts = -1;
-
-	  /* Look at the parent directory.  */
-	  if (dotp == dotlist)
-	    {
-# ifdef NO_ALLOCATION
-	      __set_errno (ENOMEM);
-	      goto lose;
-# else
-	      /* My, what a deep directory tree you have, Grandma.  */
-	      char *new;
-	      if (dotlist == dots)
-		{
-		  new = malloc (dotsize * 2 + 1);
-		  if (new == NULL)
-		    goto lose;
-#  ifdef HAVE_MEMPCPY
-		  dotp = mempcpy (new, dots, dotsize);
-#  else
-		  memcpy (new, dots, dotsize);
-		  dotp = &new[dotsize];
-#  endif
-		}
-	      else
-		{
-		  new = realloc ((void *) dotlist, dotsize * 2 + 1);
-		  if (new == NULL)
-		    goto lose;
-		  dotp = &new[dotsize];
-		}
-#  ifdef HAVE_MEMPCPY
-	      *((char *) mempcpy ((char *) dotp, new, dotsize)) = '\0';
-	      dotsize *= 2;
-#  else
-	      memcpy ((char *) dotp, new, dotsize);
-	      dotsize *= 2;
-	      new[dotsize] = '\0';
-#  endif
-	      dotlist = new;
-# endif
-	    }
-
-	  dotp -= 3;
+      struct dirent *d;
+      dev_t dotdev;
+      ino_t dotino;
+      bool mount_point;
+      int parent_status;
+      size_t dirroom;
+      size_t namlen;
+      bool use_d_ino = true;
 
-	  /* Figure out if this directory is a mount point.  */
-	  if (__lstat64 (dotp, &st) < 0)
-	    goto lose;
-	}
+      /* Look at the parent directory.  */
+#if HAVE_OPENAT_SUPPORT
+      fd = __openat64 (fd, "..", O_RDONLY);
+      if (fd < 0)
+        goto lose;
+      fd_needs_closing = true;
+      parent_status = __fstat64 (fd, &st);
+#else
+      dotlist[dotlen++] = '.';
+      dotlist[dotlen++] = '.';
+      dotlist[dotlen] = '\0';
+      parent_status = __lstat64 (dotlist, &st);
 #endif
-      else
-	goto lose;
+      if (parent_status != 0)
+        goto lose;
 
       if (dirstream && __closedir (dirstream) != 0)
-	{
-	  dirstream = NULL;
-	  goto lose;
-       }
+        {
+          dirstream = NULL;
+          goto lose;
+        }
 
-      dev_t dotdev = st.st_dev;
-      ino_t dotino = st.st_ino;
-      bool mount_point = dotdev != thisdev;
+      /* Figure out if this directory is a mount point.  */
+      dotdev = st.st_dev;
+      dotino = st.st_ino;
+      mount_point = dotdev != thisdev;
 
       /* Search for the last directory.  */
-      if (__have_atfcts >= 0)
-	dirstream = __fdopendir (fd);
-#ifndef __ASSUME_ATFCTS
-      else
-	dirstream = __opendir (dotp);
-#endif
+#if HAVE_OPENAT_SUPPORT
+      dirstream = __fdopendir (fd);
       if (dirstream == NULL)
-	goto lose;
+        goto lose;
       fd_needs_closing = false;
-
-      struct dirent *d;
-      bool use_d_ino = true;
-      while (1)
-	{
-	  /* Clear errno to distinguish EOF from error if readdir returns
-	     NULL.  */
-	  __set_errno (0);
-	  d = __readdir (dirstream);
-	  if (d == NULL)
-	    {
-	      if (errno == 0)
-		{
-		  /* When we've iterated through all directory entries
-		     without finding one with a matching d_ino, rewind the
-		     stream and consider each name again, but this time, using
-		     lstat64.  This is necessary in a chroot on at least one
-		     system.  */
-		  if (use_d_ino)
-		    {
-		      use_d_ino = false;
-		      __rewinddir (dirstream);
-		      continue;
-		    }
-
-		  /* EOF on dirstream, which means that the current directory
-		     has been removed.  */
-		  __set_errno (ENOENT);
-		}
-	      goto lose;
-	    }
-
-#ifdef _DIRENT_HAVE_D_TYPE
-	  if (d->d_type != DT_DIR && d->d_type != DT_UNKNOWN)
-	    continue;
-#endif
-	  if (d->d_name[0] == '.'
-	      && (d->d_name[1] == '\0'
-		  || (d->d_name[1] == '.' && d->d_name[2] == '\0')))
-	    continue;
-	  if (use_d_ino && !mount_point && (ino_t) d->d_ino != thisino)
-	    continue;
-
-	  if (__have_atfcts >= 0)
-	    {
-	      /* We don't fail here if we cannot stat64() a directory entry.
-		 This can happen when (network) filesystems fail.  If this
-		 entry is in fact the one we are looking for we will find
-		 out soon as we reach the end of the directory without
-		 having found anything.  */
-	      if (__fstatat64 (fd, d->d_name, &st, AT_SYMLINK_NOFOLLOW) < 0)
-		continue;
-	    }
-#ifndef __ASSUME_ATFCTS
-	  else
-	    {
-	      char name[dotlist + dotsize - dotp + 1 + _D_ALLOC_NAMLEN (d)];
-# ifdef HAVE_MEMPCPY
-	      char *tmp = mempcpy (name, dotp, dotlist + dotsize - dotp);
-	      *tmp++ = '/';
-	      strcpy (tmp, d->d_name);
-# else
-	      memcpy (name, dotp, dotlist + dotsize - dotp);
-	      name[dotlist + dotsize - dotp] = '/';
-	      strcpy (&name[dotlist + dotsize - dotp + 1], d->d_name);
-# endif
-	      /* We don't fail here if we cannot stat64() a directory entry.
-		 This can happen when (network) filesystems fail.  If this
-		 entry is in fact the one we are looking for we will find
-		 out soon as we reach the end of the directory without
-		 having found anything.  */
-	      if (__lstat64 (name, &st) < 0)
-		continue;
-	    }
+#else
+      dirstream = __opendir (dotlist);
+      if (dirstream == NULL)
+        goto lose;
+      dotlist[dotlen++] = '/';
 #endif
-	  if (S_ISDIR (st.st_mode)
-	      && st.st_dev == thisdev && st.st_ino == thisino)
-	    break;
-	}
-
-      size_t namlen = _D_EXACT_NAMLEN (d);
-
-      if ((size_t) (pathp - path) <= namlen)
-	{
-#ifndef NO_ALLOCATION
-	  if (size == 0)
-	    {
-	      size_t oldsize = allocated;
-
-	      allocated = 2 * MAX (allocated, namlen);
-	      char *tmp = realloc (path, allocated);
-	      if (tmp == NULL)
-		goto lose;
-
-	      /* Move current contents up to the end of the buffer.
-		 This is guaranteed to be non-overlapping.  */
-	      pathp = memcpy (tmp + allocated - (path + oldsize - pathp),
-			      tmp + (pathp - path),
-			      path + oldsize - pathp);
-	      path = tmp;
-	    }
-	  else
+      for (;;)
+        {
+          /* Clear errno to distinguish EOF from error if readdir returns
+             NULL.  */
+          __set_errno (0);
+          d = __readdir (dirstream);
+
+          /* When we've iterated through all directory entries without finding
+             one with a matching d_ino, rewind the stream and consider each
+             name again, but this time, using lstat.  This is necessary in a
+             chroot on at least one system (glibc-2.3.6 + linux 2.6.12), where
+             .., ../.., ../../.., etc. all had the same device number, yet the
+             d_ino values for entries in / did not match those obtained
+             via lstat.  */
+          if (d == NULL && errno == 0 && use_d_ino)
+            {
+              use_d_ino = false;
+              __rewinddir (dirstream);
+              d = __readdir (dirstream);
+            }
+
+          if (d == NULL)
+            {
+              if (errno == 0)
+                /* EOF on dirstream, which can mean e.g., that the current
+                   directory has been removed.  */
+                __set_errno (ENOENT);
+              goto lose;
+            }
+          if (d->d_name[0] == '.' &&
+              (d->d_name[1] == '\0' ||
+               (d->d_name[1] == '.' && d->d_name[2] == '\0')))
+            continue;
+
+          if (use_d_ino)
+            {
+              bool match = (MATCHING_INO (d, thisino) || mount_point);
+              if (! match)
+                continue;
+            }
+
+          {
+            int entry_status;
+#if HAVE_OPENAT_SUPPORT
+            entry_status = __fstatat64 (fd, d->d_name, &st, AT_SYMLINK_NOFOLLOW);
+#else
+            /* Compute size needed for this file name, or for the file
+               name ".." in the same directory, whichever is larger.
+               Room for ".." might be needed the next time through
+               the outer loop.  */
+            size_t name_alloc = _D_ALLOC_NAMLEN (d);
+            size_t filesize = dotlen + MAX (sizeof "..", name_alloc);
+
+            if (filesize < dotlen)
+              goto memory_exhausted;
+
+            if (dotsize < filesize)
+              {
+                /* My, what a deep directory tree you have, Grandma.  */
+                size_t newsize = MAX (filesize, dotsize * 2);
+                size_t i;
+                if (newsize < dotsize)
+                  goto memory_exhausted;
+                if (dotlist != dots)
+                  free (dotlist);
+                dotlist = malloc (newsize);
+                if (dotlist == NULL)
+                  goto lose;
+                dotsize = newsize;
+
+                i = 0;
+                do
+                  {
+                    dotlist[i++] = '.';
+                    dotlist[i++] = '.';
+                    dotlist[i++] = '/';
+                  }
+                while (i < dotlen);
+              }
+
+            memcpy (dotlist + dotlen, d->d_name, _D_ALLOC_NAMLEN (d));
+            entry_status = __lstat64 (dotlist, &st);
 #endif
-	    {
-	      __set_errno (ERANGE);
-	      goto lose;
-	    }
-	}
-      pathp -= namlen;
-      (void) memcpy (pathp, d->d_name, namlen);
-      *--pathp = '/';
+            /* We don't fail here if we cannot stat() a directory entry.
+               This can happen when (network) file systems fail.  If this
+               entry is in fact the one we are looking for we will find
+               out soon as we reach the end of the directory without
+               having found anything.  */
+            if (entry_status == 0 && S_ISDIR (st.st_mode)
+                && st.st_dev == thisdev && st.st_ino == thisino)
+              break;
+          }
+        }
+
+      dirroom = dirp - dir;
+      namlen = _D_EXACT_NAMLEN (d);
+
+      if (dirroom <= namlen)
+        {
+          if (size != 0)
+            {
+              __set_errno (ERANGE);
+              goto lose;
+            }
+          else
+            {
+              char *tmp;
+              size_t oldsize = allocated;
+
+              allocated += MAX (allocated, namlen);
+              if (allocated < oldsize
+                  || ! (tmp = realloc (dir, allocated)))
+                goto memory_exhausted;
+
+              /* Move current contents up to the end of the buffer.
+                 This is guaranteed to be non-overlapping.  */
+              dirp = memcpy (tmp + allocated - (oldsize - dirroom),
+                             tmp + dirroom,
+                             oldsize - dirroom);
+              dir = tmp;
+            }
+        }
+      dirp -= namlen;
+      memcpy (dirp, d->d_name, namlen);
+      *--dirp = '/';
 
       thisdev = dotdev;
       thisino = dotino;
     }
 
-  if (dirstream != NULL && __closedir (dirstream) != 0)
+  if (dirstream && __closedir (dirstream) != 0)
     {
       dirstream = NULL;
       goto lose;
     }
 
-  if (pathp == &path[allocated - 1])
-    *--pathp = '/';
+  if (dirp == &dir[allocated - 1])
+    *--dirp = '/';
 
-#ifndef __ASSUME_ATFCTS
+#if ! HAVE_OPENAT_SUPPORT
   if (dotlist != dots)
-    free ((void *) dotlist);
+    free (dotlist);
 #endif
 
-  size_t used = path + allocated - pathp;
-  memmove (path, pathp, used);
+  used = dir + allocated - dirp;
+  memmove (dir, dirp, used);
 
   if (size == 0)
     /* Ensure that the buffer is only as large as necessary.  */
-    buf = realloc (path, used);
+    buf = (used < allocated ? realloc (dir, used) : dir);
 
   if (buf == NULL)
-    /* Either buf was NULL all along, or `realloc' failed but
+    /* Either buf was NULL all along, or 'realloc' failed but
        we still have the original string.  */
-    buf = path;
-
-  /* Restore errno on successful return.  */
-  __set_errno (prev_errno);
+    buf = dir;
 
   return buf;
 
- lose:;
-  int save_errno = errno;
-#ifndef __ASSUME_ATFCTS
-  if (dotlist != dots)
-    free ((void *) dotlist);
-#endif
-  if (dirstream != NULL)
-    __closedir (dirstream);
-  if (fd_needs_closing)
-    __close_nocancel_nostatus (fd);
-#ifndef NO_ALLOCATION
-  if (buf == NULL)
-    free (path);
+ memory_exhausted:
+  __set_errno (ENOMEM);
+ lose:
+  {
+    int save = errno;
+    if (dirstream)
+      __closedir (dirstream);
+#if HAVE_OPENAT_SUPPORT
+    if (fd_needs_closing)
+       __close_nocancel_nostatus (fd);
+#else
+    if (dotlist != dots)
+      free (dotlist);
 #endif
-  __set_errno (save_errno);
+    if (buf == NULL)
+      free (dir);
+    __set_errno (save);
+  }
   return NULL;
 }