[newlib-cygwin/cygwin-3_4-branch] Cygwin: Fix and streamline AT_EMPTY_PATH handling

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=b56b4d7fd85b4caa04593b20320468de12bfcec3

commit b56b4d7fd85b4caa04593b20320468de12bfcec3
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Wed Jul 12 13:47:49 2023 +0200
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Wed Jul 26 15:19:59 2023 +0200

    Cygwin: Fix and streamline AT_EMPTY_PATH handling
    
    The GLIBC extension AT_EMPTY_PATH allows the functions fchownat
    and fstatat to operate on dirfd alone, if the given pathname is an
    empty string.  This also allows to operate on any file type, not
    only directories.
    
    Commit fa84aa4dd2fb4 broke this.  It only allows dirfd to be a
    directory in calls to these two functions.
    
    Fix that by handling AT_EMPTY_PATH right in gen_full_path_at.
    A valid dirfd and an empty pathname is now a valid combination
    and, noticably, this returns a valid path in path_ret.  That
    in turn allows to remove the additional path generation code
    from the callers.
    
    Fixes: fa84aa4dd2fb ("Cygwin: fix errno values set by readlinkat")
    Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
    Tested-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/syscalls.cc | 47 +++++++++++------------------------------------
 1 file changed, 11 insertions(+), 36 deletions(-)

diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 33cf4f07212c..a0ad3c2d0b29 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -4439,7 +4439,7 @@ gen_full_path_at (char *path_ret, int dirfd, const char *pathname,
 	  cygheap_fdget cfd (dirfd);
 	  if (cfd < 0)
 	    return -1;
-	  if (!cfd->pc.isdir ())
+	  if (!cfd->pc.isdir () && !(flags & AT_EMPTY_PATH))
 	    {
 	      set_errno (ENOTDIR);
 	      return -1;
@@ -4450,6 +4450,8 @@ gen_full_path_at (char *path_ret, int dirfd, const char *pathname,
 	{
 	  if (!*pathname)
 	    {
+	      if (flags & AT_EMPTY_PATH)
+		return 0;
 	      set_errno (ENOENT);
 	      return -1;
 	    }
@@ -4571,29 +4573,14 @@ fchownat (int dirfd, const char *pathname, uid_t uid, gid_t gid, int flags)
 	  __leave;
 	}
       char *path = tp.c_get ();
-      int res = gen_full_path_at (path, dirfd, pathname);
+      int res = gen_full_path_at (path, dirfd, pathname, flags);
       if (res)
+	__leave;
+      if (!*pathname) /* Implies AT_EMPTY_PATH */
 	{
-	  if (!(errno == ENOENT && (flags & AT_EMPTY_PATH)))
-	    __leave;
-	  /* pathname is an empty string.  Operate on dirfd. */
-	  if (dirfd == AT_FDCWD)
-	    {
-	      cwdstuff::acquire_read ();
-	      strcpy (path, cygheap->cwd.get_posix ());
-	      cwdstuff::release_read ();
-	    }
-	  else
-	    {
-	      cygheap_fdget cfd (dirfd);
-	      if (cfd < 0)
-		__leave;
-	      strcpy (path, cfd->get_name ());
-	      /* If dirfd refers to a symlink (which was necessarily
-		 opened with O_PATH | O_NOFOLLOW), we must operate
-		 directly on that symlink.. */
-	      flags = AT_SYMLINK_NOFOLLOW;
-	    }
+	  /* If dirfd refers to a symlink (which was necessarily opened with
+	     O_PATH | O_NOFOLLOW), we must operate directly on that symlink. */
+	  flags = AT_SYMLINK_NOFOLLOW;
 	}
       return chown_worker (path, (flags & AT_SYMLINK_NOFOLLOW)
 				 ? PC_SYM_NOFOLLOW : PC_SYM_FOLLOW, uid, gid);
@@ -4616,21 +4603,9 @@ fstatat (int dirfd, const char *__restrict pathname, struct stat *__restrict st,
 	  __leave;
 	}
       char *path = tp.c_get ();
-      int res = gen_full_path_at (path, dirfd, pathname);
+      int res = gen_full_path_at (path, dirfd, pathname, flags);
       if (res)
-	{
-	  if (!(errno == ENOENT && (flags & AT_EMPTY_PATH)))
-	    __leave;
-	  /* pathname is an empty string.  Operate on dirfd. */
-	  if (dirfd == AT_FDCWD)
-	    {
-	      cwdstuff::acquire_read ();
-	      strcpy (path, cygheap->cwd.get_posix ());
-	      cwdstuff::release_read ();
-	    }
-	  else
-	    return fstat (dirfd, st);
-	}
+	  __leave;
       path_conv pc (path, ((flags & AT_SYMLINK_NOFOLLOW)
 			   ? PC_SYM_NOFOLLOW : PC_SYM_FOLLOW)
 			  | PC_POSIX | PC_KEEP_HANDLE, stat_suffixes);