[newlib-cygwin] Cygwin: simplify linkat with AT_EMPTY_PATH

[Ken Brown <kbrown@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=182ba1f022fb42b22fd2c7d925bf687227b2b3c7

commit 182ba1f022fb42b22fd2c7d925bf687227b2b3c7
Author: Ken Brown <kbrown@cornell.edu>
Date:   Thu Feb 25 07:54:10 2021 -0500

    Cygwin: simplify linkat with AT_EMPTY_PATH
    
    linkat(olddirfd, oldpath, oldname, newdirfd, newname, AT_EMPTY_PATH)
    is supposed to create a link to the file referenced by olddirfd if
    oldname is the empty string.  Currently this is done via the /proc
    filesystem by converting the call to
    
      linkat(AT_FDCWD, "/proc/self/fd/<olddirfd>", newdirfd, newname,
             AT_SYMLINK_FOLLOW),
    
    which ultimately leads to a call to the appropriate fhandler's link
    method.  Simplify this by using cygheap_fdget to obtain the fhandler
    directly.

Diff:
---
 winsup/cygwin/syscalls.cc | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 460fe6801..6ba4f10f7 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -4962,6 +4962,8 @@ linkat (int olddirfd, const char *oldpathname,
 	int flags)
 {
   tmp_pathbuf tp;
+  fhandler_base *fh = NULL;
+
   __try
     {
       if (flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH))
@@ -4970,21 +4972,25 @@ linkat (int olddirfd, const char *oldpathname,
 	  __leave;
 	}
       char *oldpath = tp.c_get ();
-      /* AT_EMPTY_PATH with an empty oldpathname is equivalent to
-
-	   linkat(AT_FDCWD, "/proc/self/fd/<olddirfd>", newdirfd,
-		  newname, AT_SYMLINK_FOLLOW);
-
-	 Convert the request accordingly. */
       if ((flags & AT_EMPTY_PATH) && oldpathname && oldpathname[0] == '\0')
 	{
+	  /* Operate directly on olddirfd, which can be anything
+	     except a directory. */
 	  if (olddirfd == AT_FDCWD)
 	    {
 	      set_errno (EPERM);
 	      __leave;
 	    }
-	  __small_sprintf (oldpath, "/proc/%d/fd/%d", myself->pid, olddirfd);
-	  flags = AT_SYMLINK_FOLLOW;
+	  cygheap_fdget cfd (olddirfd);
+	  if (cfd < 0)
+	    __leave;
+	  if (cfd->pc.isdir ())
+	    {
+	      set_errno (EPERM);
+	      __leave;
+	    }
+	  fh = cfd;
+	  flags = 0;		/* In case AT_SYMLINK_FOLLOW was set. */
 	}
       else if (gen_full_path_at (oldpath, olddirfd, oldpathname))
 	__leave;
@@ -5003,6 +5009,8 @@ linkat (int olddirfd, const char *oldpathname,
 	    }
 	  strcpy (oldpath, old_name.get_posix ());
 	}
+      if (fh)
+	return fh->link (newpath);
       return link (oldpath, newpath);
     }
   __except (EFAULT) {}