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changeset 15326:52719068f9c2

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pipe, pipe2: don't corrupt fd on error I noticed a potential subtle double-close bug in libvirt. There, a common idiom is to initialize an int fd[2]={-1,-1}, then have multiple error paths goto common cleanup code. In the cleanup code, the fds are closed if they are not already -1; this works if the error label is reached before the pipe call, or after pipe succeeds, but if it was the pipe call itself that jumped to the error label, then it is relying on failed pipe() not altering the values already in fd array prior to the failure. Our pipe2 replacement violated this assumption, and could leave a non-negative value in the array, which in turn would let libvirt close an already-closed fd, possibly nuking an unrelated fd opened by another thread that happened to get the same value. As a result, I raised a POSIX issue regarding the behavior of pipe on failure: http://austingroupbugs.net/view.php?id=467 Using that test program, I learned that most systems leave fd unchanged on error, but that mingw always assigns -1 into the array. This fixes the mingw pipe() replacement, as well as the gnulib pipe2() replacement. I don't know of any race-free way to work around a cygwin crash: http://cygwin.com/ml/cygwin/2011-06/msg00328.html - we could always open() and then close() two fds to guess whether two spare fd still remain before calling pipe(), but that is racy. * lib/pipe.c (pipe): Leave fd unchanged on error. * lib/pipe2.c (pipe2): Likewise. * doc/posix-functions/pipe.texi (pipe): Document cygwin issue. * doc/glibc-functions/pipe2.texi (pipe2): Likewise. Signed-off-by: Eric Blake <eblake@redhat.com>
author Eric Blake <eblake@redhat.com>
date Wed, 29 Jun 2011 15:46:50 -0600
parents 9c73c1ba1864
children 5e07722935cf
files ChangeLog doc/glibc-functions/pipe2.texi doc/posix-functions/pipe.texi lib/pipe.c lib/pipe2.c
diffstat 5 files changed, 38 insertions(+), 2 deletions(-) [+]
line wrap: on
line diff
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2011-06-29  Eric Blake  <eblake@redhat.com>
+
+	pipe, pipe2: don't corrupt fd on error
+	* lib/pipe.c (pipe): Leave fd unchanged on error.
+	* lib/pipe2.c (pipe2): Likewise.
+	* doc/posix-functions/pipe.texi (pipe): Document cygwin issue.
+	* doc/glibc-functions/pipe2.texi (pipe2): Likewise.
+
 2011-06-27  Paolo Bonzini  <bonzini@gnu.org>
 
 	mmap-anon: do not use regular expressions inadvertently
--- a/doc/glibc-functions/pipe2.texi
+++ b/doc/glibc-functions/pipe2.texi
@@ -14,6 +14,10 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
+@item
+This function crashes rather than failing with @code{EMFILE} if no
+resources are left on some platforms:
+Cygwin 1.7.9.
 @end itemize
 
 Note: This function portably supports the @code{O_NONBLOCK} flag only if the
--- a/doc/posix-functions/pipe.texi
+++ b/doc/posix-functions/pipe.texi
@@ -15,4 +15,8 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
+@item
+This function crashes rather than failing with @code{EMFILE} if no
+resources are left on some platforms:
+Cygwin 1.7.9.
 @end itemize
--- a/lib/pipe.c
+++ b/lib/pipe.c
@@ -32,7 +32,16 @@
 int
 pipe (int fd[2])
 {
-  return _pipe (fd, 4096, _O_BINARY);
+  /* Mingw changes fd to {-1,-1} on failure, but this violates
+     http://austingroupbugs.net/view.php?id=467 */
+  int tmp[2];
+  int result = _pipe (tmp, 4096, _O_BINARY);
+  if (!result)
+    {
+      fd[0] = tmp[0];
+      fd[1] = tmp[1];
+    }
+  return result;
 }
 
 #else
--- a/lib/pipe2.c
+++ b/lib/pipe2.c
@@ -40,6 +40,11 @@
 int
 pipe2 (int fd[2], int flags)
 {
+  /* Mingw _pipe() corrupts fd on failure; also, if we succeed at
+     creating the pipe but later fail at changing fcntl, we want
+     to leave fd unchanged: http://austingroupbugs.net/view.php?id=467  */
+  int tmp[2] = { fd[0], fd[1] };
+
 #if HAVE_PIPE2
 # undef pipe2
   /* Try the system call first, if it exists.  (We may be running with a glibc
@@ -71,7 +76,11 @@
 /* Native Woe32 API.  */
 
   if (_pipe (fd, 4096, flags & ~O_NONBLOCK) < 0)
-    return -1;
+    {
+      fd[0] = tmp[0];
+      fd[1] = tmp[1];
+      return -1;
+    }
 
   /* O_NONBLOCK handling.
      On native Windows platforms, O_NONBLOCK is defined by gnulib.  Use the
@@ -145,6 +154,8 @@
     int saved_errno = errno;
     close (fd[0]);
     close (fd[1]);
+    fd[0] = tmp[0];
+    fd[1] = tmp[1];
     errno = saved_errno;
     return -1;
   }