[Jamie Lokier <jamie.lokier@cern.ch>] libc/1068: [glibc] Possible erroneous return code from Linux __getdirentries

[Andreas Jaeger <aj@arthur.rhein-neckar.de>]

We've received the appended bug report about a problem with
getdirentries.

The linux man page states:
RETURN VALUE
       getdirentries returns the number of  bytes  read  or  zero
       when  at the end of the directory.  If an error occurs, -1
       is returned, and errno is set appropriately.

and <dirent.h> has:
/* Read directory entries from FD into BUF, reading at most NBYTES.
   Reading starts at offset *BASEP, and *BASEP is updated with the new
   position after reading.  Returns the number of bytes read; zero when at
   end of directory; or -1 for errors.  */


We could (as my appended patch does) directly return the -1 from
getdents for errors.  Is this the right semantic?

Andreas


1999-04-07  Andreas Jaeger  <aj@arthur.rhein-neckar.de>

	* sysdeps/unix/sysv/linux/getdents.c (__getdirentries): Return
	directly if getdents returns with error set.

--- sysdeps/unix/sysv/linux/getdents.c.~1~	Thu Oct 22 07:37:46 1998
+++ sysdeps/unix/sysv/linux/getdents.c	Wed Apr  7 13:29:55 1999
@@ -1,4 +1,4 @@
-/* Copyright (C) 1993, 1995, 1996, 1997, 1998 Free Software Foundation, Inc.
+/* Copyright (C) 1993, 1995, 1996, 1997, 1998, 1999 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -82,6 +82,9 @@
 
   retval = INLINE_SYSCALL (getdents, 3, fd, (char *) kdp, red_nbytes);
 
+  if (retval == -1)
+    return -1;
+  
   while ((char *) kdp < (char *) skdp + retval)
     {
       const size_t alignment = __alignof__ (struct dirent);

---

• To: bugs@gnu.org
• Subject: libc/1068: [glibc] Possible erroneous return code from Linux __getdirentries
• From: Jamie Lokier <jamie.lokier@cern.ch>
• Date: Wed, 7 Apr 1999 05:32:54 +0200
• Xref: arthur.rhein-neckar.de mail.gnats-libc-bugs:2925

>Number:         1068
>Category:       libc
>Synopsis:       Possible erroneous return code from Linux __getdirentries
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    libc-gnats
>State:          open
>Class:          sw-bug
>Submitter-Id:   unknown
>Arrival-Date:   Tue Apr 06 23:40:02 EDT 1999
>Last-Modified:
>Originator:     Jamie Lokier
>Organization:
 CERN, Geneva
>Release:        libc-2.1.1
>Environment:
	
Host type: i386-redhat-linux-gnu
System: Linux pcep-jamie 2.2.5 #9 Wed Mar 31 18:35:38 CEST 1999 i686 unknown
Architecture: i686

Addons: crypt glibc-compat linuxthreads
Build CFLAGS: -O3 -Wall -Winline -Wstrict-prototypes -Wwrite-strings -g
Build CC: egcs
Compiler version: egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)
Kernel headers: 2.2.5
Symbol versioning: yes
Build static: yes
Build shared: yes
Build pic-default: no
Build profile: yes
Build omitfp: no
Build bounded: no
Build static-nss: no
Stdio: libio

>Description:
	(On Linux).
	When __getdirentries is called with a buffer that is too small
	for even one directory entry, it correctly sets errno to EINVAL.
	However it then returns 0, which is the same as it returns when
	there are no more directory entries to read.

	As luck would have it, the code happens to do this even though
	the getdents system call, which it calls, returns -1.  It is the
	system call that sets errno.

>How-To-Repeat:
	Call __getdirentries with a very small buffer size, for example 1.
	Or ltrace a program reading a directory with an obscenely large
	name in it.  See the zero return value.

>Fix:
	The workaround for an application is to set errno to zero first,
	and check it afterwards if __getdirentries returns zero.

	But perhaps __getdirentries should be returning -1 in this case,
	like the getdents system call it uses?

	I'm not entirely sure this is required.  __getdirentries is not
	very well documented (only in the <dirent.h> header, not the
	manual).  And other targets, for example BSD, simply call __read
	to implement __getdirentries.  For all I know, _those_ calls
	may return zero when there isn't room.
>Audit-Trail:
>Unformatted:

---

-- 
 Andreas Jaeger   aj@arthur.rhein-neckar.de    jaeger@informatik.uni-kl.de
  for pgp-key finger ajaeger@aixd1.rhrk.uni-kl.de