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From: Ed Schouten <ed@FreeBSD.org>
Date: Sat, 11 Feb 2012 11:11:13 +0000 (UTC)
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Subject: svn commit: r231514 - head/lib/libc/gen
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Author: ed
Date: Sat Feb 11 11:11:13 2012
New Revision: 231514
URL: http://svn.freebsd.org/changeset/base/231514

Log:
  Set read buffer size to multiple of sizeof(struct futx).
  
  If the utmpx database gets updated while an application is reading it,
  there is a chance the reading application processes partially
  overwritten entries. To solve this, make sure we always read a multiple
  of sizeof(struct futx) at a time.
  
  MFC after:	2 weeks

Modified:
  head/lib/libc/gen/getutxent.c

Modified: head/lib/libc/gen/getutxent.c
==============================================================================
--- head/lib/libc/gen/getutxent.c	Sat Feb 11 08:58:12 2012	(r231513)
+++ head/lib/libc/gen/getutxent.c	Sat Feb 11 11:11:13 2012	(r231514)
@@ -70,13 +70,18 @@ setutxdb(int db, const char *file)
 	if (uf == NULL)
 		return (-1);
 
-	/* Safety check: never use broken files. */
-	if (db != UTXDB_LOG && _fstat(fileno(uf), &sb) != -1 &&
-	    sb.st_size % sizeof(struct futx) != 0) {
-		fclose(uf);
-		uf = NULL;
-		errno = EFTYPE;
-		return (-1);
+	if (db != UTXDB_LOG) {
+		/* Safety check: never use broken files. */
+		if (_fstat(fileno(uf), &sb) != -1 &&
+		    sb.st_size % sizeof(struct futx) != 0) {
+			fclose(uf);
+			uf = NULL;
+			errno = EFTYPE;
+			return (-1);
+		}
+		/* Prevent reading of partial records. */
+		(void)setvbuf(uf, NULL, _IOFBF,
+		    rounddown(BUFSIZ, sizeof(struct futx)));
 	}
 
 	udb = db;