Regression fix for realpath() buffer.

Let the system allocate realpath() buffers intead
of trying to portably derive a buffer size.

This was found with default GCC fortify settings on
Ubuntu 18.04 and showed with -D_FORTIFY_SOURCE=2 on Fedora, too.

2 files changed, 26 insertions, 11 deletions

diff --git a/NEWS b/NEWS
index dee02e2a..f2a7329f 100644
--- a/NEWS
+++ b/NEWS
@@ -65,11 +65,12 @@ removed from a 6.5.0 or newer release.)
 
 fetchmail-6.4.1 (released 2019-09-28, 27459 LoC):
 
-## REGRESSION FIX:
-* The bug fix Debian Bug#941129 was incomplete and caused a regression
-  in the default file locations, so that fetchmail was no longer able to
-  find it configuration files in some situations.
-  Reported by Cy Schubert.
+## REGRESSION FIXES:
+* The bug fix Debian Bug#941129 was incomplete and caused
+  + a regression in the default file locations, so that fetchmail was no longer
+    able to find it configuration files in some situations.
+    Reported by Cy Schubert.
+  + a regression under _FORTIFY_SOURCE where PATH_MAX > minimal _POSIX_PATH_MAX.
 
 --------------------------------------------------------------------------------
 
diff --git a/env.c b/env.c
index d181945a..a26cf908 100644
--- a/env.c
+++ b/env.c
@@ -137,9 +137,16 @@ void envquery(int argc, char **argv)
      * without changing behaviour.
      */
     {
-	static char _home_abs[_POSIX_PATH_MAX];
-        char *tmp = realpath(home, _home_abs);
-        if (tmp) home = _home_abs;
+	static char *_home_abs;
+	char *tmp;
+	if (_home_abs) free(_home_abs), _home_abs = 0;
+        tmp = realpath(home, NULL);
+        if (tmp) {
+		home = _home_abs = tmp;
+	} else {
+		report(stderr, GT_("Cannot find absolute path for user's home directory.\n"));
+		exit(PS_UNDEFINED);
+	}
     }
 
     /* compute fetchmail's home directory */
@@ -154,9 +161,16 @@ void envquery(int argc, char **argv)
      * This is to fix Debian Bug#941129 by Alex Andreotti.
      */
     {
-	static char _fmhome_abs[_POSIX_PATH_MAX];
-        char *tmp = realpath(fmhome, _fmhome_abs);
-        if (tmp) fmhome = _fmhome_abs;
+	static char *_fmhome_abs;
+	char *tmp;
+	if (_fmhome_abs) free(_fmhome_abs), _fmhome_abs = 0;
+        tmp = realpath(fmhome, NULL);
+        if (tmp) {
+		fmhome = _fmhome_abs = tmp;
+	} else {
+		report(stderr, GT_("Cannot find absolute path for fetchmail's home directory.\n"));
+		exit(PS_UNDEFINED);
+	}
     }
 
 #define RCFILE_NAME	"fetchmailrc"