Make do even if PATH_MAX isn't defined

As explained in `man realpath(3)`:

> The POSIX.1-2001 standard version of this function is broken by design,
> since it is impossible  to determine  a  suitable  size  for  the  output
> buffer, resolved_path.  According to POSIX.1-2001 a buffer of size PATH_MAX
> suffices, but PATH_MAX need not be a defined constant, and may have to  be
> obtained  using  pathconf(3).  And asking pathconf(3) does not really help,
> since, on the one hand POSIX warns that the result of pathconf(3) may be huge
> and unsuitable for mallocing memory, and on the other hand pathconf(3) may
> return -1 to  signify  that  PATH_MAX  is  not  bounded.   The  re‐ solved_path
> == NULL  feature,  not standardized in POSIX.1-2001, but standardized in
> POSIX.1-2008, allows this design problem to be avoided.

So we can either not compile, or be pragmatic, and define PATH_MAX to a sane
value, like 4096, which is the one used on Linux and some/most BSD.

This commit also adds two tests to ensure that things aren't catastrophically
broken by this change.

4 files changed, 27 insertions, 3 deletions

diff --git a/include/stdlib.h b/include/stdlib.h
index 0336d56..c95bb55 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -43,8 +43,9 @@ __fortify_warning_if(__p == NULL, "'realpath' called with path set to `NULL`; di
 _FORTIFY_FN(realpath) char *realpath(const char *__p, char *__r)
 {
 #ifndef PATH_MAX
-#error PATH_MAX unset. A fortified realpath will not work.
-#else
+/* see man realpath(3) */
+#define PATH_MAX 4096
+#endif
         if (__r && PATH_MAX > __bos(__r, 2)) {
                 char __buf[PATH_MAX], *__ret;
                 size_t __l;
@@ -59,7 +60,6 @@ _FORTIFY_FN(realpath) char *realpath(const char *__p, char *__r)
                 return __r;
         }
         return __orig_realpath(__p, __r);
-#endif
 }
 #endif
 
diff --git a/tests/Makefile b/tests/Makefile
index 81f7f6e..9c2dede 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -61,6 +61,8 @@ RUNTIME_TARGETS= \
         test_read_static              \
         test_readlink_dynamic         \
         test_readlink_static          \
+        test_realpath_null            \
+        test_realpath                 \
         test_recv_dynamic             \
         test_recv_static              \
         test_recvfrom_dynamic         \
diff --git a/tests/test_realpath.c b/tests/test_realpath.c
new file mode 100644
index 0000000..2a17708
--- /dev/null
+++ b/tests/test_realpath.c
@@ -0,0 +1,11 @@
+#include "common.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(int argc, char** argv) {
+  char buf[PATH_MAX];
+  char* rpath = realpath("/root/../", buf);
+  printf("%s\n", rpath);
+  return 0;
+}
diff --git a/tests/test_realpath_null.c b/tests/test_realpath_null.c
new file mode 100644
index 0000000..b62cee8
--- /dev/null
+++ b/tests/test_realpath_null.c
@@ -0,0 +1,11 @@
+#include "common.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(int argc, char** argv) {
+  char* rpath = realpath("/root/../", NULL);
+  printf("%s\n", rpath);
+  free(rpath);
+  return 0;
+}