From a41789af204856c3a83a75b645cd24ecebc26047 Mon Sep 17 00:00:00 2001
From: Laszlo Budai <laszlo.budai@balabit.com>
Date: Tue, 2 Jul 2019 18:48:02 +0200
Subject: [PATCH] alloc: improve mi_realpath on Linux

When PATH_MAX defined, the buffer is allocated on the stack, otherwise we need to allocate
the buffer on the heap, size of the buffer is determined based on the method from Advanced
Programming in the UNIX Environment, 3rd Edition by W. Richard Stevens and Stephen A. Rago.

Signed-off-by: Laszlo Budai <laszlo.budai@outlook.com>
---
 src/alloc.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 49 insertions(+), 3 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index e8543a96..28d94add 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -425,10 +425,8 @@ char* mi_heap_realpath(mi_heap_t* heap, const char* fname, char* resolved_name)
 }
 #else
 #include <limits.h>
-#ifndef PATH_MAX
-#define PATH_MAX 260
-#endif
 
+#ifdef PATH_MAX
 char* mi_heap_realpath(mi_heap_t* heap, const char* fname, char* resolved_name) mi_attr_noexcept {
   if (resolved_name != NULL) {
     return realpath(fname,resolved_name);
@@ -439,6 +437,54 @@ char* mi_heap_realpath(mi_heap_t* heap, const char* fname, char* resolved_name)
     return mi_heap_strndup(heap,rname,PATH_MAX); // ok if `rname==NULL`
   }
 }
+#else
+#include <unistd.h>
+#define PATH_MAX_GUESS 4096  // Linux default
+
+static long _mi_path_max(void) {
+  static long path_max = 0;
+
+  if (path_max == 0) {
+    /* This code based on example from the Advanced Programming in the UNIX environment
+     * on how to determine the max path length
+     */
+    static long posix_version = 0;
+    static long xsi_version = 0;
+    if (posix_version == 0)
+      posix_version = sysconf(_SC_VERSION);
+
+    if (xsi_version == 0)
+      xsi_version = sysconf(_SC_XOPEN_VERSION);
+
+    if ((path_max = pathconf("/",_PC_PATH_MAX)) < 0)
+      path_max = PATH_MAX_GUESS;
+    else
+      path_max++;    /* add one since it's relative to root */
+
+    /*
+     * Before POSIX.1-2001, we aren't guaranteed that PATH_MAX includes
+     * the terminating null byte.  Same goes for XPG3.
+     */
+    if ((posix_version < 200112L) && (xsi_version < 4))
+      path_max++;
+  }
+
+  return path_max;
+}
+
+char* mi_heap_realpath(mi_heap_t* heap, const char* fname, char* resolved_name) mi_attr_noexcept {
+  if (resolved_name != NULL) {
+    return realpath(fname,resolved_name);
+  }
+  else {
+    char* buf= mi_heap_malloc(heap,_mi_path_max());
+    char* rname = realpath(fname,buf);
+    char* mi_rname = mi_heap_strndup(heap,rname,PATH_MAX); // ok if `rname==NULL`
+    mi_free(buf);
+    return mi_rname;
+  }
+}
+#endif
 #endif
 
 char* mi_realpath(const char* fname, char* resolved_name) mi_attr_noexcept {