Merge: #12839: Fix crash in zlib module due to version mismatch.

If the version of zlib used to compile the zlib module is incompatible
with the one that is actually linked in, then calls into zlib will fail.
This can leave attributes of the z_stream uninitialized, so we must take
care to avoid segfaulting by trying to use an invalid pointer.

Fix by Richard M. Tew.

3 files changed, 11 insertions, 1 deletions

diff --git a/Misc/ACKS b/Misc/ACKS
index 77baff9..dd9e9f6 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -933,6 +933,7 @@ Monty Taylor
 Amy Taylor
 Anatoly Techtonik
 Mikhail Terekhov
+Richard M. Tew
 Tobias Thelen
 James Thomas
 Robin Thomas
diff --git a/Misc/NEWS b/Misc/NEWS
index 85dbe25..b852219 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -268,6 +268,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #12839: Fix crash in zlib module due to version mismatch.
+  Fix by Richard M. Tew.
+
 - Issue #9923: The mailcap module now correctly uses the platform path
   separator for the MAILCAP environment variable on non-POSIX platforms.
 
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index 711004e..f1f84e3 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -53,7 +53,13 @@ typedef struct
 static void
 zlib_error(z_stream zst, int err, char *msg)
 {
-    const char *zmsg = zst.msg;
+    const char *zmsg = Z_NULL;
+    /* In case of a version mismatch, zst.msg won't be initialized.
+       Check for this case first, before looking at zst.msg. */
+    if (err == Z_VERSION_ERROR)
+        zmsg = "library version mismatch";
+    if (zmsg == Z_NULL)
+        zmsg = zst.msg;
     if (zmsg == Z_NULL) {
         switch (err) {
         case Z_BUF_ERROR: