From 0b0041b942f7ecca56efec4cb882caec0247984f Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Fri, 13 Mar 2026 16:06:50 +0900
Subject: [PATCH] xml2: Fix failure with xslt_process() under
 -fsanitize=undefined

The logic of xslt_process() has never considered the fact that
xsltSaveResultToString() would return NULL for an empty string (the
upstream code has always done so, with a string length of 0).  This
would cause memcpy() to be called with a NULL pointer, something
forbidden by POSIX.

Like 46ab07ffda9d and similar fixes, this is backpatched down to all the
supported branches, with a test case to cover this scenario.  An empty
string has been always returned in xml2 in this case, based on the
history of the module, so this is an old issue.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/c516a0d9-4406-47e3-9087-5ca5176ebcf9@gmail.com
Backpatch-through: 14
---
 contrib/xml2/expected/xml2.out   | 10 ++++++++++
 contrib/xml2/expected/xml2_1.out |  6 ++++++
 contrib/xml2/sql/xml2.sql        |  6 ++++++
 contrib/xml2/xslt_proc.c         |  9 ++++++++-
 4 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/contrib/xml2/expected/xml2.out b/contrib/xml2/expected/xml2.out
index eba6ae60364..3027e4df868 100644
--- a/contrib/xml2/expected/xml2.out
+++ b/contrib/xml2/expected/xml2.out
@@ -222,3 +222,13 @@ $$<xsl:stylesheet version="1.0"
   </xsl:template>
 </xsl:stylesheet>$$);
 ERROR:  failed to apply stylesheet
+-- empty output
+select xslt_process('<aaa/>',
+$$<xsl:stylesheet version="1.0"
+      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+</xsl:stylesheet>$$);
+ xslt_process 
+--------------
+ 
+(1 row)
+
diff --git a/contrib/xml2/expected/xml2_1.out b/contrib/xml2/expected/xml2_1.out
index bac90e5a2a9..ed3e399aa31 100644
--- a/contrib/xml2/expected/xml2_1.out
+++ b/contrib/xml2/expected/xml2_1.out
@@ -166,3 +166,9 @@ $$<xsl:stylesheet version="1.0"
   </xsl:template>
 </xsl:stylesheet>$$);
 ERROR:  xslt_process() is not available without libxslt
+-- empty output
+select xslt_process('<aaa/>',
+$$<xsl:stylesheet version="1.0"
+      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+</xsl:stylesheet>$$);
+ERROR:  xslt_process() is not available without libxslt
diff --git a/contrib/xml2/sql/xml2.sql b/contrib/xml2/sql/xml2.sql
index ac49cfa7c52..c7fe0c102da 100644
--- a/contrib/xml2/sql/xml2.sql
+++ b/contrib/xml2/sql/xml2.sql
@@ -137,3 +137,9 @@ $$<xsl:stylesheet version="1.0"
     </sax:output>
   </xsl:template>
 </xsl:stylesheet>$$);
+
+-- empty output
+select xslt_process('<aaa/>',
+$$<xsl:stylesheet version="1.0"
+      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+</xsl:stylesheet>$$);
diff --git a/contrib/xml2/xslt_proc.c b/contrib/xml2/xslt_proc.c
index f30a3a42c03..9f3bf527d80 100644
--- a/contrib/xml2/xslt_proc.c
+++ b/contrib/xml2/xslt_proc.c
@@ -179,7 +179,14 @@ xslt_process(PG_FUNCTION_ARGS)
 	if (resstat < 0)
 		PG_RETURN_NULL();
 
-	result = cstring_to_text_with_len((char *) resstr, reslen);
+	/*
+	 * If an empty string has been returned, resstr would be NULL. In
+	 * this case, assume that the result is an empty string.
+	 */
+	if (reslen == 0)
+		result = cstring_to_text("");
+	else
+		result = cstring_to_text_with_len((char *) resstr, reslen);
 
 	if (resstr)
 		xmlFree(resstr);