Content-Type: multipart/mixed; boundary="------------G8H2lPU0UXhj2MHsfzNvLNYd"
Message-ID: <777bd201-6e3a-4da0-a922-4ea9de46a3ee@gmail.com>
Date: Tue, 20 Jan 2026 07:00:00 +0200
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Content-Language: en-US
To: pgsql-hackers <pgsql-hackers@postgresql.org>
From: Alexander Lakhin <exclusion@gmail.com>
Subject: Undefined behavior detected by new clang's ubsan
Archived-At: 
 <https://www.postgresql.org/message-id/777bd201-6e3a-4da0-a922-4ea9de46a3ee%40gmail.com>
Precedence: bulk

This is a multi-part message in MIME format.
--------------G8H2lPU0UXhj2MHsfzNvLNYd
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

Hello hackers.

When trying to run `make check` for a build made by clang-21 with
sanitizers enabled:
CFLAGS="-Og  -fsanitize=address -fsanitize=undefined -fno-sanitize-recover -fno-sanitize=function" 
LDFLAGS="-static-libsan" ...

  I hit into:
../../src/include/lib/sort_template.h:314:15: runtime error: applying non-zero offset 8 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../src/include/lib/sort_template.h:314:15

I could workaround that with:
--- a/src/include/lib/sort_template.h
+++ b/src/include/lib/sort_template.h
@@ -307,6 +307,9 @@ ST_SORT(ST_ELEMENT_TYPE * data, size_t n
      int         r,
                  presorted;

+if (!data && n == 0)
+    return;
+
  loop:

But then there was:
heaptoast.c:770:26: runtime error: addition of unsigned offset to 0x7395fbd3d204 overflowed to 0x7395fbd3d142
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior heaptoast.c:770:26

sharedtuplestore.c:326:30: runtime error: applying non-zero offset 24 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior sharedtuplestore.c:326:30

and
trgm_gist.c:702:40: runtime error: applying non-zero offset 16 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior trgm_gist.c:702:40

With the attached patch applied, `make check-world` passes for me.

Reproduced with clang 20.1, but not reproduced with clang 20.0, so maybe
this note is relevant here:
https://releases.llvm.org/20.1.0/tools/clang/docs/ReleaseNotes.html#sanitizers
Changed -fsanitize=pointer-overflow to no longer report NULL + 0 as undefined behavior in C, in line with N3322, and 
matching the previous behavior for C++. NULL + non_zero continues to be reported as undefined behavior.

Best regards,
Alexander
--------------G8H2lPU0UXhj2MHsfzNvLNYd
Content-Type: text/x-patch; charset=UTF-8;
 name="fixes-for-new-clang-sanitizers.patch"
Content-Disposition: attachment;
 filename="fixes-for-new-clang-sanitizers.patch"
Content-Transfer-Encoding: base64
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--------------G8H2lPU0UXhj2MHsfzNvLNYd--