From: Jim Meyering <meyering@fb.com>
Date: Wed, 25 Nov 2015 19:06:24 +0000 (-0800)
Subject: folly/detail/ThreadLocalDetail.h: avoid UBSAN-detected memcpy abuse
X-Git-Tag: deprecate-dynamic-initializer~230
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=f6f52f3e575f1c7dc75bddcfa39db37e79753836;p=folly.git

folly/detail/ThreadLocalDetail.h: avoid UBSAN-detected memcpy abuse

Summary: [technically, the existing code is probably a no-op on
all systems we care about, but since it is officially UB, switching
to a more strict platform could cause trouble, so it's worth fixing]

Calling memcpy with "nullptr" as 2nd argument is undefined, even when
the third argument is zero, and causes a failure when testing with an
UBSAN-enabled binary (-fsanitize=undefined).
Before this change, the buck-run test below would evoke this failure:

  [ RUN      ] ThreadLocalPtr.BasicDestructor
  folly/detail/ThreadLocalDetail.h:533:29: runtime error: null pointer passed as argument 2, which is declared to never be null
  third-party-buck/build/glibc/include/string.h:47:45: note: nonnull attribute specified here

Ironically, the failure of the target-determinator-buck_push_blocking test (due to an unrelated proxygen dep problem) would block me from landing this, so I am adding this line to override it.

Reviewed By: luciang, alexshap

Differential Revision: D2692625

fb-gh-sync-id: 8bdc5cd2899705f39c9565d640921de1f363807d
---

diff --git a/folly/detail/ThreadLocalDetail.h b/folly/detail/ThreadLocalDetail.h
index c97083ff..999d169d 100644
--- a/folly/detail/ThreadLocalDetail.h
+++ b/folly/detail/ThreadLocalDetail.h
@@ -530,10 +530,11 @@ struct StaticMeta {
         * destructing a ThreadLocal and writing to the elements vector
         * of this thread.
         */
-        memcpy(reallocated, threadEntry->elements,
-               sizeof(ElementWrapper) * prevCapacity);
-        using std::swap;
-        swap(reallocated, threadEntry->elements);
+        if (prevCapacity != 0) {
+          memcpy(reallocated, threadEntry->elements,
+                 sizeof(*reallocated) * prevCapacity);
+        }
+        std::swap(reallocated, threadEntry->elements);
       }
       threadEntry->elementsCapacity = newCapacity;
     }