Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -1376,7 +1376,6 @@
     }
 
     ESym->st_name = Ent.StrTabOffset;
-    ESym->st_size = Body->getSize<ELFT>();
 
     // Set a section index.
     if (const OutputSection *OutSec = Body->getOutputSection())
@@ -1386,6 +1385,13 @@
     else if (isa<DefinedCommon>(Body))
       ESym->st_shndx = SHN_COMMON;
 
+    // Do not set size for undefined symbols. This size is unreliable for case
+    // when linking DSO which links to another DSO. In that case it is possible
+    // that low level DSO may be changed in the way that alters symbol size. In
+    // such case if high-level DSO is rebuilt, its CRC will change, what may be
+    // undesirable. It is possible to avoid if we set st_size to zero.
+    ESym->st_size = (ESym->st_shndx == SHN_UNDEF) ? 0 : Body->getSize<ELFT>();
+
     // st_value is usually an address of a symbol, but that has a
     // special meaining for uninstantiated common symbols (this can
     // occur if -r is given).
Index: test/ELF/dso-undef-size.s
===================================================================
--- test/ELF/dso-undef-size.s
+++ test/ELF/dso-undef-size.s
@@ -0,0 +1,32 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %S/Inputs/dso-undef-size.s -o %t1.o
+# RUN: ld.lld -shared %t1.o -o %t1.so
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2.o
+# RUN: ld.lld -shared %t2.o %t1.so -o %t2.so
+# RUN: llvm-readobj -symbols -dyn-symbols %t2.so
+
+# CHECK:      Symbols [
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: foo
+# CHECK-NEXT:     Value:
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding:
+# CHECK-NEXT:     Type:
+# CHECK-NEXT:     Other:
+# CHECK-NEXT:     Section: Undefined
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+# CHECK:      DynamicSymbols [
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: foo
+# CHECK-NEXT:     Value:
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding:
+# CHECK-NEXT:     Type:
+# CHECK-NEXT:     Other:
+# CHECK-NEXT:     Section: Undefined
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+
+.text
+.global foo