[ELF] - Do not set st_size field of SHT_UNDEF symbols.

This fixes PR33598.

Size field for undefined symbols is not significant.
Setting it to fixed value, like zero, may be useful though.

For example when we have 2 DSO's, like in this PR, if lower level DSO may
change slightly (in part of some symbol's st_size)  and higher-level DSO is
rebuilt, then tools that monitoring checksum of high level DSO file can notice
it and trigger cascade of some other unnecessary actions. 
If we set st_size to zero, that can be avoided.

Differential revision: https://reviews.llvm.org/D34673

llvm-svn: 306526

3 files changed, 44 insertions, 1 deletions

diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index cb1494d427a..4c5b4f30f2c 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -1376,7 +1376,6 @@ template <class ELFT> void SymbolTableSection<ELFT>::writeTo(uint8_t *Buf) {
     }
 
     ESym->st_name = Ent.StrTabOffset;
-    ESym->st_size = Body->getSize<ELFT>();
 
     // Set a section index.
     if (const OutputSection *OutSec = Body->getOutputSection())
@@ -1386,6 +1385,14 @@ template <class ELFT> void SymbolTableSection<ELFT>::writeTo(uint8_t *Buf) {
     else if (isa<DefinedCommon>(Body))
       ESym->st_shndx = SHN_COMMON;
 
+    // Copy symbol size if it is a defined symbol. st_size is not significant
+    // for undefined symbols, so whether copying it or not is up to us if that's
+    // the case. We'll leave it as zero because by not setting a value, we can
+    // get the exact same outputs for two sets of input files that differ only
+    // in undefined symbol size in DSOs.
+    if (ESym->st_shndx != SHN_UNDEF)
+      ESym->st_size = 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).
diff --git a/lld/test/ELF/Inputs/dso-undef-size.s b/lld/test/ELF/Inputs/dso-undef-size.s
new file mode 100644
index 00000000000..424f56f82b7
--- /dev/null
+++ b/lld/test/ELF/Inputs/dso-undef-size.s
@@ -0,0 +1,4 @@
+.text
+.global foo
+.size foo, 4
+foo:
diff --git a/lld/test/ELF/dso-undef-size.s b/lld/test/ELF/dso-undef-size.s
new file mode 100644
index 00000000000..5a235565bbc
--- /dev/null
+++ b/lld/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