Don't mess up RelIplt symbols during relocatable processing

[dim]

Summary

During upgrading of the FreeBSD source tree with lld 7.0.0, I noticed
that it started complaining about crt1.o having an "index past the
end of the symbol table".

Such a symbol table looks approximately like this, viewed with `readelf
-s` (note the Ndx field being messed up):

Symbol table '.symtab' contains 4 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 SECTION LOCAL  DEFAULT    1
     2: 00000000     0 NOTYPE  WEAK   HIDDEN  RSV[0xffff] __rel_iplt_end
     3: 00000000     0 NOTYPE  WEAK   HIDDEN  RSV[0xffff] __rel_iplt_start

At first, it seemed that recent ifunc relocation work had caused this:
https://reviews.freebsd.org/rS339351, but it turned out that it was
due to incorrect processing of the object files by lld, when using -r
(a.k.a. --relocatable).

Bisecting showed that rL324421 ("Convert a use of Config->Static") was
the commit where this new behavior began. Simply reverting it solved
the issue, and the __rel_iplt symbols had an index of UND again.

Looking at Rafael's commit message, I think he simply missed the
possibility of --relocatable being in effect, so I have added an
additional check for it.

I also added a simple regression test case.

[ruiu]

LGTM

Thank you for fixing this!

ELF/Writer.cpp

 
 // The beginning and the ending of .rel[a].plt section are marked
 // with __rel[a]_iplt_{start,end} symbols if it is a statically linked
 // executable. The runtime needs these symbols in order to resolve
 // all IRELATIVE relocs on startup. For dynamic executables, we don't
 // need these symbols, since IRELATIVE relocs are resolved through GOT
 // and PLT. For details, see http://www.airs.com/blog/archives/403.
 template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
-  if (needsInterpSection())
+  if (Config->Relocatable || needsInterpSection())
     return;
   StringRef S = Config->IsRela ? "__rela_iplt_start" : "__rel_iplt_start";
   addOptionalRegular(S, In.RelaIplt, 0, STV_HIDDEN, STB_WEAK);
 
   S = Config->IsRela ? "__rela_iplt_end" : "__rel_iplt_end";
   ElfSym::RelaIpltEnd =
       addOptionalRegular(S, In.RelaIplt, 0, STV_HIDDEN, STB_WEAK);
 }

test/ELF/relocatable-rel-iplt.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i686-- %s -o %t1.o
+# RUN: ld.lld -r %t1.o -o %t2.o
+# RUN: llvm-readobj -t %t2.o | FileCheck %s
+
+// CHECK:      Symbols [
+// CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name:  (0)
+// CHECK-NEXT:     Value: 0x0
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Local (0x0)
+// CHECK-NEXT:     Type: None (0x0)
+// CHECK-NEXT:     Other: 0
+// CHECK-NEXT:     Section: Undefined (0x0)
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name:  (0)
+// CHECK-NEXT:     Value: 0x0
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Local (0x0)
+// CHECK-NEXT:     Type: Section (0x3)
+// CHECK-NEXT:     Other: 0
+// CHECK-NEXT:     Section: .text (0x1)
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name: __rel_iplt_end (1)
+// CHECK-NEXT:     Value: 0x0
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Weak (0x2)
+// CHECK-NEXT:     Type: None (0x0)
+// CHECK-NEXT:     Other [ (0x2)
+// CHECK-NEXT:       STV_HIDDEN (0x2)
+// CHECK-NEXT:     ]
+// CHECK-NEXT:     Section: Undefined (0x0)
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name: __rel_iplt_start (16)
+// CHECK-NEXT:     Value: 0x0
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Weak (0x2)
+// CHECK-NEXT:     Type: None (0x0)
+// CHECK-NEXT:     Other [ (0x2)
+// CHECK-NEXT:       STV_HIDDEN (0x2)
+// CHECK-NEXT:     ]
+// CHECK-NEXT:     Section: Undefined (0x0)
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]
+
+  movl  __rel_iplt_start, %eax
+  movl  __rel_iplt_end, %eax
+  ret
+
+  .hidden __rel_iplt_start
+  .hidden __rel_iplt_end
+  .weak __rel_iplt_start
+  .weak __rel_iplt_end