[ELF] Fix .gnu.version crash when .dynsym is discarded

Fix #88650

In addition, delete the unneeded comment.
https://sourceware.org/gnu-gabi/program-loading-and-dynamic-linking.txt

Author: MaskRay

lld/ELF/SyntheticSections.cpp

@@ -3798,9 +3798,8 @@ VersionTableSection::VersionTableSection(Ctx &ctx)
 }
 
 void VersionTableSection::finalizeContents() {
-  // At the moment of june 2016 GNU docs does not mention that sh_link field
-  // should be set, but Sun docs do. Also readelf relies on this field.
-  getParent()->link = getPartition(ctx).dynSymTab->getParent()->sectionIndex;
+  if (OutputSection *osec = getPartition(ctx).dynSymTab->getParent())
+    getParent()->link = osec->sectionIndex;
 }
 
 size_t VersionTableSection::getSize() const {

lld/test/ELF/linkerscript/discard-section-dynsym.s

@@ -1,24 +1,43 @@
 # REQUIRES: aarch64
 
 ## We allow discarding .dynsym, check we don't crash.
-# RUN: llvm-mc -filetype=obj -triple=aarch64 %s -o %t.o
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=aarch64 a.s -o a.o
+# RUN: llvm-mc -filetype=obj -triple=aarch64 c.s -o c.o
+# RUN: ld.lld -shared --version-script=c.ver c.o -o c.so
 
-# RUN: echo 'SECTIONS { /DISCARD/ : { *(.dynsym) } }' > %t.lds
-# RUN: ld.lld -shared -T %t.lds %t.o -o %t.so
-# RUN: llvm-readelf -r %t.so | FileCheck %s
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.dynsym) } }' > 1.lds
+# RUN: ld.lld -shared -T 1.lds a.o c.so -o out1.so
+# RUN: llvm-readelf -Sr out1.so | FileCheck %s --check-prefixes=CHECK,CHECK1
 
-# RUN: echo 'SECTIONS { /DISCARD/ : { *(.dynsym .dynstr) } }' > %t.lds
-# RUN: ld.lld -shared -T %t.lds %t.o -o %t.so
-# RUN: llvm-readelf -r %t.so | FileCheck %s
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.dynsym .dynstr) } }' > 2.lds
+# RUN: ld.lld -shared -T 2.lds a.o c.so -o out2.so
+# RUN: llvm-readelf -Sr out2.so | FileCheck %s --check-prefixes=CHECK,CHECK2
+
+# CHECK:       [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
+# CHECK-NEXT:  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
+# CHECK-NEXT:  [ 1] .gnu.version      VERSYM          0000000000000000 {{.*}} 000006 02   A  0   0  2
+# CHECK1-NEXT: [ 2] .gnu.version_r    VERNEED         0000000000000008 {{.*}} 000020 00   A  5   1  4
+# CHECK2-NEXT: [ 2] .gnu.version_r    VERNEED         0000000000000008 {{.*}} 000020 00   A  0   1  4
+# CHECK1:      [ 5] .dynstr           STRTAB
 
 # CHECK:      contains 2 entries:
 # CHECK:      R_AARCH64_RELATIVE  [[#]]
 # CHECK-NEXT: R_AARCH64_GLOB_DAT  0{{$}}
 
+#--- a.s
   adrp x9, :got:var
   ldr  x9, [x9, :got_lo12:var]
+  bl __libc_start_main
 
 .data
 .align 8
 foo:
 .quad foo
+
+#--- c.s
+.globl __libc_start_main
+__libc_start_main:
+
+#--- c.ver
+GLIBC_2.34 { __libc_start_main; };