[ELF,RISCV] Fix TLSDESC=>IE when there is no TLS section

See the comment in handleTlsRelocation. For TLSDESC=>IE (the TLS symbol
is defined in another DSO), R_RISCV_TLSDESC_{LOAD_LO12,ADD_LO12_I,CALL}
referencing a non-preemptible label uses the `R_RELAX_TLS_GD_TO_LE` code
path.

If there is no TLS section, `getTlsTpOffset` will be called with null
`Out::tlsPhdr`, leading to a null pointer dereference. Since the return
value is used by `RISCV::relocateAlloc` and ignored there, just return
0.

LoongArch TLSDESC doesn't use STT_NOTYPE labels. The `if (..) return 0;`
is a no-op for LoongArch.

This patch is a follow-up to llvm#79239 and fixes some comments.

Pull Request: llvm#98569

Author: MaskRay

lld/ELF/Arch/RISCV.cpp

@@ -631,8 +631,8 @@ void RISCV::relocateAlloc(InputSectionBase &sec, uint8_t *buf) const {
       continue;
     case R_RELAX_TLS_GD_TO_LE:
       // See the comment in handleTlsRelocation. For TLSDESC=>IE,
-      // R_RISCV_TLSDESC_{LOAD_LO12,ADD_LO12,CALL} also reach here. If isToIe is
-      // true, this is actually TLSDESC=>IE optimization.
+      // R_RISCV_TLSDESC_{LOAD_LO12,ADD_LO12,CALL} also reach here. If isToLe is
+      // false, this is actually TLSDESC=>IE optimization.
       if (rel.type == R_RISCV_TLSDESC_HI20) {
         tlsdescVal = val;
         isToLe = true;

lld/ELF/InputSection.cpp

@@ -660,6 +660,11 @@ static int64_t getTlsTpOffset(const Symbol &s) {
     return s.getVA(0) + (tls->p_vaddr & (tls->p_align - 1)) - 0x7000;
   case EM_LOONGARCH:
   case EM_RISCV:
+    // See the comment in handleTlsRelocation. For TLSDESC=>IE,
+    // R_RISCV_TLSDESC_{LOAD_LO12,ADD_LO12_I,CALL} also reach here. While
+    // `tls` may be null, the return value is ignored.
+    if (s.type != STT_TLS)
+      return 0;
     return s.getVA(0) + (tls->p_vaddr & (tls->p_align - 1));
 
     // Variant 2.

lld/ELF/Relocations.cpp

@@ -1399,8 +1399,8 @@ static unsigned handleTlsRelocation(RelType type, Symbol &sym,
     // depending on the symbol being locally defined or not.
     //
     // R_RISCV_TLSDESC_{LOAD_LO12,ADD_LO12_I,CALL} reference a non-preemptible
-    // label, so the LE optimization will be categorized as
-    // R_RELAX_TLS_GD_TO_LE. We fix the categorization in RISCV::relocateAlloc.
+    // label, so TLSDESC=>IE will be categorized as R_RELAX_TLS_GD_TO_LE. We fix
+    // the categorization in RISCV::relocateAlloc.
     if (sym.isPreemptible) {
       sym.setFlags(NEEDS_TLSGD_TO_IE);
       c.addReloc({target->adjustTlsExpr(type, R_RELAX_TLS_GD_TO_IE), type,

lld/test/ELF/riscv-tlsdesc.s

@@ -37,6 +37,11 @@
 # RUN: llvm-mc -triple=riscv32 -filetype=obj d.s -o d.32.o --defsym ELF32=1
 # RUN: ld.lld -shared -soname=d.32.so -o d.32.so d.32.o --fatal-warnings
 
+## The output has a TLS reference but no TLS section.
+# RUN: llvm-mc -filetype=obj -triple=riscv64 a1.s -o a1.64.o
+# RUN: ld.lld -pie a1.64.o c.64.so -o a1.64
+# RUN: llvm-objdump --no-show-raw-insn -M no-aliases -Rd a1.64 | FileCheck %s --check-prefix=IE64A
+
 # GD64-RELA:      .rela.dyn {
 # GD64-RELA-NEXT:   0x2408 R_RISCV_TLSDESC - 0x7FF
 # GD64-RELA-NEXT:   0x23E8 R_RISCV_TLSDESC a 0x0
@@ -164,6 +169,17 @@
 # IE32-NEXT:         lw      a0, 0x80(a0)
 # IE32-NEXT:         add     a0, a0, tp
 
+# IE64A:       OFFSET           TYPE                     VALUE
+# IE64A-NEXT:  0000000000002340 R_RISCV_TLS_TPREL64      c
+# IE64A-EMPTY:
+## &.got[c]-. = 0x2340 - 0x1258 = 0x10e8
+# IE64A-LABEL: <.Ltlsdesc_hi2>:
+# IE64A-NEXT:         addi    zero, zero, 0x0
+# IE64A-NEXT:         addi    zero, zero, 0x0
+# IE64A-NEXT:   1258: auipc   a0, 0x1
+# IE64A-NEXT:         ld      a0, 0xe8(a0)
+# IE64A-NEXT:         add     a0, a0, tp
+
 #--- a.s
 .macro load dst, src
 .ifdef ELF32
@@ -202,6 +218,15 @@ a:
 b:
 .zero 1
 
+#--- a1.s
+## a.s without TLS definitions.
+.Ltlsdesc_hi2:
+  auipc a4, %tlsdesc_hi(c)
+  ld    a5, %tlsdesc_load_lo(.Ltlsdesc_hi2)(a4)
+  addi  a0, a4, %tlsdesc_add_lo(.Ltlsdesc_hi2)
+  jalr  t0, 0(a5), %tlsdesc_call(.Ltlsdesc_hi2)
+  add   a0, a0, tp
+
 #--- c.s
 .tbss
 .globl c