[ELF][MIPS] Fix crash in LLD when linking code that needs PIC thunks

[arichardson]

Summary

The bug triggers when the following conditions are met:

• A thunk is created in a given input section S
• A linker script is specified
• There is at least one matcher in the linker script .text section output that does not match any of the sections in the input files, before the matcher that matches section S.

The issue was found when linking the FreeBSD kernel for MIPS when built
with -fPIC. Patch by Alfredo Mazzinghi.

[peter.smith]

(Adding my Linaro email address as a reviewer) My apologies for missing that case. This looks good to me as by definition we can't find our target section in an empty InputSectionDescription. Will be worth seeing if Rui has any style comments.

[arichardson]

test/ELF/linkerscript/thunk-gen-mips.s

41 ↗

(On Diff #123324)

I will add this newline before comitting.

[ruiu]

LGTM

lld/trunk/ELF/Relocations.cpp

ThunkSection *ThunkCreator::getISThunkSec(InputSection *IS) {
   if (TS)
     return TS;
 
   // Find InputSectionRange within Target Output Section (TOS) that the
   // InputSection (IS) that we need to precede is in.
   OutputSection *TOS = IS->getParent();
   for (BaseCommand *BC : TOS->SectionCommands)
     if (auto *ISD = dyn_cast<InputSectionDescription>(BC)) {
+      if (ISD->Sections.empty())
+        continue;
       InputSection *first = ISD->Sections.front();
       InputSection *last = ISD->Sections.back();
       if (IS->OutSecOff >= first->OutSecOff &&
           IS->OutSecOff <= last->OutSecOff) {
         TS = addThunkSection(TOS, ISD, IS->OutSecOff);
         ThunkedSections[IS] = TS;
         break;
       }

lld/trunk/test/ELF/linkerscript/thunk-gen-mips.s

+# REQUIRES: mips
+# RUN:  llvm-mc -filetype=obj -defsym=MAIN=1 -triple=mips-unknown-freebsd %s -o %t
+# RUN:  llvm-mc -filetype=obj -defsym=TARGET=1 -triple=mips-unknown-freebsd %s -o %t1
+
+# SECTIONS command with the first pattern that does not match.
+# Linking a PIC and non-PIC object files triggers the LA25 thunk generation.
+# RUN:    echo "SECTIONS { \
+# RUN:    .text : { \
+# RUN:      *(.nomatch) \
+# RUN:      %t(.text) \
+# RUN:      . = . + 0x100000 ; \
+# RUN:      %t1(.text) \
+# RUN:    } \
+# RUN:  }" > %t.script
+# RUN: ld.lld -o %t.exe --script %t.script %t %t1
+# RUN: llvm-objdump -t %t.exe | FileCheck %s
+# CHECK: SYMBOL TABLE:
+# CHECK-ANY: 00000000         .text           00000000 _start
+# CHECK-ANY: 0010000c l     F .text           00000010 __LA25Thunk_too_far
+# CHECK-ANY: 00100020 g     F .text           00000024 too_far
+
+.ifdef MAIN
+.global _start
+_start:
+  j too_far
+  nop
+.endif
+
+.ifdef TARGET
+  .text
+  .abicalls
+  .set    noreorder
+  .globl  too_far
+  .ent    too_far
+too_far:
+  nop
+  jr      $ra
+  nop
+  .end    too_far
+.endif