[LLD] [COFF] Error out if the runtime pseudo relocation function is missing (#88573)

When then linker creates runtime pseudo relocations, it places them in a
list with the assumption that the runtime will fix these relocations
later, when the image gets loaded. If the relevant runtime function
doesn't seem to be present in the linked image, error out.

Normally when linking the mingw-w64 runtime libraries, this function
always is available. However, if linking without including the mingw-w64
CRT startup files, and the image needs runtime pseudo relocations, make
it clear that this won't work as expected at runtime.

With ld.bfd, this situation is a hard error too; ld.bfd adds an
undefined reference to this symbol if runtime pseudo relocations are
needed.

A later alternative would be to actually try to pull in the symbol (if
seen in a static library, but not included yet). This would allow
decoupling the function from the main mingw-w64 CRT startup code (making
it optional, only running if the linker actually produced runtime pseudo
relocations).

Doing that would require restructuring the lld code (gathering pseudo
relocations earlier, then loading the relocator function, then pulling in
more object files to satisfy the dependencies of the relocator) though.

Also, ld.bfd doesn't currently successfully pull in more object files to
satisfy the dependency on _pei386_runtime_relocator, so with that in
mind, there's not much extra value in making LLD do it currently either;
we can't make such a change in mingw-w64's CRT until both linkers
handle it.

This fixes one issue brought up in
#84424.

Author: mstorsjo

lld/COFF/Writer.cpp

@@ -2072,8 +2072,16 @@ void Writer::createRuntimePseudoRelocs() {
     return;
   }
 
-  if (!rels.empty())
+  if (!rels.empty()) {
     log("Writing " + Twine(rels.size()) + " runtime pseudo relocations");
+    const char *symbolName = "_pei386_runtime_relocator";
+    Symbol *relocator = ctx.symtab.findUnderscore(symbolName);
+    if (!relocator)
+      error("output image has runtime pseudo relocations, but the function " +
+            Twine(symbolName) +
+            " is missing; it is needed for fixing the relocations at runtime");
+  }
+
   PseudoRelocTableChunk *table = make<PseudoRelocTableChunk>(rels);
   rdataSec->addChunk(table);
   EmptyChunk *endOfList = make<EmptyChunk>();

lld/test/COFF/autoimport-arm-data.s

@@ -33,6 +33,9 @@
     .text
     .thumb
 main:
+    bx lr
+    .global _pei386_runtime_relocator
+_pei386_runtime_relocator:
     bx lr
     .data
 ptr:

lld/test/COFF/autoimport-arm64-data.s

@@ -33,6 +33,9 @@
     .global main
     .text
 main:
+    ret
+    .global _pei386_runtime_relocator
+_pei386_runtime_relocator:
     ret
     .data
 ptr:

lld/test/COFF/autoimport-gnu-implib.s

@@ -27,5 +27,8 @@
     .text
 main:
     movl data(%rip), %eax
+    ret
+    .global _pei386_runtime_relocator
+_pei386_runtime_relocator:
     ret
     .data

lld/test/COFF/autoimport-handler-func.s

@@ -0,0 +1,36 @@
+# REQUIRES: x86
+# RUN: split-file %s %t.dir
+
+# RUN: llvm-dlltool -m i386:x86-64 -d %t.dir/lib.def -D lib.dll -l %t.dir/lib.lib
+
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.dir/main.s -filetype=obj -o %t.dir/main.obj
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.dir/func.s -filetype=obj -o %t.dir/func.obj
+# RUN: env LLD_IN_TEST=1 not lld-link -lldmingw -out:%t.dir/main.exe -entry:main %t.dir/main.obj %t.dir/lib.lib 2>&1 | FileCheck %s --check-prefix=ERR
+
+# RUN: lld-link -lldmingw -out:%t.dir/main.exe -entry:main %t.dir/main.obj %t.dir/func.obj %t.dir/lib.lib 2>&1 | FileCheck %s --check-prefix=NOERR --allow-empty
+
+# ERR: error: output image has runtime pseudo relocations, but the function _pei386_runtime_relocator is missing; it is needed for fixing the relocations at runtime
+
+# NOERR-NOT: error
+
+#--- main.s
+    .global main
+    .text
+main:
+    ret
+
+    .data
+    .long 1
+    .quad variable
+    .long 2
+
+#--- func.s
+    .global _pei386_runtime_relocator
+    .text
+_pei386_runtime_relocator:
+    ret
+
+#--- lib.def
+EXPORTS
+variable DATA
+

lld/test/COFF/autoimport-warn.s

@@ -18,6 +18,9 @@ main:
     movl variable2(%rip), %ecx
     addl %ecx, %eax
     ret
+    .global _pei386_runtime_relocator
+_pei386_runtime_relocator:
+    ret
 
     .section .rdata$.refptr.variable1,"dr",discard,.refptr.variable1
     .global .refptr.variable1

lld/test/COFF/autoimport-x86.s

@@ -55,6 +55,9 @@
     .text
 main:
     movl variable(%rip), %eax
+    ret
+    .global _pei386_runtime_relocator
+_pei386_runtime_relocator:
     ret
     .data
 ptr: