[lld-macho] Don't support relocations in cstring sections

[int3]

Summary

We can technically handle them, but since they shouldn't come up in any
real-world programs (since ld64 dedups strings unconditionally), there's
no reason to support them.

It's a thoroughly untested code path too -- as evidenced by the fact
that the only test this change breaks is one that verifies that we
reject relocations when dedup'ing. There is no test that covers the case
where we handle relocations in cstring sections when dedup is disabled.

[oontvoo]

LG

[thakis]

Could we have a diag that's more helpful for users? This looks like a "so what?" diag 🙂 ("Make sure to deduplicate strings" or what)

[int3]

Could we have a diag that's more helpful for users? This looks like a "so what?" diag

The point kind of is that there is no user-level remediation though. I don't expect this to actually happen in the wild, and if it does I'd like to have a closer look at the input files to figure out the right solution

I guess I emit the a "file a bug" message like we do for uncaught exceptions but that seems a bit too much?

[thakis]

"file bug" seems more actionable to me, but fair enough 🙂

lld/MachO/InputFiles.cpp

auto splitRecords = [&](size_t recordSize) -> void {
         auto *isec = make<ConcatInputSection>(
             section, data.slice(off, std::min(data.size(), recordSize)), align);
         subsections.push_back({off, isec});
       }
       section.doneSplitting = true;
     };
 
     if (sectionType(sec.flags) == S_CSTRING_LITERALS) {
-      if (sec.nreloc && config->dedupStrings)
-        fatal(toString(this) + " contains relocations in " + sec.segname + "," +
-              sec.sectname +
-              ", so LLD cannot deduplicate strings. Try re-running with "
-              "--no-deduplicate-strings.");
-
-      InputSection *isec = make<CStringInputSection>(
-          section, data, align,
-          /*dedupLiterals=*/name == section_names::objcMethname ||
-              config->dedupStrings);
+      if (sec.nreloc)
+        fatal(toString(this) + ": " + sec.segname + "," + sec.sectname +
+              " contains relocations, which is unsupported");
+      bool dedupLiterals =
+          name == section_names::objcMethname || config->dedupStrings;
+      InputSection *isec =
+          make<CStringInputSection>(section, data, align, dedupLiterals);
       // FIXME: parallelize this?
       cast<CStringInputSection>(isec)->splitIntoPieces();
       section.subsections.push_back({0, isec});
     } else if (isWordLiteralSection(sec.flags)) {
       if (sec.nreloc)
-        fatal(toString(this) + " contains unsupported relocations in " +
-              sec.segname + "," + sec.sectname);
+        fatal(toString(this) + ": " + sec.segname + "," + sec.sectname +
+              " contains relocations, which is unsupported");
       InputSection *isec = make<WordLiteralInputSection>(section, data, align);
       section.subsections.push_back({0, isec});
     } else if (auto recordSize = getRecordSize(segname, name)) {
       splitRecords(*recordSize);
     } else if (name == section_names::ehFrame &&
                segname == segment_names::text) {
       splitEhFrames(data, *sections.back());
     } else if (segname == segment_names::llvm) {

lld/test/MachO/invalid/cstring-dedup.s

 ## We're intentionally testing fatal errors (for malformed input files), and
 ## fatal errors aren't supported for testing when main is run twice.
 # XFAIL: main-run-twice
 
 # REQUIRES: x86
 # RUN: rm -rf %t; split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/not-terminated.s -o %t/not-terminated.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/relocs.s -o %t/relocs.o
 
 # RUN: not %lld -dylib %t/not-terminated.o 2>&1 | FileCheck %s --check-prefix=TERM
 # RUN: not %lld -dylib %t/relocs.o 2>&1 | FileCheck %s --check-prefix=RELOCS
 
 # TERM:   not-terminated.o:(__cstring+0x4): string is not null terminated
-# RELOCS: relocs.o contains relocations in __TEXT,__cstring, so LLD cannot deduplicate strings. Try re-running with --no-deduplicate-strings.
+# RELOCS: error: {{.*}}relocs.o: __TEXT,__cstring contains relocations, which is unsupported
 
 #--- not-terminated.s
 .cstring
 .asciz "foo"
 .ascii "oh no"
 
 #--- relocs.s
 .cstring
 _str:
 .asciz "foo"
 .quad _str