[ELF] Fail the link early if a map file path is invalid

[jhenderson]

Summary

As with the changes made in rL297645, we do not want a potentially long link to be run, if it will ultimately fail because the map file is not writable. This change reuses the same functionality as the output file path check. See D30449 for further justification and explanations.

[ruiu]

LGTM

ELF/Filesystem.cpp

73

We generally use short variable names unlike JavaIshLongVariableName, so let's name this Desc.

ELF/Driver.cpp

template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
 
   Config->MaxPageSize = getMaxPageSize(Args);
   Config->ImageBase = getImageBase(Args);
 
   // Default output filename is "a.out" by the Unix tradition.
   if (Config->OutputFile.empty())
     Config->OutputFile = "a.out";
 
-  // Fail early if the output file is not writable. If a user has a long link,
-  // e.g. due to a large LTO link, they do not wish to run it and find that it
-  // failed because there was a mistake in their command-line.
-  if (!isFileWritable(Config->OutputFile))
+  // Fail early if the output file or map file is not writable. If a user has a
+  // long link, e.g. due to a large LTO link, they do not wish to run it and
+  // find that it failed because there was a mistake in their command-line.
+  if (!isFileWritable(Config->OutputFile, "output file"))
+    return;
+  if (!isFileWritable(Config->MapFile, "map file"))
     return;
 
   // Use default entry point name if no name was given via the command
   // line nor linker scripts. For some reason, MIPS entry point name is
   // different from others.
   Config->WarnMissingEntry =
       (!Config->Entry.empty() || (!Config->Shared && !Config->Relocatable));
   if (Config->Entry.empty() && !Config->Relocatable)

ELF/Filesystem.h

 #ifndef LLD_ELF_FILESYSTEM_H
 #define LLD_ELF_FILESYSTEM_H
 
 #include "lld/Core/LLVM.h"
 
 namespace lld {
 namespace elf {
 void unlinkAsync(StringRef Path);
-bool isFileWritable(StringRef Path);
+bool isFileWritable(StringRef Path, StringRef FileDescription);
 }
 }
 
 #endif

ELF/Filesystem.cpp

 // create a file. But we don't want to do that in this function
 // because LLD shouldn't update any file if it will end in a failure.
 // We also don't want to reimplement heuristics. So we'll let
 // FileOutputBuffer do the work.
 //
 // FileOutputBuffer doesn't touch a desitnation file until commit()
 // is called. We use that class without calling commit() to predict
 // if the given file is writable.
-bool elf::isFileWritable(StringRef Path) {
+bool elf::isFileWritable(StringRef Path, StringRef FileDescription) {
   if (auto EC = FileOutputBuffer::create(Path, 1).getError()) {
-    error("cannot open output file " + Path + ": " + EC.message());
+    error("cannot open " + FileDescription + " " + Path + ": " + EC.message());
     return false;
   }
   return true;
 }

test/ELF/early-exit-for-bad-paths.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 
 # RUN: not ld.lld %t.o -o does_not_exist/output 2>&1 | \
-# RUN:   FileCheck %s -check-prefixes=NO-DIR,CHECK
+# RUN:   FileCheck %s -check-prefixes=NO-DIR-OUTPUT,CHECK
 # RUN: not ld.lld %t.o -o %s/dir_is_a_file 2>&1 | \
-# RUN:   FileCheck %s -check-prefixes=DIR-IS-FILE,CHECK
+# RUN:   FileCheck %s -check-prefixes=DIR-IS-OUTPUT,CHECK
 
 # RUN: echo "OUTPUT(\"does_not_exist/output\")" > %t.script
 # RUN: not ld.lld %t.o %t.script 2>&1 | \
-# RUN:   FileCheck %s -check-prefixes=NO-DIR,CHECK
+# RUN:   FileCheck %s -check-prefixes=NO-DIR-OUTPUT,CHECK
 # RUN: echo "OUTPUT(\"%s/dir_is_a_file\")" > %t.script
 # RUN: not ld.lld %t.o %t.script 2>&1 | \
-# RUN:   FileCheck %s -check-prefixes=DIR-IS-FILE,CHECK
+# RUN:   FileCheck %s -check-prefixes=DIR-IS-OUTPUT,CHECK
 
-# NO-DIR: error: cannot open output file does_not_exist/output:
-# DIR-IS-FILE: error: cannot open output file {{.*}}/dir_is_a_file:
+# RUN: not ld.lld %t.o -o %t -Map=does_not_exist/output 2>&1 | \
+# RUN:   FileCheck %s -check-prefixes=NO-DIR-MAP,CHECK
+# RUN: not ld.lld %t.o -o %t -Map=%s/dir_is_a_file 2>&1 | \
+# RUN:   FileCheck %s -check-prefixes=DIR-IS-MAP,CHECK
+
+# NO-DIR-OUTPUT: error: cannot open output file does_not_exist/output:
+# DIR-IS-OUTPUT: error: cannot open output file {{.*}}/dir_is_a_file:
+# NO-DIR-MAP: error: cannot open map file does_not_exist/output:
+# DIR-IS-MAP: error: cannot open map file {{.*}}/dir_is_a_file:
 
 # We should exit before doing the actual link. If an undefined symbol error is
 # discovered we haven't bailed out early as expected.
 # CHECK-NOT: undefined_symbol
 
-# RUN: not ld.lld %t.o -o / 2>&1 | FileCheck %s -check-prefixes=ROOT
+# RUN: not ld.lld %t.o -o / 2>&1 | FileCheck %s -check-prefixes=ROOT,CHECK
 # ROOT: error: cannot open output file /
 
   .globl _start
 _start:
   call undefined_symbol

test/ELF/map-file.s

 // CHECK-NEXT: 0000000000000000 00000000000000f0     8         .symtab
 // CHECK-NEXT: 0000000000000000 0000000000000039     1 .shstrtab
 // CHECK-NEXT: 0000000000000000 0000000000000039     1         .shstrtab
 // CHECK-NEXT: 0000000000000000 000000000000002f     1 .strtab
 // CHECK-NEXT: 0000000000000000 000000000000002f     1         .strtab
 
 // RUN: not ld.lld %t1.o %t2.o %t3.o %t4.a -o %t -Map=/ 2>&1 \
 // RUN:  | FileCheck -check-prefix=FAIL %s
-// FAIL: cannot open /
+// FAIL: cannot open map file /