[ELF] - Fix for: Bug 30521 - lld exits non-zero return value linking a library with no entry symbol

[grimar]

Summary

PR30521 was about linking shared library. After r282295 code when linking -shared produced
"entry symbol not found" warning, what in combination with --fatal-errors failed linkage.

Patch fixes logic (and adds testcases) to follow next rules:

• If entry was specified and not found report warning.
• If entry was not specified then:
  1. Emit warning if not -shared.
  2. Do not emit warning if -shared.

[emaste]

I'm not sure this is right: shared libraries normally don't use entry but they can in some cases - for example I think glibc's rtld is built with an entry point and can be executed directly. FreeBSD's rtld has some commented-out build infrastructure to build with entry but it is not used in the default case.

My guess at the logic would be: not shared produces a warning for missing entry. Shared case never emits a warning. Set entry if specified, if not specified set to zero with no warning.

[grimar]

In D24913#552139, @emaste wrote:

I'm not sure this is right: shared libraries normally don't use entry but they can in some cases - for example I think glibc's rtld is built with an entry point and can be executed directly. FreeBSD's rtld has some commented-out build infrastructure to build with entry but it is not used in the default case.

My guess at the logic would be: not shared produces a warning for missing entry. Shared case never emits a warning. Set entry if specified, if not specified set to zero with no warning.

I am not sure we do not want to print something in case if entry was specified but not found then. That can hide a mistype for example.

So ideally behavior that seems to be the most correct is:

• If entry was specified and not found report warning.
• If entry was not specified then:
  1. Emit warning if not -shared.
  2. Do not emit warning if -shared.

[grimar]

• Updated.

[ruiu]

I think this conflicts with https://reviews.llvm.org/D24899.

[ruiu]

LGTM

lld/trunk/ELF/Driver.cpp

if (!Config->Entry.empty()) {
     // It is either "-e <addr>" or "-e <symbol>".
     HasEntryAddr = !Config->Entry.getAsInteger(0, Config->EntryAddr);
   } else if (!Config->Shared && !Config->Relocatable &&
              Config->EMachine != EM_AMDGPU) {
     // -e was not specified. Use the default start symbol name
     // if it is resolvable.
     Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
   }
-  if (!HasEntryAddr) {
+  if (!HasEntryAddr && !Config->Entry.empty()) {
     if (Symtab.find(Config->Entry))
       Config->EntrySym = Symtab.addUndefined(Config->Entry);
     else
       warning("entry symbol " + Config->Entry + " not found, assuming 0");
   }
 
   if (HasError)
     return; // There were duplicate symbols or incompatible files

lld/trunk/test/ELF/entry.s

 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
+
 # RUN: ld.lld -e foobar %t1 -o %t2 2>&1 | FileCheck -check-prefix=WARN %s
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=NOENTRY %s
+# RUN: ld.lld %t1 -o %t2 2>&1 | FileCheck -check-prefix=WARN2 %s
+
+# RUN: ld.lld -shared -e foobar %t1 -o %t2 2>&1 | FileCheck -check-prefix=WARN %s
+# RUN: ld.lld -shared --fatal-warnings -e entry %t1 -o %t2
+# RUN: ld.lld -shared --fatal-warnings %t1 -o %t2
+
 # RUN: ld.lld %t1 -o %t2 -e entry
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=SYM %s
-# RUN: ld.lld %t1 -shared -o %t2 -e entry
+# RUN: ld.lld %t1 --fatal-warnings -shared -o %t2 -e entry
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=DSO %s
 # RUN: ld.lld %t1 -o %t2 --entry=4096
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=DEC %s
 # RUN: ld.lld %t1 -o %t2 --entry 0xcafe
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=HEX %s
 # RUN: ld.lld %t1 -o %t2 -e 0777
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=OCT %s
 
 # WARN: entry symbol foobar not found, assuming 0
+# WARN2: entry symbol _start not found, assuming 0
 
 # NOENTRY: Entry: 0x0
 # SYM: Entry: 0x11000
 # DSO: Entry: 0x1000
 # DEC: Entry: 0x1000
 # HEX: Entry: 0xCAFE
 # OCT: Entry: 0x1FF
 
 .globl entry
 entry: