[ELF] Don't fall back to .text for e_entry

[MaskRay]

Summary

We have the rule to simulate
(https://sourceware.org/binutils/docs/ld/Entry-Point.html),
but the behavior is questionable
(https://sourceware.org/pipermail/binutils/2021-September/117929.html).

gold doesn't fall back to .text.
The behavior is unlikely relied by projects (there is even a warning for
executable links), so let's just delete this fallback path.

[jhenderson]

Seems reasonable to me, but should definitely be release-noted.

lld/test/ELF/entry.s

8

Should we be checking this entry address in the other cases too?

[MaskRay]

improve test

[jhenderson]

LGTM, but please wait for someone else too.

[MaskRay]

add release note

[jhenderson]

lld/docs/ReleaseNotes.rst

29

Perhaps add "Instead, a value of 0 will be written."

[peter.smith]

No objections from me. I agree with James that it should be release noted as it is user visible behaviour change.

[MaskRay]

update release note

lld/ELF/Writer.cpp

 }
 
 // The entry point address is chosen in the following ways.
 //
 // 1. the '-e' entry command-line option;
 // 2. the ENTRY(symbol) command in a linker control script;
 // 3. the value of the symbol _start, if present;
 // 4. the number represented by the entry symbol, if it is a number;
-// 5. the address of the first byte of the .text section, if present;
-// 6. the address 0.
+// 5. the address 0.
 static uint64_t getEntryAddr() {
   // Case 1, 2 or 3
   if (Symbol *b = symtab->find(config->entry))
     return b->getVA();
 
   // Case 4
   uint64_t addr;
   if (to_integer(config->entry, addr))
     return addr;
 
   // Case 5
-  if (OutputSection *sec = findSection(".text")) {
-    if (config->warnMissingEntry)
-      warn("cannot find entry symbol " + config->entry + "; defaulting to 0x" +
-           utohexstr(sec->addr));
-    return sec->addr;
-  }
-
-  // Case 6
   if (config->warnMissingEntry)
     warn("cannot find entry symbol " + config->entry +
          "; not setting start address");
   return 0;
 }
 
 static uint16_t getELFType() {
   if (config->isPic)

lld/docs/ReleaseNotes.rst

 Non-comprehensive list of changes in this release
 =================================================
 
 ELF Improvements
 ----------------
 
 * ``--export-dynamic-symbol-list`` has been added.
   (`D107317 <https://reviews.llvm.org/D107317>`_)
+* ``e_entry`` no longer falls back to the address of ``.text`` if the entry symbol does not exist.
+  Instead, a value of 0 will be written.
+  (`D110014 <https://reviews.llvm.org/D110014>`_)
 
 Breaking changes
 ----------------
 
 * ...
 
 COFF Improvements
 -----------------

lld/test/ELF/basic-ppc.s

 // CHECK-NEXT:     FileVersion: 1
 // CHECK-NEXT:     OS/ABI: FreeBSD (0x9)
 // CHECK-NEXT:     ABIVersion: 0
 // CHECK-NEXT:     Unused: (00 00 00 00 00 00 00)
 // CHECK-NEXT:   }
 // CHECK-NEXT:   Type: Executable (0x2)
 // CHECK-NEXT:   Machine: EM_PPC (0x14)
 // CHECK-NEXT:   Version: 1
-// CHECK-NEXT:   Entry: 0x100100B4
+// CHECK-NEXT:   Entry: 0x0
 // CHECK-NEXT:   ProgramHeaderOffset: 0x34
 // CHECK-NEXT:   SectionHeaderOffset: 0x104
 // CHECK-NEXT:   Flags [ (0x0)
 // CHECK-NEXT:   ]
 // CHECK-NEXT:   HeaderSize: 52
 // CHECK-NEXT:   ProgramHeaderEntrySize: 32
 // CHECK-NEXT:   ProgramHeaderCount: 4
 // CHECK-NEXT:   SectionHeaderEntrySize: 40

lld/test/ELF/basic-ppc64.s

 // CHECK-NEXT:    FileVersion: 1
 // CHECK-NEXT:    OS/ABI: SystemV (0x0)
 // CHECK-NEXT:    ABIVersion: 0
 // CHECK-NEXT:    Unused: (00 00 00 00 00 00 00)
 // CHECK-NEXT:  }
 // CHECK-NEXT:  Type: SharedObject (0x3)
 // CHECK-NEXT:  Machine: EM_PPC64 (0x15)
 // CHECK-NEXT:  Version: 1
-// CHECK-NEXT:  Entry: 0x1022C
+// CHECK-NEXT:  Entry: 0x0
 // CHECK-NEXT:  ProgramHeaderOffset: 0x40
 // CHECK-NEXT:  SectionHeaderOffset: 0x330
 // CHECK-NEXT:  Flags [ (0x2)
 // CHECK-NEXT:    0x2
 // CHECK-NEXT:  ]
 // CHECK-NEXT:  HeaderSize: 64
 // CHECK-NEXT:  ProgramHeaderEntrySize: 56
 // CHECK-NEXT:  ProgramHeaderCount: 7

lld/test/ELF/entry.s

 # REQUIRES: x86
 # 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=WARN1 %s
-# RUN: llvm-readobj --file-headers %t2 | FileCheck -check-prefix=TEXT %s
+# RUN: llvm-readobj --file-headers %t2 | FileCheck -check-prefix=NOENTRY %s
 
-# WARN1: warning: cannot find entry symbol foobar; defaulting to 0x201120
-# TEXT: Entry: 0x201120
+# WARN1: warning: cannot find entry symbol foobar; not setting start address
 
 # RUN: ld.lld %t1 -o %t2 2>&1 | FileCheck -check-prefix=WARN2 %s
-# WARN2: warning: cannot find entry symbol _start; defaulting to 0x201120
+# RUN: llvm-readobj --file-headers %t2 | FileCheck -check-prefix=NOENTRY %s
+# WARN2: warning: cannot find entry symbol _start; not setting start address
 
 # RUN: ld.lld -shared -e foobar %t1 -o %t2 2>&1 | FileCheck -check-prefix=WARN3 %s
-# WARN3: warning: cannot find entry symbol foobar; defaulting to 0x1238
+# RUN: llvm-readobj --file-headers %t2 | FileCheck -check-prefix=NOENTRY %s
+# WARN3: warning: cannot find entry symbol foobar; not setting start address
 
 # RUN: ld.lld -shared --fatal-warnings -e entry %t1 -o %t2
 # RUN: ld.lld -shared --fatal-warnings %t1 -o %t2
 
 # RUN: echo .data > %t.s
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux -n %t.s -o %t3
 # RUN: ld.lld %t3 -o %t4 2>&1 | FileCheck -check-prefix=WARN4 %s
 # RUN: llvm-readobj --file-headers %t4 | FileCheck -check-prefix=NOENTRY %s