[ELF] - Allow moving location counter backward in some cases.

[grimar]

Summary

PHDRS {
 text PT_LOAD FLAGS(5); /* R_E */
 data PT_LOAD FLAGS(6); /* RW_ */
 percpu PT_LOAD FLAGS(6); /* RW_ */
 init PT_LOAD FLAGS(7); /* RWE */
 note PT_NOTE FLAGS(0); /* ___ */
}
SECTIONS
{
 . = (0xffffffff80000000 + ALIGN(0x1000000, 0x200000));

 .text : AT(ADDR(.text) - 0xffffffff80000000) {...} :text = 0x9090
 .notes : AT(ADDR(.notes) - 0xffffffff80000000) {...} :text :note
 __ex_table : AT(ADDR(__ex_table) - 0xffffffff80000000) {...} :text = 0x9090
 ...
 .data..percpu 0 : AT(__per_cpu_load - 0xffffffff80000000) { } :percpu
 . = __per_cpu_load + SIZEOF(.data..percpu);
...
}

When trying to assign address 0x0 to .data..percpu LLD fails with
"unable to move location counter backward" error. Though it should be safe to do
because script controls segments via PHDRS command by itself.

This patch suggests to change logic we use currently. I think what we can do instead of
restricting moving counter always is to check that all sections in PT_LOAD are in assending order.

[grimar]

• Removed usused conditions included by mistake.
• Addressed review comment about comment grammar.

[ruiu]

Could you give me a program you are trying to link with this patch?

[grimar]

In D34977#805861, @ruiu wrote:

Could you give me a program you are trying to link with this patch?

It is linux kernel. Script is:
https://github.com/torvalds/linux/blob/master/arch/x86/kernel/vmlinux.lds.S

Reproduce file uploaded here:
https://drive.google.com/file/d/0B_OWr6ld9gUmNG4xbW93bDBDWWM/view

[ruiu]

ELF/Writer.cpp

1652–1668 ↗

(On Diff #105393)

Can you remove this? It seems this is not directly related to the change you want to make.

[grimar]

• Addressed review comments (removed sections VA's ordering verification logic).

[ruiu]

LGTM

lld/trunk/ELF/LinkerScript.cpp

LinkerScript::getOrCreateOutputSectionCommand(StringRef Name) {
   OutputSectionCommand *&CmdRef = NameToOutputSectionCommand[Name];
   if (!CmdRef)
     CmdRef = make<OutputSectionCommand>(Name);
   return CmdRef;
 }
 
 void LinkerScript::setDot(Expr E, const Twine &Loc, bool InSec) {
   uint64_t Val = E().getValue();
-  if (Val < Dot) {
-    if (InSec)
-      error(Loc + ": unable to move location counter backward for: " +
-            CurAddressState->OutSec->Name);
-    else
-      error(Loc + ": unable to move location counter backward");
-  }
+  if (Val < Dot && InSec)
   Dot = Val;
   // Update to location counter means update to section size.
   if (InSec)
     CurAddressState->OutSec->Size = Dot - CurAddressState->OutSec->Addr;
 }
 
 // Sets value of a symbol. Two kinds of symbols are processed: synthetic
 // symbols, whose value is an offset from beginning of section and regular

lld/trunk/test/ELF/linkerscript/locationcountererr2.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: echo "SECTIONS {" > %t.script
 # RUN: echo ". = 0x20; . = 0x10; .text : {} }" >> %t.script
-# RUN: not ld.lld %t.o --script %t.script -o %t -shared 2>&1 | FileCheck %s
-# CHECK: {{.*}}.script:2: unable to move location counter backward
+# RUN: ld.lld %t.o --script %t.script -o %t -shared
+# RUN: llvm-objdump -section-headers %t | FileCheck %s
+# CHECK: Idx Name   Size      Address
+# CHECK:  1 .text 00000000 0000000000000010
 
 # RUN: echo "SECTIONS { . = 0x20; . = ASSERT(0x1, "foo"); }" > %t2.script
 # RUN: ld.lld %t.o --script %t2.script -o %t -shared

lld/trunk/test/ELF/linkerscript/out-of-order.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-linux %s -o %t.o
 # RUN: echo "SECTIONS { .data 0x4000 : { *(.data) } .text 0x2000 : { *(.text) } }" > %t.script
-# RUN: not ld.lld -o %t.so --script %t.script %t.o -shared 2>&1 | FileCheck %s
+# RUN: ld.lld -o %t.so --script %t.script %t.o -shared
+# RUN: llvm-objdump -section-headers %t.so | FileCheck %s
 
-# CHECK:  error: {{.*}}.script:1: unable to move location counter backward
+# CHECK:      Sections:
+# CHECK-NEXT: Idx Name          Size      Address          Type
+# CHECK-NEXT:   0               00000000 0000000000000000
+# CHECK-NEXT:   1 .data         00000008 0000000000004000  DATA
+# CHECK-NEXT:   2 .dynamic      00000060 0000000000004008
+# CHECK-NEXT:   3 .text         00000008 0000000000002000  TEXT DATA
+# CHECK-NEXT:   4 .dynsym       00000018 0000000000002008
+# CHECK-NEXT:   5 .hash         00000010 0000000000002020
+# CHECK-NEXT:   6 .dynstr       00000001 0000000000002030
 
 .quad 0
 .data
 .quad 0