lld: elf: Fix sections with explict addresses in regions

[yodaldevoid]

Summary

The address for a section would be evaluated before the region was
switched to. Because of this, the position within the region would not
be updated. After the region is swapped to the dot would be set to the
out of date position within the region, undoing the section address
evaluation.

To fix this, the region is swapped to before the section's address is
evaluated. As part of the fallout of this, expandMemoryRegions needed
to be gated in setDot on the condition that the evaluated address is
less than the dot. This is for the case where sections are not listed
from lowest address to highest address.

Finally, a test for the case where sections are listed "out of order"
was added.

[ruiu]

LGTM

This change seems correct, although it is a bit tricky and perhaps unnecessary to move . backwards in many linker scripts.

lld/ELF/LinkerScript.cpp

769 ↗

(On Diff #195286)

Add () to group & operator for those who don't remember which has a higher predence, & or &&.

[yodaldevoid]

I have addressed the comment regarding the grouping of operators.

I agree that it is not often that the dot needs to be moved backwards, but if I had not run into this in the wild I would not have looked to fix this issue. GCC's linker handles this fine, for what it's worth.

[ruiu]

LGTM, please commit.

My last comment was not to you but to generally mention that this is a bit tricky case, which is true, but we need to support that anyway.

Thank you for doing this.

[yodaldevoid]

I myself do not have commit access. If someone who does could commit this it would be appreciated.

[ruiu]

Committed as r358638.

[grimar]

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

12

When the test has a large script, we usually make it as a *.test not as *.s.
I did that conversion in rL358659.

lld/trunk/ELF/LinkerScript.cpp

void LinkerScript::setDot(Expr E, const Twine &Loc, bool InSec) {
   uint64_t Val = E().getValue();
   if (Val < Dot && InSec)
     error(Loc + ": unable to move location counter backward for: " +
           Ctx->OutSec->Name);
 
   // Update to location counter means update to section size.
   if (InSec)
     expandOutputSection(Val - Dot);
-  else
+  else if (Val > Dot)
     expandMemoryRegions(Val - Dot);
 
   Dot = Val;
 }
 
 // Used for handling linker symbol assignments, for both finalizing
 // their values and doing early declarations. Returns true if symbol
 // should be defined from linker script.
static OutputSection *findFirstSection(PhdrEntry *Load) {
   return nullptr;
 }
 
 // This function assigns offsets to input sections and an output section
 // for a single sections command (e.g. ".text { *(.text); }").
 void LinkerScript::assignOffsets(OutputSection *Sec) {
   if (!(Sec->Flags & SHF_ALLOC))
     Dot = 0;
-  else if (Sec->AddrExpr)
-    setDot(Sec->AddrExpr, Sec->Location, false);
 
   Ctx->MemRegion = Sec->MemRegion;
   Ctx->LMARegion = Sec->LMARegion;
   if (Ctx->MemRegion)
     Dot = Ctx->MemRegion->CurPos;
 
+  if ((Sec->Flags & SHF_ALLOC) && Sec->AddrExpr)
+    setDot(Sec->AddrExpr, Sec->Location, false);
+
   switchTo(Sec);
 
   if (Sec->LMAExpr)
     Ctx->LMAOffset = Sec->LMAExpr().getValue() - Dot;
 
   if (MemoryRegion *MR = Sec->LMARegion)
     Ctx->LMAOffset = MR->CurPos - Dot;
 

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

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
+# RUN: echo "MEMORY {                                       \
+# RUN:   REGION (rwx) : ORIGIN = 0x1000, LENGTH = 0x100     \
+# RUN: }                                                    \
+# RUN:                                                      \
+# RUN: SECTIONS {                                           \
+# RUN:   .aaa ORIGIN(REGION) + 0x8 : { *(.aaa) } > REGION   \
+# RUN:   _stext = .;                                        \
+# RUN:   .bbb ORIGIN(REGION) : { *(.bbb) } > REGION         \
+# RUN:   . = _stext;                                        \
+# RUN: }" > %t.script
+# RUN: ld.lld %t --script %t.script -o %t2
+# RUN: llvm-objdump -section-headers %t2 | FileCheck %s
+# CHECK: .aaa        00000008 0000000000001008 DATA
+# CHECK: .bbb        00000008 0000000000001000 DATA
+
+.section .aaa, "a"
+.quad 0
+
+.section .bbb, "a"
+.quad 0