[ELF] Fix incorrect physical address on self-referencing AT command

[erickreyes]

Summary

When a section placement (AT) command references the section itself, the physical address of the section in the ELF header was calculated incorrectly due to alignment happening right after the location pointer's value was captured.

[ruiu]

This looks good, but I wonder in what situation you want to define a self-referencing LMA. (I just want to understand the problem better.)

[erickreyes]

In D41403#964012, @ruiu wrote:

This looks good, but I wonder in what situation you want to define a self-referencing LMA. (I just want to understand the problem better.)

I ran into this problem when trying to link the Linux kernel with lld. There are a lot of sections defined like this in arm64:

.rodata : AT(ADDR(.rodata) - 0)

Since the linker script is generated, I guess there are situations where an offset is applied to the section address (zero in this case).

[ruiu]

LGTM

[erickreyes]

Replaced the patch with Rafael's version, which avoids computing section alignment twice

[• espindola]

Do you have commit access?

[erickreyes]

No, I don't

[• espindola]

322421

ELF/LinkerScript.cpp

 
   Ctx->OutSec = Sec;
   Ctx->OutSec->Addr = advance(0, Ctx->OutSec->Alignment);
-
-  // If neither AT nor AT> is specified for an allocatable section, the linker
-  // will set the LMA such that the difference between VMA and LMA for the
-  // section is the same as the preceding output section in the same region
-  // https://sourceware.org/binutils/docs-2.20/ld/Output-Section-LMA.html
-  if (Ctx->LMAOffset)
-    Ctx->OutSec->LMAOffset = Ctx->LMAOffset();
 }
 
 // This function searches for a memory region to place the given output

   if (Ctx->MemRegion)
     Dot = Ctx->MemRegionOffset[Ctx->MemRegion];
 
+  switchTo(Sec);
+
   if (Sec->LMAExpr) {
     uint64_t D = Dot;
     Ctx->LMAOffset = [=] { return Sec->LMAExpr().getValue() - D; };
   }
+  // If neither AT nor AT> is specified for an allocatable section, the linker
+  // will set the LMA such that the difference between VMA and LMA for the
+  // section is the same as the preceding output section in the same region
+  // https://sourceware.org/binutils/docs-2.20/ld/Output-Section-LMA.html
+  if (Ctx->LMAOffset)
+    Ctx->OutSec->LMAOffset = Ctx->LMAOffset();
 
-  switchTo(Sec);
-
   // The Size previously denoted how many InputSections had been added to this
   // section, and was used for sorting SHF_LINK_ORDER sections. Reset it to
   // compute the actual size value.

test/ELF/linkerscript/at-self-reference.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { \
+# RUN:  . = 0x1000; \
+# RUN:  .aaa : AT(ADDR(.aaa)) { *(.aaa) } \
+# RUN:  .bbb : AT(ADDR(.bbb)) { *(.bbb) } \
+# RUN: }" > %t.script
+# RUN: ld.lld %t --script %t.script -o %t2
+# RUN: llvm-readobj -program-headers %t2 | FileCheck %s
+
+# CHECK:      ProgramHeaders [
+# CHECK-NEXT:  ProgramHeader {
+# CHECK-NEXT:    Type: PT_LOAD (0x1)
+# CHECK-NEXT:    Offset: 0x1000
+# CHECK-NEXT:    VirtualAddress: 0x1000
+# CHECK-NEXT:    PhysicalAddress: 0x1000
+# CHECK-NEXT:    FileSize: 3
+# CHECK-NEXT:    MemSize: 3
+# CHECK-NEXT:    Flags [ (0x5)
+# CHECK-NEXT:      PF_R (0x4)
+# CHECK-NEXT:      PF_X (0x1)
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Alignment: 4096
+# CHECK-NEXT:  }
+# CHECK-NEXT:  ProgramHeader {
+# CHECK-NEXT:    Type: PT_LOAD (0x1)
+# CHECK-NEXT:    Offset: 0x1008
+# CHECK-NEXT:    VirtualAddress: 0x1008
+# CHECK-NEXT:    PhysicalAddress: 0x1008
+# CHECK-NEXT:    FileSize: 9
+# CHECK-NEXT:    MemSize: 9
+# CHECK-NEXT:    Flags [ (0x5)
+# CHECK-NEXT:      PF_R (0x4)
+# CHECK-NEXT:      PF_X (0x1)
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Alignment: 4096
+# CHECK-NEXT:  }
+# CHECK-NEXT:  ProgramHeader {
+# CHECK-NEXT:    Type: PT_GNU_STACK (0x6474E551)
+# CHECK-NEXT:    Offset: 0x0
+# CHECK-NEXT:    VirtualAddress: 0x0
+# CHECK-NEXT:    PhysicalAddress: 0x0
+# CHECK-NEXT:    FileSize: 0
+# CHECK-NEXT:    MemSize: 0
+# CHECK-NEXT:    Flags [ (0x6)
+# CHECK-NEXT:      PF_R (0x4)
+# CHECK-NEXT:      PF_W (0x2)
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Alignment: 0
+# CHECK-NEXT:  }
+# CHECK-NEXT:]
+
+.global _start
+_start:
+ nop
+
+
+.section .aaa, "a"
+.asciz "aa"
+
+.section .bbb, "a"
+.align 8
+.quad 0