[PATCH] D67032: [ELF] Align SHT_LLVM_PART_EHDR to a maximum page size boundary

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: hans, pcc, rnk, ruiu, thakis.
Herald added subscribers: llvm-commits, arichardson, emaste.
Herald added a reviewer: espindola.
Herald added a reviewer: alexshap.
Herald added a project: LLVM.
MaskRay updated this revision to Diff 218204.
MaskRay added a comment.

Delete a stray line from the test


This is a speculative fix for https://bugs.chromium.org/p/chromium/issues/detail?id=998712

SHT_LLVM_PART_EHDR marks the start of a partition. The partition
sections will be extracted to a separate file. ELF header not at the
start will be weird, and we cannot benefit from overlapping p_offset
ranges with the previous segment anyway, so align to the next maximum
page size boundary.

It seems we lack some llvm-objcopy --extract-main-partition and
--extract-partition sanity checks. It may place EHDR at the start
even if p_offset if non zero. Anyway, the lld change is justified for
the reasons above.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D67032

Files:
  ELF/Writer.cpp
  test/ELF/partitions.s


Index: test/ELF/partitions.s
===================================================================
--- test/ELF/partitions.s
+++ test/ELF/partitions.s
@@ -1,13 +1,20 @@
-// REQUIRES: x86
+// REQUIRES: aarch64, x86
 // RUN: llvm-mc %s -o %t.o -filetype=obj --triple=x86_64-unknown-linux
+// RUN: ld.lld %t.o -o %t --export-dynamic --gc-sections -z max-page-size=65536
+// RUN: llvm-readelf -S -s %t | FileCheck %s
+
+// RUN: llvm-mc %s -o %t.o -filetype=obj --triple=aarch64
 // RUN: ld.lld %t.o -o %t --export-dynamic --gc-sections
 // RUN: llvm-readelf -S -s %t | FileCheck %s
 
 // This is basically lld/docs/partitions.dot in object file form.
 // Test that the sections are correctly allocated to partitions.
+// part1 and part2 should be aligned to a maximum page size boundary.
 
 // CHECK: [[MAIN:[0-9]+]]] .text
+// CHECK: part1 LOOS+0xFFF4C06 {{[0-9a-f]+}}0000 {{[0-9a-f]+}}0000
 // CHECK: [[P1:[0-9]+]]] .text
+// CHECK: part2 LOOS+0xFFF4C06 {{[0-9a-f]+}}0000 {{[0-9a-f]+}}0000
 // CHECK: [[P2:[0-9]+]]] .text
 
 // CHECK: Symbol table '.symtab'
@@ -30,21 +37,21 @@
 .section .text._start,"ax", at progbits
 .globl _start
 _start:
-call f3
+.quad f3
 
 .section .text.f1,"ax", at progbits
 .globl f1
 f1:
-call f3
-call f4
-call f5
+.quad f3
+.quad f4
+.quad f5
 
 .section .text.f2,"ax", at progbits
 .globl f2
 f2:
-call f3
-call f5
-call f6
+.quad f3
+.quad f5
+.quad f6
 
 .section .text.f3,"ax", at progbits
 f3:
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -2223,13 +2223,21 @@
       // the next maximum page size boundary on transitions between executable
       // and non-executable segments.
       //
+      // SHT_LLVM_PART_EHDR marks the start of a partition. The partition
+      // sections will be extracted to a separate file. ELF header not at the
+      // start will be weird, and we cannot benefit from overlapping p_offset
+      // ranges with the previous segment anyway, so align to the next maximum
+      // page size boundary.
+      //
       // TODO Enable this technique on all targets.
       bool enable = config->emachine != EM_HEXAGON &&
                     config->emachine != EM_MIPS &&
                     config->emachine != EM_X86_64;
 
-      if (!enable || (config->zSeparateCode && prev &&
-                      (prev->p_flags & PF_X) != (p->p_flags & PF_X)))
+      if (!enable ||
+          (config->zSeparateCode && prev &&
+           (prev->p_flags & PF_X) != (p->p_flags & PF_X)) ||
+          cmd->type == SHT_LLVM_PART_EHDR)
         cmd->addrExpr = [] {
           return alignTo(script->getDot(), config->maxPageSize);
         };


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