[lld/ELF] Place large executable sections at beginning or end

Author: aeubanks

lld/ELF/Writer.cpp

@@ -653,15 +653,17 @@ enum RankFlags {
   RF_NOT_ADDR_SET = 1 << 27,
   RF_NOT_ALLOC = 1 << 26,
   RF_PARTITION = 1 << 18, // Partition number (8 bits)
+  RF_LARGE_EXEC_WRITE = 1 << 16,
   RF_LARGE_ALT = 1 << 15,
   RF_WRITE = 1 << 14,
   RF_EXEC_WRITE = 1 << 13,
   RF_EXEC = 1 << 12,
   RF_RODATA = 1 << 11,
-  RF_LARGE = 1 << 10,
-  RF_NOT_RELRO = 1 << 9,
-  RF_NOT_TLS = 1 << 8,
-  RF_BSS = 1 << 7,
+  RF_LARGE_EXEC = 1 << 10,
+  RF_LARGE = 1 << 9,
+  RF_NOT_RELRO = 1 << 8,
+  RF_NOT_TLS = 1 << 7,
+  RF_BSS = 1 << 6,
 };
 
 unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
@@ -691,14 +693,15 @@ unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
   // places.
   bool isExec = osec.flags & SHF_EXECINSTR;
   bool isWrite = osec.flags & SHF_WRITE;
+  bool isLarge = osec.flags & SHF_X86_64_LARGE && ctx.arg.emachine == EM_X86_64;
 
   if (!isWrite && !isExec) {
     // Among PROGBITS sections, place .lrodata further from .text.
     // For -z lrodata-after-bss, place .lrodata after .lbss like GNU ld. This
     // layout has one extra PT_LOAD, but alleviates relocation overflow
     // pressure for absolute relocations referencing small data from -fno-pic
     // relocatable files.
-    if (osec.flags & SHF_X86_64_LARGE && ctx.arg.emachine == EM_X86_64)
+    if (isLarge)
       rank |= ctx.arg.zLrodataAfterBss ? RF_LARGE_ALT : 0;
     else
       rank |= ctx.arg.zLrodataAfterBss ? 0 : RF_LARGE;
@@ -722,7 +725,16 @@ unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
     else
       rank |= RF_RODATA;
   } else if (isExec) {
-    rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
+    // Place readonly .ltext before .lrodata and writable .ltext after .lbss to
+    // keep writable and readonly segments separate.
+    if (isLarge) {
+      if (isWrite)
+        rank |= RF_LARGE_EXEC_WRITE;
+      else
+        rank |= RF_LARGE_EXEC;
+    } else {
+      rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
+    }
   } else {
     rank |= RF_WRITE;
     // The TLS initialization block needs to be a single contiguous block. Place
@@ -737,7 +749,7 @@ unsigned elf::getSectionRank(Ctx &ctx, OutputSection &osec) {
     // alleviates relocation overflow pressure.
     // For -z lrodata-after-bss, place .lbss/.lrodata/.ldata after .bss.
     // .bss/.lbss being adjacent reuses the NOBITS size optimization.
-    if (osec.flags & SHF_X86_64_LARGE && ctx.arg.emachine == EM_X86_64) {
+    if (isLarge) {
       rank |= ctx.arg.zLrodataAfterBss
                   ? (osec.type == SHT_NOBITS ? 1 : RF_LARGE_ALT)
                   : RF_LARGE;

lld/test/ELF/x86-64-section-layout.s

@@ -18,6 +18,10 @@
 # RUN: ld.lld --section-start=.note=0x200300 a.o -z lrodata-after-bss -o a3
 # RUN: llvm-readelf -S -l -sX a3 | FileCheck %s --check-prefix=CHECK3
 
+# RUN: llvm-mc -filetype=obj -triple=x86_64 c.s -o c.o
+# RUN: ld.lld c.o -o c
+# RUN: llvm-readelf -S -l c | FileCheck %s --check-prefix=CHECK4
+
 # CHECK:       Name              Type            Address          Off    Size   ES Flg Lk Inf Al
 # CHECK-NEXT:                    NULL            0000000000000000 000000 000000 00      0   0  0
 # CHECK-NEXT:  .note             NOTE            0000000000200300 000300 000001 00   A  0   0  1
@@ -116,6 +120,18 @@
 # CHECK3-NEXT:  0000000000203307     0 NOTYPE  GLOBAL DEFAULT [[#]] (.data)   _edata
 # CHECK3-NEXT:  0000000000207d0d     0 NOTYPE  GLOBAL DEFAULT [[#]] (.ldata2) _end
 
+# CHECK4:      .note      NOTE
+# CHECK4-NEXT: .ltext     PROGBITS
+# CHECK4-NEXT: .lrodata   PROGBITS
+# CHECK4-NEXT: .rodata    PROGBITS
+# CHECK4-NEXT: .text      PROGBITS
+# CHECK4-NEXT: .data      PROGBITS
+# CHECK4-NEXT: .bss       NOBITS
+# CHECK4-NEXT: .ldata     PROGBITS
+# CHECK4-NEXT: .lbss      NOBITS
+# CHECK4-NEXT: .ltext_w   PROGBITS
+# CHECK4-NEXT: .comment   PROGBITS
+
 #--- a.s
 .globl _start, _etext, _edata, _end
 _start:
@@ -155,3 +171,27 @@ SECTIONS {
   .ldata2 : {}
   .lbss : { *(.lbss .lbss.*) }
 }
+
+#--- c.s
+.section .ltext,"axl",@progbits
+.globl f
+f:
+  ret
+
+.section .ltext_w,"awxl",@progbits
+.globl g
+g:
+  ret
+
+.section .text,"ax",@progbits
+.globl h
+h:
+  ret
+
+.section .note,"a",@note; .space 1
+.section .rodata,"a",@progbits; .space 1
+.section .data,"aw",@progbits; .space 1
+.section .bss,"aw",@nobits; .space 1
+.section .lrodata,"al"; .space 1
+.section .ldata,"awl"; .space 1
+.section .lbss,"awl",@nobits; .space 1