[PATCH] D44775: [ELF] - Fill executable segments with trap instructions.

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
grimar added reviewers: ruiu, espindola.
Herald added subscribers: arichardson, emaste.

LLD currently fills the last page of each executable segment with
trap instructions. It seems to be not enough, see PR36853.

Patch changes logic to fill the whole segment.
With that any zero areas which exist because of alignment
paddings are also covered.


https://reviews.llvm.org/D44775

Files:
  ELF/Writer.cpp
  test/ELF/fill-trap2.s


Index: test/ELF/fill-trap2.s
===================================================================
--- test/ELF/fill-trap2.s
+++ test/ELF/fill-trap2.s
@@ -0,0 +1,27 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t2
+# RUN: llvm-readobj -program-headers %t2 | FileCheck %s
+# RUN: od -Ax -t x1 -N16 -j0x1000 %t2 | FileCheck %s -check-prefix=FILL
+
+# CHECK: ProgramHeader {
+# CHECK:   Type: PT_LOAD
+# CHECK:   Offset: 0x1000
+# CHECK-NEXT:   VirtualAddress:
+# CHECK-NEXT:   PhysicalAddress:
+# CHECK-NEXT:   FileSize: 8192
+# CHECK-NEXT:   MemSize:
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     PF_R
+# CHECK-NEXT:     PF_X
+# CHECK-NEXT:   ]
+
+# FILL: 001000 90 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
+
+nop
+
+.section .foo,"ax"
+.align 16
+nop
+.zero 0x1000
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -2189,21 +2189,19 @@
     memcpy(I, &Target->TrapInstr, 4);
 }
 
-// Fill the last page of executable segments with trap instructions
-// instead of leaving them as zero. Even though it is not required by any
-// standard, it is in general a good thing to do for security reasons.
-//
-// We'll leave other pages in segments as-is because the rest will be
-// overwritten by output sections.
+// Fill the executable segments with trap instructions instead of leaving them
+// as zero. Even though it is not required by any standard, it is in general a
+// good thing to do for security reasons. We have to fill all pages to properly
+// fill paddings created during applying alignment to sections.
 template <class ELFT> void Writer<ELFT>::writeTrapInstr() {
   if (Script->HasSectionsCommand)
     return;
 
-  // Fill the last page.
+  // Fill executable segments with trap instructions.
   uint8_t *Buf = Buffer->getBufferStart();
   for (PhdrEntry *P : Phdrs)
     if (P->p_type == PT_LOAD && (P->p_flags & PF_X))
-      fillTrap(Buf + alignDown(P->p_offset + P->p_filesz, Target->PageSize),
+      fillTrap(Buf + P->p_offset,
                Buf + alignTo(P->p_offset + P->p_filesz, Target->PageSize));
 
   // Round up the file size of the last segment to the page boundary iff it is


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