Address review comments

Created using spr 1.3.6-beta.1

Author: pcc

lld/ELF/Config.h

@@ -320,6 +320,7 @@ struct Config {
   bool printGcSections;
   bool printIcfSections;
   bool printMemoryUsage;
+  std::optional<uint64_t> randomizeSectionPadding;
   bool rejectMismatch;
   bool relax;
   bool relaxGP;
@@ -329,7 +330,6 @@ struct Config {
   bool relrPackDynRelocs = false;
   llvm::DenseSet<llvm::StringRef> saveTempsArgs;
   llvm::SmallVector<std::pair<llvm::GlobPattern, uint32_t>, 0> shuffleSections;
-  std::optional<uint64_t> shufflePadding;
   bool singleRoRx;
   bool shared;
   bool symbolic;

lld/ELF/Driver.cpp

@@ -1410,8 +1410,9 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
   ctx.arg.searchPaths = args::getStrings(args, OPT_library_path);
   ctx.arg.sectionStartMap = getSectionStartMap(ctx, args);
   ctx.arg.shared = args.hasArg(OPT_shared);
-  if (args.hasArg(OPT_shuffle_padding))
-    ctx.arg.shufflePadding = args::getInteger(args, OPT_shuffle_padding, 0);
+  if (args.hasArg(OPT_randomize_section_padding))
+    ctx.arg.randomizeSectionPadding =
+        args::getInteger(args, OPT_randomize_section_padding, 0);
   ctx.arg.singleRoRx = !args.hasFlag(OPT_rosegment, OPT_no_rosegment, true);
   ctx.arg.soName = args.getLastArgValue(OPT_soname);
   ctx.arg.sortSection = getSortSection(ctx, args);

lld/ELF/Options.td

@@ -434,7 +434,7 @@ defm section_start: Eq<"section-start", "Set address of section">,
 
 def shared: F<"shared">, HelpText<"Build a shared object">;
 
-def shuffle_padding: JJ<"shuffle-padding=">,
+def randomize_section_padding: JJ<"randomize-section-padding=">,
   HelpText<"Randomly insert padding between input sections using given seed">;
 
 defm soname: Eq<"soname", "Set DT_SONAME">;

lld/ELF/SyntheticSections.cpp

@@ -2753,14 +2753,14 @@ RelroPaddingSection::RelroPaddingSection(Ctx &ctx)
     : SyntheticSection(ctx, ".relro_padding", SHT_NOBITS, SHF_ALLOC | SHF_WRITE,
                        1) {}
 
-ShufflePaddingSection::ShufflePaddingSection(Ctx &ctx, uint64_t size,
-                                             OutputSection *parent)
-    : SyntheticSection(ctx, ".shuffle_padding", SHF_ALLOC, SHT_PROGBITS, 1),
+RandomizePaddingSection::RandomizePaddingSection(Ctx &ctx, uint64_t size,
+                                                 OutputSection *parent)
+    : SyntheticSection(ctx, ".randomize_padding", SHT_PROGBITS, SHF_ALLOC, 1),
       size(size) {
   this->parent = parent;
 }
 
-void ShufflePaddingSection::writeTo(uint8_t *buf) {
+void RandomizePaddingSection::writeTo(uint8_t *buf) {
   std::array<uint8_t, 4> filler = getParent()->getFiller(ctx);
   uint8_t *end = buf + size;
   for (; buf + 4 <= end; buf += 4)

lld/ELF/SyntheticSections.h

@@ -796,11 +796,11 @@ class RelroPaddingSection final : public SyntheticSection {
   void writeTo(uint8_t *buf) override {}
 };
 
-class ShufflePaddingSection final : public SyntheticSection {
+class RandomizePaddingSection final : public SyntheticSection {
   uint64_t size;
 
 public:
-  ShufflePaddingSection(Ctx &ctx, uint64_t size, OutputSection *parent);
+  RandomizePaddingSection(Ctx &ctx, uint64_t size, OutputSection *parent);
   size_t getSize() const override { return size; }
   void writeTo(uint8_t *buf) override;
 };

lld/ELF/Writer.cpp

@@ -1450,8 +1450,8 @@ static bool canInsertPadding(OutputSection *sec) {
          s.starts_with(".text");
 }
 
-static void shufflePadding(Ctx &ctx) {
-  std::mt19937 g(*ctx.arg.shufflePadding);
+static void randomizeSectionPadding(Ctx &ctx) {
+  std::mt19937 g(*ctx.arg.randomizeSectionPadding);
   PhdrEntry *curPtLoad = nullptr;
   for (OutputSection *os : ctx.outputSections) {
     if (!canInsertPadding(os))
@@ -1460,14 +1460,15 @@ static void shufflePadding(Ctx &ctx) {
       if (auto *isd = dyn_cast<InputSectionDescription>(bc)) {
         SmallVector<InputSection *, 0> tmp;
         if (os->ptLoad != curPtLoad) {
-          tmp.push_back(
-              make<ShufflePaddingSection>(ctx, g() % ctx.arg.maxPageSize, os));
+          tmp.push_back(make<RandomizePaddingSection>(
+              ctx, g() % ctx.arg.maxPageSize, os));
           curPtLoad = os->ptLoad;
         }
         for (InputSection *isec : isd->sections) {
-          if (g() < (1 << 28))
+          // Probability of inserting padding is 1 in 16.
+          if (g() % 16 == 0)
             tmp.push_back(
-                make<ShufflePaddingSection>(ctx, isec->addralign, os));
+                make<RandomizePaddingSection>(ctx, isec->addralign, os));
           tmp.push_back(isec);
         }
         isd->sections = std::move(tmp);
@@ -1502,8 +1503,8 @@ template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
   if (ctx.arg.emachine == EM_HEXAGON)
     hexagonTLSSymbolUpdate(ctx);
 
-  if (ctx.arg.shufflePadding)
-    shufflePadding(ctx);
+  if (ctx.arg.randomizeSectionPadding)
+    randomizeSectionPadding(ctx);
 
   uint32_t pass = 0, assignPasses = 0;
   for (;;) {

lld/docs/ld.lld.1

@@ -529,6 +529,15 @@ and
 .It Fl -pop-state
 Restore the states saved by
 .Fl -push-state.
+.It Fl -randomize-section-padding Ns = Ns Ar seed
+Randomly insert padding between input sections using the given seed.
+Padding is inserted into output sections with names matching the following patterns:
+.Cm .bss ,
+.Cm .data ,
+.Cm .data.rel.ro ,
+.Cm .rodata
+and
+.Cm .text* .
 .It Fl --relax-gp
 Enable global pointer relaxation for RISC-V.
 .It Fl -relocatable , Fl r
@@ -577,15 +586,6 @@ were concatenated in the order they appeared on the command line.
 Set address of section.
 .It Fl -shared , Fl -Bsharable
 Build a shared object.
-.It Fl -shuffle-padding Ns = Ns Ar seed
-Randomly insert padding between input sections using the given seed.
-Padding is inserted into output sections with names matching the following patterns:
-.Cm .bss ,
-.Cm .data ,
-.Cm .data.rel.ro ,
-.Cm .rodata
-and
-.Cm .text* .
 .It Fl -shuffle-sections Ns = Ns Ar seed
 Shuffle matched sections using the given seed before mapping them to the output sections.
 If -1, reverse the section order. If 0, use a random seed.

lld/test/ELF/randomize-section-padding.s

@@ -0,0 +1,88 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+
+## --randomize-section-padding= inserts segment offset padding and pre-section
+## padding, and does not affect flags. Segment offset padding is only inserted
+## when PT_LOAD changes, as shown by .bss size (.data and .bss share a PT_LOAD).
+
+# RUN: ld.lld --randomize-section-padding=6 %t.o -o %t.out
+# RUN: llvm-readelf -sS -x .rodata -x .text -x .data %t.out | FileCheck --check-prefix=PAD6 %s
+
+# PAD6: .rodata           PROGBITS        0000000000200158 000158 000b8d 00   A  0   0  1
+# PAD6: .text             PROGBITS        0000000000201ce8 000ce8 000270 00  AX  0   0  4
+# PAD6: .data             PROGBITS        0000000000202f58 000f58 000941 00  WA  0   0  1
+# PAD6: .bss              NOBITS          0000000000203899 001899 000003 00  WA  0   0  1
+
+# PAD6: 0000000000203899     0 NOTYPE  LOCAL  DEFAULT     4 a
+# PAD6: 000000000020389a     0 NOTYPE  LOCAL  DEFAULT     4 b
+# PAD6: 000000000020389b     0 NOTYPE  LOCAL  DEFAULT     4 c
+
+# PAD6: Hex dump of section '.rodata':
+# PAD6: 0x00200cd8 00000000 00000000 00000102 03
+# PAD6: Hex dump of section '.text':
+# PAD6: 0x00201f48 cccccccc cccccccc cccccccc 0405cc06
+# PAD6: Hex dump of section '.data':
+# PAD6: 0x00203888 00000000 00000000 00000000 00000708
+# PAD6: 0x00203898 09
+
+## Size of segment offset padding and location of pre-section padding is
+## dependent on the seed.
+
+# RUN: ld.lld --randomize-section-padding=46 %t.o -o %t.out
+# RUN: llvm-readelf -sS -x .rodata -x .text -x .data %t.out | FileCheck --check-prefix=PAD46 %s
+
+# PAD46: .rodata           PROGBITS        0000000000200158 000158 000cc0 00   A  0   0  1
+# PAD46: .text             PROGBITS        0000000000201e18 000e18 0009bf 00  AX  0   0  4
+# PAD46: .data             PROGBITS        00000000002037d7 0017d7 000540 00  WA  0   0  1
+# PAD46: .bss              NOBITS          0000000000203d17 001d17 000004 00  WA  0   0  1
+
+# PAD46: 0000000000203d17     0 NOTYPE  LOCAL  DEFAULT     4 a
+# PAD46: 0000000000203d18     0 NOTYPE  LOCAL  DEFAULT     4 b
+# PAD46: 0000000000203d1a     0 NOTYPE  LOCAL  DEFAULT     4 c
+
+# PAD46: Hex dump of section '.rodata':
+# PAD46: 0x00200e08 00000000 00000000 00000000 00010203
+# PAD46: Hex dump of section '.text':
+# PAD46: 0x002027c8 cccccccc cccccccc cccccccc 040506
+# PAD46: Hex dump of section '.data':
+# PAD46: 0x00203d07 00000000 00000000 00000000 07000809
+
+.section .rodata.a,"a",@progbits
+.byte 1
+
+.section .rodata.b,"a",@progbits
+.byte 2
+
+.section .rodata.c,"a",@progbits
+.byte 3
+
+.section .text.a,"ax",@progbits
+.byte 4
+
+.section .text.b,"ax",@progbits
+.byte 5
+
+.section .text.c,"ax",@progbits
+.byte 6
+
+.section .data.a,"aw",@progbits
+.byte 7
+
+.section .data.b,"aw",@progbits
+.byte 8
+
+.section .data.c,"aw",@progbits
+.byte 9
+
+.section .bss.a,"a",@nobits
+a:
+.zero 1
+
+.section .bss.b,"a",@nobits
+b:
+.zero 1
+
+.section .bss.c,"a",@nobits
+c:
+.zero 1
+