[ELF2] -omagic implementation

[grimar]

Summary

-N, --omagic
Set the text and data sections to be readable and writable. Also, do not page-align the data segment, and disable linking against shared libraries.

Notes: bfd looks just to create single PT_LOAD and tries to keep minimal possible set of flags for it (just concatinates all sections flags that take part in PT_LOAD). Also bfd does not output empty sections, but we do. So this ones are also affect on minimal set of flags for new lld.

[grimar]

От: [e-mail] [[e-mail]] от имени Davide Italiano [[e-mail]]
Отправлено: 6 ноября 2015 г. 20:10
Кому: George Rimar; [e-mail]
Копия: Rafael Ávila de Espíndola; Rui Ueyama; [e-mail]
Тема: Re: [PATCH] D14449: [ELF2] -omagic implementation

Is somebody really using this?

Well, it could be used is for -r patch (relocatable output) which status is still unclear. Its description mentions that "As a side effect, in environments that support standard Unix magic numbers, this option also sets the output file’s magic number to "OMAGIC"".
Second possible use (I guessing here) is for writing self modifiying apps.

[grimar]

От: George Rimar
Отправлено: 6 ноября 2015 г. 22:21
Кому: Davide Italiano; [e-mail]
Копия: Rafael Ávila de Espíndola; Rui Ueyama; [e-mail]
Тема: HA: [PATCH] D14449: [ELF2] -omagic implementation

От: [e-mail] [[e-mail]] от имени Davide Italiano [[e-mail]]
Отправлено: 6 ноября 2015 г. 20:10
Кому: George Rimar; [e-mail]
Копия: Rafael Ávila de Espíndola; Rui Ueyama; [e-mail]
Тема: Re: [PATCH] D14449: [ELF2] -omagic implementation

Is somebody really using this?

Well, it could be used is for -r patch (relocatable output) which status is still unclear. Its description mentions that "As a side effect, in environments that support standard Unix magic numbers, this option also sets the output file’s magic number to "OMAGIC"".

Actually I am not sure that "set magic number to OMAGIC" is equals to applying -omagic here. That needs checking.
But I saw that flag usings during search about it in web, so I think it can be useful for some people.

Second possible use (I guessing here) is for writing self modifiying apps.

[ruiu]

I'm reluctant to add this option unless it's proved to be in use by many real-world use cases. This option seems to be for a compatibility for ancient Unix systems.

[grimar]

-N, --omagic
Set the text and data sections to be readable and writable. Also, do
not page-align the data segment, and disable linking against shared libraries.

I can't find any user of this in pkgsrc at least, so it really looks
like historic baggage. Most of the use cases I can think of are better
handled e.g. using objcopy.

Joerg

Ok, thank you for info.

ELF/Config.h

struct Configuration {
   bool DiscardNone;
   bool EnableNewDtags;
   bool ExportDynamic;
   bool GcSections;
   bool GnuHash = false;
   bool Mips64EL = false;
   bool NoInhibitExec;
   bool NoUndefined;
+  bool OMagic = false;
   bool Shared;
   bool Static = false;
   bool StripAll;
   bool SysvHash = true;
   bool Verbose;
   bool ZNodelete = false;
   bool ZNow = false;
   bool ZOrigin = false;

ELF/Driver.cpp

case OPT_Bdynamic:
       Config->Static = false;
       break;
     case OPT_whole_archive:
       WholeArchive = true;
       break;
     case OPT_no_whole_archive:
       WholeArchive = false;
       break;
+    case OPT_omagic:
+      Config->OMagic = true;
+      Config->Static = true;
+      break;
     }
   }
 
   if (Files.empty())
     error("no input files.");
 }
 
 template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {

ELF/Options.td

def noinhibit_exec : Flag<["--"], "noinhibit-exec">,
   HelpText<"Retain the executable output file whenever it is still usable">;
 
 def no_undefined : Flag<["--"], "no-undefined">,
   HelpText<"Report unresolved symbols even if the linker is creating a shared library">;
 
 def o : Separate<["-"], "o">, MetaVarName<"<path>">,
   HelpText<"Path to file to write output">;
 
+def omagic : Flag<["--"], "omagic">,
+  HelpText<"Set the text and data sections to be readable and writable. Also, do not "
+           "page-align the data segment, and disable linking against shared libraries.">;
+
 def rpath : Separate<["-"], "rpath">,
   HelpText<"Add a DT_RUNPATH to the output">;
 
 def script : Separate<["--"], "script">, HelpText<"Read linker script">;
 
 def shared : Flag<["-"], "shared">,
   HelpText<"Build a shared object">;
 

 def alias_discard_all_x: Flag<["-"], "x">, Alias<discard_all>;
 def alias_discard_locals_X: Flag<["-"], "X">, Alias<discard_locals>;
 def alias_entry_e : Separate<["-"], "e">, Alias<entry>;
 def alias_fini_fini : Joined<["-"], "fini=">, Alias<fini>;
 def alias_hash_style_hash_style : Joined<["--", "-"], "hash-style=">, Alias<hash_style>;
 def alias_init_init : Joined<["-"], "init=">, Alias<init>;
 def alias_l__library : Joined<["--"], "library=">, Alias<l>;
 def alias_o_output : Joined<["--"], "output=">, Alias<o>;
+def alias_omagic : Flag<["-"], "N">, Alias<omagic>;
 def alias_rpath_rpath : Joined<["-"], "rpath=">, Alias<rpath>;
 def alias_shared_Bshareable : Flag<["-"], "Bshareable">, Alias<shared>;
 def alias_soname_h : Separate<["-"], "h">, Alias<soname>;
 def alias_soname_soname : Separate<["-"], "soname">, Alias<soname>;
 def alias_script_T : Separate<["-"], "T">, Alias<script>;
 def alias_strip_all: Flag<["-"], "s">, Alias<strip_all>;
 def alias_undefined_u : Separate<["-"], "u">, Alias<undefined>;
 

ELF/Writer.cpp

for (InputSectionBase<ELFT> *C : F->getSections()) {
       if (!C || !C->isLive() || C == &InputSection<ELFT>::Discarded)
         continue;
       const Elf_Shdr *H = C->getSectionHdr();
       uintX_t OutFlags = H->sh_flags & ~SHF_GROUP;
       // For SHF_MERGE we create different output sections for each sh_entsize.
       // This makes each output section simple and keeps a single level
       // mapping from input to output.
       auto *IS = dyn_cast<InputSection<ELFT>>(C);
+      if (IS && Config->OMagic && getOutputName(C->getSectionName()) == ".text")
+        OutFlags |= SHF_WRITE;
       uintX_t EntSize = IS ? 0 : H->sh_entsize;
       SectionKey<ELFT::Is64Bits> Key{getOutputName(C->getSectionName()),
                                      H->sh_type, OutFlags, EntSize};
       OutputSectionBase<ELFT> *&Sec = Map[Key];
       if (!Sec) {
         if (IS)
           Sec = new (SecAlloc.Allocate())
               OutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
template <class ELFT> void Writer<ELFT>::assignAddresses() {
 
   // PT_INTERP must be the second entry if exists.
   int PhdrIdx = 0;
   Elf_Phdr *Interp = nullptr;
   if (needsInterpSection())
     Interp = &Phdrs[++PhdrIdx];
 
   // Add the first PT_LOAD segment for regular output sections.
-  setPhdr(&Phdrs[++PhdrIdx], PT_LOAD, PF_R, 0, getVAStart(), FileOff,
-          Target->getPageSize());
+  uintX_t Flags = PF_R;
+  uint64_t PageAlign = Config->OMagic ? 1 : Target->getPageSize();
+  setPhdr(&Phdrs[++PhdrIdx], PT_LOAD, Flags, 0, getVAStart(), FileOff,
+          PageAlign);
 
   Elf_Phdr TlsPhdr{};
   uintX_t ThreadBSSOffset = 0;
   // Create phdrs as we assign VAs and file offsets to all output sections.
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     if (needsPhdr<ELFT>(Sec)) {
       uintX_t Flags = toPhdrFlags(Sec->getFlags());
-      if (Phdrs[PhdrIdx].p_flags != Flags) {
+      if (!Config->OMagic && Phdrs[PhdrIdx].p_flags != Flags) {
         // Flags changed. Create a new PT_LOAD.
-        VA = RoundUpToAlignment(VA, Target->getPageSize());
-        FileOff = RoundUpToAlignment(FileOff, Target->getPageSize());
+        VA = RoundUpToAlignment(VA, PageAlign);
+        FileOff = RoundUpToAlignment(FileOff, PageAlign);
         Elf_Phdr *PH = &Phdrs[++PhdrIdx];
-        setPhdr(PH, PT_LOAD, Flags, FileOff, VA, 0, Target->getPageSize());
+        setPhdr(PH, PT_LOAD, Flags, FileOff, VA, 0, PageAlign);
       }
 
+      if (Config->OMagic)
+        Phdrs[PhdrIdx].p_flags |= Flags;
+
       if (Sec->getFlags() & SHF_TLS) {
         if (!TlsPhdr.p_vaddr) {
           setPhdr(&TlsPhdr, PT_TLS, PF_R, FileOff, VA, 0, Sec->getAlign());
           Out<ELFT>::TlsInitImageVA = VA;
         }
         if (Sec->getType() != SHT_NOBITS)
           VA = RoundUpToAlignment(VA, Sec->getAlign());
         uintX_t TVA = RoundUpToAlignment(VA + ThreadBSSOffset, Sec->getAlign());
if (isOutputDynamic())
     ++I;
   uintX_t Last = PF_R;
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     if (!needsPhdr<ELFT>(Sec))
       continue;
     if (Sec->getFlags() & SHF_TLS)
       Tls = true;
     uintX_t Flags = toPhdrFlags(Sec->getFlags());
-    if (Last != Flags) {
+    if (!Config->OMagic && Last != Flags) {
       Last = Flags;
       ++I;
     }
   }
   if (Tls)
     ++I;
   return I;
 }

test/elf2/omagic.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
+# RUN: ld.lld2 -N %t1 -o %t2
+# RUN: ld.lld2 %t1 -o %t3
+# RUN: llvm-readobj -file-headers -sections -program-headers -symbols %t2 | FileCheck %s
+# RUN: llvm-readobj -file-headers -sections -program-headers -symbols %t3 | FileCheck -check-prefix=CHECKNOMAGIC %s
+
+// Check alias
+# RUN: ld.lld2 --omagic %t1 -o %t2
+# RUN: llvm-readobj -file-headers -sections -program-headers -symbols %t2 | FileCheck %s
+
+# CHECK:       ProgramHeaders [
+# CHECK-NEXT:  ProgramHeader {
+# CHECK-NEXT:    Type: PT_PHDR
+# CHECK-NEXT:    Offset: 0x40
+# CHECK-NEXT:    VirtualAddress: 0x10040
+# CHECK-NEXT:    PhysicalAddress: 0x10040
+# CHECK-NEXT:    FileSize: 168
+# CHECK-NEXT:    MemSize: 168
+# CHECK-NEXT:    Flags [
+# CHECK-NEXT:      PF_R
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Alignment: 8
+# CHECK-NEXT:  }
+# CHECK-NEXT:  ProgramHeader {
+# CHECK-NEXT:    Type: PT_LOAD
+# CHECK-NEXT:    Offset: 0x0
+# CHECK-NEXT:    VirtualAddress: 0x10000
+# CHECK-NEXT:    PhysicalAddress: 0x10000
+# CHECK-NEXT:    FileSize: 288
+# CHECK-NEXT:    MemSize: 288
+# CHECK-NEXT:    Flags [ (0x7)
+# CHECK-NEXT:      PF_R (0x4)
+# CHECK-NEXT:      PF_W (0x2)
+# CHECK-NEXT:      PF_X (0x1)
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Alignment: 1
+# CHECK-NEXT:  }
+# CHECK-NEXT:  ProgramHeader {
+# CHECK-NEXT:    Type: PT_TLS
+# CHECK-NEXT:    Offset: 0xE8
+# CHECK-NEXT:    VirtualAddress: 0x100E8
+# CHECK-NEXT:    PhysicalAddress: 0x100E8
+# CHECK-NEXT:    FileSize: 8
+# CHECK-NEXT:    MemSize: 8
+# CHECK-NEXT:    Flags [
+# CHECK-NEXT:      PF_R (0x4)
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Alignment: 1
+# CHECK-NEXT:  }
+
+# CHECKNOMAGIC:       ProgramHeaders [
+# CHECKNOMAGIC-NEXT:  ProgramHeader {
+# CHECKNOMAGIC-NEXT:    Type: PT_PHDR
+# CHECKNOMAGIC-NEXT:    Offset: 0x40
+# CHECKNOMAGIC-NEXT:    VirtualAddress: 0x10040
+# CHECKNOMAGIC-NEXT:    PhysicalAddress: 0x10040
+# CHECKNOMAGIC-NEXT:    FileSize: 280
+# CHECKNOMAGIC-NEXT:    MemSize: 280
+# CHECKNOMAGIC-NEXT:    Flags [
+# CHECKNOMAGIC-NEXT:      PF_R
+# CHECKNOMAGIC-NEXT:    ]
+# CHECKNOMAGIC-NEXT:    Alignment: 8
+# CHECKNOMAGIC-NEXT:  }
+# CHECKNOMAGIC-NEXT:  ProgramHeader {
+# CHECKNOMAGIC-NEXT:    Type: PT_LOAD
+# CHECKNOMAGIC-NEXT:    Offset: 0x0
+# CHECKNOMAGIC-NEXT:    VirtualAddress: 0x10000
+# CHECKNOMAGIC-NEXT:    PhysicalAddress: 0x10000
+# CHECKNOMAGIC-NEXT:    FileSize: 376
+# CHECKNOMAGIC-NEXT:    MemSize: 376
+# CHECKNOMAGIC-NEXT:    Flags [
+# CHECKNOMAGIC-NEXT:      PF_R
+# CHECKNOMAGIC-NEXT:    ]
+# CHECKNOMAGIC-NEXT:    Alignment: 4096
+# CHECKNOMAGIC-NEXT:  }
+# CHECKNOMAGIC-NEXT:  ProgramHeader {
+# CHECKNOMAGIC-NEXT:    Type: PT_LOAD
+# CHECKNOMAGIC-NEXT:    Offset: 0x1000
+# CHECKNOMAGIC-NEXT:    VirtualAddress: 0x11000
+# CHECKNOMAGIC-NEXT:    PhysicalAddress: 0x11000
+# CHECKNOMAGIC-NEXT:    FileSize: 8
+# CHECKNOMAGIC-NEXT:    MemSize: 8
+# CHECKNOMAGIC-NEXT:    Flags [
+# CHECKNOMAGIC-NEXT:      PF_R
+# CHECKNOMAGIC-NEXT:      PF_X
+# CHECKNOMAGIC-NEXT:    ]
+# CHECKNOMAGIC-NEXT:    Alignment: 4096
+# CHECKNOMAGIC-NEXT:  }
+# CHECKNOMAGIC-NEXT:  ProgramHeader {
+# CHECKNOMAGIC-NEXT:    Type: PT_LOAD
+# CHECKNOMAGIC-NEXT:    Offset: 0x2000
+# CHECKNOMAGIC-NEXT:    VirtualAddress: 0x12000
+# CHECKNOMAGIC-NEXT:    PhysicalAddress: 0x12000
+# CHECKNOMAGIC-NEXT:    FileSize: 16
+# CHECKNOMAGIC-NEXT:    MemSize: 16
+# CHECKNOMAGIC-NEXT:    Flags [
+# CHECKNOMAGIC-NEXT:      PF_R
+# CHECKNOMAGIC-NEXT:      PF_W
+# CHECKNOMAGIC-NEXT:    ]
+# CHECKNOMAGIC-NEXT:    Alignment: 4096
+# CHECKNOMAGIC-NEXT:  }
+# CHECKNOMAGIC-NEXT:  ProgramHeader {
+# CHECKNOMAGIC-NEXT:    Type: PT_TLS
+# CHECKNOMAGIC-NEXT:    Offset: 0x158
+# CHECKNOMAGIC-NEXT:    VirtualAddress: 0x10158
+# CHECKNOMAGIC-NEXT:    PhysicalAddress: 0x10158
+# CHECKNOMAGIC-NEXT:    FileSize: 8
+# CHECKNOMAGIC-NEXT:    MemSize: 8
+# CHECKNOMAGIC-NEXT:    Flags [
+# CHECKNOMAGIC-NEXT:      PF_R
+# CHECKNOMAGIC-NEXT:    ]
+# CHECKNOMAGIC-NEXT:    Alignment: 1
+# CHECKNOMAGIC-NEXT:  }
+
+.section .r,"ax"
+.globl _start
+_start:
+.quad 0
+
+.section .aw1,"aw"
+.quad 1
+
+.section .a,"a"
+.quad 2
+
+.section .tls,"aT"
+.quad 3
+
+.section .s1,"aS"
+.quad 4
+
+.section .aw2,"aw"
+.quad 5
+
+.section .s2,"aS"
+.quad 6