[PATCH] D26888: [ELF] - Implemented -N.

[Ed Maste via Phabricator via llvm-commits]

emaste accepted this revision.
emaste added a comment.

This gives me a working hello world and reasonable looking phdr:

  % cat hello_data.c
  #include <stdio.h>
  
  int bss_var;
  int data_var = 3;
  
  int
  main(int argc, char *argv[])
  {
          printf("hello bss %d data %d\n", bss_var, data_var);
          return 0;
  }
  % cc -fuse-ld=lld -Wl,-N,-z,norelro hello_data.c
  % ./a.out
  hello bss 0 data 3
  % readelf -l a.out
  
  Elf file type is EXEC (Executable file)
  Entry point 0x2004d0
  There are 7 program headers, starting at offset 64
  
  Program Headers:
    Type           Offset             VirtAddr           PhysAddr
                   FileSiz            MemSiz              Flags  Align
    PHDR           0x0000000000000040 0x0000000000200040 0x0000000000200040
                   0x0000000000000188 0x0000000000000188  R      8
    INTERP         0x00000000000001c8 0x00000000002001c8 0x00000000002001c8
                   0x0000000000000015 0x0000000000000015  R      1
        [Requesting program interpreter: /libexec/ld-elf.so.1]
    LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
                   0x00000000000009c0 0x00000000000009cc  RWE    1000
    DYNAMIC        0x0000000000000868 0x0000000000200868 0x0000000000200868
                   0x0000000000000120 0x0000000000000120  RW     8
    GNU_EH_FRAME   0x00000000000004a0 0x00000000002004a0 0x00000000002004a0
                   0x0000000000000024 0x0000000000000024  R      1
    GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                   0x0000000000000000 0x0000000000000000  RW     0
    NOTE           0x00000000000001e0 0x00000000002001e0 0x00000000002001e0
                   0x0000000000000030 0x0000000000000030  R      4
  
   Section to Segment mapping:
    Segment Sections...
     00     
     01     .interp 
     02     .interp .note.tag .rodata .dynsym .gnu.version .gnu.version_r .gnu.hash .hash .dynstr .eh_frame .rela.plt .eh_frame_hdr .text .init .fini .plt .data .ctors .dtors .jcr .dynamic .got.plt .bss 
     03     .dynamic 
     04     .eh_frame_hdr 
     05     
     06     .note.tag 

The RO `GNU_EH_FRAME` may cause trouble in some applications, but FreeBSD's rtld didn't seem to care (and I don't think it should matter for the actual expected use cases for -N).

I did have to use -z norelro though: maybe we should fail if -N is used with relro or force norelro? It's arguable that relro is fundamentally incompatible with -N. Either way, I think this patch should go in as is first.



================
Comment at: ELF/Options.td:165
+def omagic: F<"omagic">, MetaVarName<"<magic>">,
+  HelpText<"Set the text and data sections to be readable and writable">;
+  
----------------
Maybe "Combine text and data into one readable and writable segment"? We should definitely refer to segment here not section because the shdr still has .text .data .bss with different perms.



https://reviews.llvm.org/D26888