Re: RO Elf program headers

[Dimitry Andric]

On 2020-02-23 16:31, Paul Floyd wrote:
> I’ve been working on getting Valgrind to work again on FreeBSD.
> Probably the biggest problem at the moment is that Valgrind doesn’t (yet) 
> handle the read-only program headers.
> If I run “readelf -l libc++.so.1.0” on Linux I see two LOAD headers, the 
> first one “R E” (read-execute) and the second one “RW” (read-write). If I do 
> the same on FreeBSD then I see a third program header which is “R” 
> (read-only), before the other two.
> My questions are
> - Does anyone know when this extra header was added? (I assume that it was 
> added sometime in the past 5 years or so)

Most likely a very long time ago.


> - What is in these segments, and is it safe to assume that multiple shared 
> libraries can map the same RO content to the same address?

If you use readelf -l, you will see a "Section to Segment mapping":

$ readelf -lW /usr/lib/libc++.so.1

Elf file type is DYN (Shared object file)
Entry point 0x5c000
There are 9 program headers, starting at offset 64

Program Headers:
  Type   Offset   VirtAddr   PhysAddr   FileSiz  MemSiz 
  Flg Align
  PHDR   0x40 0x0040 0x0040 0x0001f8 
0x0001f8 R   0x8
  LOAD   0x00 0x 0x 0x05b1d4 
0x05b1d4 R   0x1000
  LOAD   0x05c000 0x0005c000 0x0005c000 0x06bbe0 
0x06bbe0 R E 0x1000
  LOAD   0x0c8000 0x000c8000 0x000c8000 0x004c78 
0x004c78 RW  0x1000
  LOAD   0x0cd000 0x000cd000 0x000cd000 0x000de0 
0x0032f8 RW  0x1000
  DYNAMIC0x0cc5c0 0x000cc5c0 0x000cc5c0 0x0001a0 
0x0001a0 RW  0x8
  GNU_RELRO  0x0c8000 0x000c8000 0x000c8000 0x004c78 
0x004c78 R   0x1
  GNU_EH_FRAME   0x046c34 0x00046c34 0x00046c34 0x003244 
0x003244 R   0x4
  GNU_STACK  0x00 0x 0x 0x00 
0x00 RW  0

 Section to Segment mapping:
  Segment Sections...
   00
   01 .dynsym .gnu.version .gnu.version_r .gnu.hash .hash .dynstr .rela.dyn 
.rela.plt .rodata .gcc_except_table .eh_frame_hdr .eh_frame
   02 .text .init .fini .plt
   03 .ctors .dtors .jcr .data.rel.ro .init_array .dynamic .got
   04 .data .got.plt .bss
   05 .dynamic
   06 .ctors .dtors .jcr .data.rel.ro .init_array .dynamic .got
   07 .eh_frame_hdr
   08

E.g, segment 01 is R, and contains things like the dynamic symbol table, 
relocations, exception handling information, and so on.  It is fairly safe to 
say that these can be shared.

-Dimitry



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RO Elf program headers

[Paul Floyd]

Hi

I’ve been working on getting Valgrind to work again on FreeBSD.

Probably the biggest problem at the moment is that Valgrind doesn’t (yet) 
handle the read-only program headers.

If I run “readelf -l libc++.so.1.0” on Linux I see two LOAD headers, the first 
one “R E” (read-execute) and the second one “RW” (read-write). If I do the same 
on FreeBSD then I see a third program header which is “R” (read-only), before 
the other two.

My questions are
- Does anyone know when this extra header was added? (I assume that it was 
added sometime in the past 5 years or so)
- What is in these segments, and is it safe to assume that multiple shared 
libraries can map the same RO content to the same address?

A+
Paul
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