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RFC: Add SHT_GNU_PHDRS

H.J. Lu hjl.tools@gmail.com
Thu Sep 27 04:35:00 GMT 2018 
On Wed, Sep 26, 2018 at 8:25 PM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 9/26/18 5:53 PM, H.J. Lu wrote:
>> When -z separate-code is used to create executable, ld won't place any
>> data in the code-only PT_LOAD segment.  If there are no data sections
>> placed before the code-only PT_LOAD segment, the program headers
>> won't be mapped into any PT_LOAD segment.  When the executable tries
>> to access it (based on the program header address passed in AT_PHDR),
>> it will lead to segfault.
>>
>> I am proposing
>>
>> #define SHT_GNU_PHDRS  0x6ffffff4  /* Dummy section for program header */
>>
>> This is a special read-only SHF_ALLOC zero-size data section.  It is the
>> first output section, which will force a data PT_LOAD segment with program
>> header before the code-only PT_LOAD segment,
>
> This seems like a huge hack to me. Either ld ensures that the page-aligned
> start of the PT_LOAD mapping includes the program headers, or we have to
> define SHT_GNU_PHDRS as having exactly that semantic and we need to clearly
> explain what a static linker needs to do to accomplish this task. What you
> have here needs more documentation.

It is to replace another hack:

[hjl@gnu-cfl-1 ld]$ readelf -SWl  tmpdir/pr23428
There are 13 section headers, starting at offset 0x2008:

Section Headers:
  [Nr] Name              Type            Address          Off    Size
 ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000
000000 00      0   0  0
  [ 1] .note.gnu.property NOTE            0000000000400120 000120
000020 00   A  0   0  8
  [ 2] .text             PROGBITS        0000000000401000 001000
00012d 00  AX  0   0 16
  [ 3] .comment          PROGBITS        0000000000000000 00112d
00002c 01  MS  0   0  1
  [ 4] .debug_aranges    PROGBITS        0000000000000000 001160
000060 00      0   0 16
  [ 5] .debug_info       PROGBITS        0000000000000000 0011c0
000482 00      0   0  1
  [ 6] .debug_abbrev     PROGBITS        0000000000000000 001642
00016d 00      0   0  1
  [ 7] .debug_line       PROGBITS        0000000000000000 0017af
000246 00      0   0  1
  [ 8] .debug_frame      PROGBITS        0000000000000000 0019f8
000040 00      0   0  8
  [ 9] .debug_str        PROGBITS        0000000000000000 001a38
000374 01  MS  0   0  1
  [10] .symtab           SYMTAB          0000000000000000 001db0
000198 18     11  11  8
  [11] .strtab           STRTAB          0000000000000000 001f48
000030 00      0   0  1
  [12] .shstrtab         STRTAB          0000000000000000 001f78
00008a 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  l (large), p (processor specific)

Elf file type is EXEC (Executable file)
Entry point 0x401000
There are 4 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr
FileSiz  MemSiz   Flg Align
  LOAD           0x000000 0x0000000000400000 0x0000000000400000
0x000140 0x000140 R   0x1000
  LOAD           0x001000 0x0000000000401000 0x0000000000401000
0x00012d 0x00012d R E 0x1000
  NOTE           0x000120 0x0000000000400120 0x0000000000400120
0x000020 0x000020 R   0x8
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000
0x000000 0x000000 RWE 0x10

 Section to Segment mapping:
  Segment Sections...
   00     .note.gnu.property
   01     .text
   02     .note.gnu.property
   03
[hjl@gnu-cfl-1 ld]$ ./tmpdir/pr23428
[hjl@gnu-cfl-1 ld]$

I added a "dummy" .note.gnu.property section in x86 backend so that
program header
can be in a data PT_LOAD segment.

-- 
H.J.
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