Re: RFC: Add GNU_PROPERTY_NEED_PHDRS

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Oct 2, 2018 at 7:52 AM Michael Matz <matz@suse.de> wrote:
>
> Hi,
>
> On Fri, 28 Sep 2018, H.J. Lu wrote:
>
> > >> AT_PHDR to main, which points to the unmmaped address.  We can ask
> > >> for kernel change or make kernel happy.
> > >
> > > Kernel change does not help because nobody is obligated to use a new
> > > kernel. Binutils would be producing binaries that don't work on
> > > existing kernels (if the note hack were reverted or if similar changes
> > > were added to other archs without a note hack; right now of course
> > > it's working again).
> >
> > True.
> >
> > >> My current .note.gnu.property patch only works for x86.  We can add
> > >>
> > >> #define GNU_PROPERTY_PHDRS 3
> > >>
> > >> so that it can be used for all targets.
> > >
> > > What would this do?
> >
> > These are what I have in mind.
>
> I don't see how the patches fix anything, in particular making sure that
> the phdrs are always mapped.  If your intention is (it would be good if
> you can explain it with words) that they only would be made mapped if this
> new property is set, then I'd disagree.  I think they should always be
> made mapped unconditionally.  There are two ways for this: (a) add a new
> PT_LOAD that covers them, (b) move the phdrs into the ro data segment.  I
> find all approaches that add properties or new section types or anything
> else that needs documentation and definition dubious.
>

A .note.gnu.property section will lead to a read-only data PT_LOAD segment
as the first PT_LOAD segment:

[hjl@gnu-17 ld]$ readelf -WlS tmpdir/pr23428
There are 16 section headers, starting at offset 0x2ed0:

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            0000000000400158 000158
000030 00   A  0   0  8
  [ 2] .text             PROGBITS        0000000000401000 001000
0000ea 00  AX  0   0 16
  [ 3] .rodata           PROGBITS        0000000000402000 002000
000006 01 AMS  0   0  1
  [ 4] .comment          PROGBITS        0000000000000000 002006
00002d 01  MS  0   0  1
  [ 5] .debug_aranges    PROGBITS        0000000000000000 002040
000060 00      0   0 16
  [ 6] .debug_info       PROGBITS        0000000000000000 0020a0
0002f7 00      0   0  1
  [ 7] .debug_abbrev     PROGBITS        0000000000000000 002397
000137 00      0   0  1
  [ 8] .debug_line       PROGBITS        0000000000000000 0024ce
0001a7 00      0   0  1
  [ 9] .debug_frame      PROGBITS        0000000000000000 002678
000040 00      0   0  8
  [10] .debug_str        PROGBITS        0000000000000000 0026b8
0002a1 01  MS  0   0  1
  [11] .debug_loc        PROGBITS        0000000000000000 002959
000295 00      0   0  1
  [12] .debug_ranges     PROGBITS        0000000000000000 002bee
000020 00      0   0  1
  [13] .symtab           SYMTAB          0000000000000000 002c10
0001e0 18     14  14  8
  [14] .strtab           STRTAB          0000000000000000 002df0
000030 00      0   0  1
  [15] .shstrtab         STRTAB          0000000000000000 002e20
0000ab 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 0x4010c0
There are 5 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr
FileSiz  MemSiz   Flg Align
  LOAD           0x000000 0x0000000000400000 0x0000000000400000
0x000188 0x000188 R   0x1000
  LOAD           0x001000 0x0000000000401000 0x0000000000401000
0x0000ea 0x0000ea R E 0x1000
  LOAD           0x002000 0x0000000000402000 0x0000000000402000
0x000006 0x000006 R   0x1000
  NOTE           0x000158 0x0000000000400158 0x0000000000400158
0x000030 0x000030 R   0x8
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000
0x000000 0x000000 RWE 0x10

 Section to Segment mapping:
  Segment Sections...
   00     .note.gnu.property
   01     .text
   02     .rodata
   03     .note.gnu.property
   04
[hjl@gnu-17 ld]$


-- 
H.J.