Re: [Minios-devel] [UNIKRAFT PATCH 3/4] plat/*: Provide section for library constructors

[Florian Schmidt <Florian.Schmidt@xxxxxxxxx>]

I'm not sure this is doing exactly the right thing. I'll take the x86 
linuxu code as example, but it should apply to the other platforms, too:
On 02/12/2018 05:01 PM, Simon Kuenzer wrote:
> diff --git a/plat/linuxu/x86/link64.ld b/plat/linuxu/x86/link64.ld
> index a7d642d..3f78b81 100644
> --- a/plat/linuxu/x86/link64.ld
> +++ b/plat/linuxu/x86/link64.ld
> @@ -56,6 +56,30 @@ SECTIONS {
>                 *(.rodata)
>                 *(.rodata.*)
>         }
> +
> +       /* Constructor table */
> +       ukplat_ctortab = .;
> +       .ctortab : {
> +               *(.ukplat_ctortab0)
> +               *(.ukplat_ctortab0.*)
> +               *(.ukplat_ctortab1)
> +               *(.ukplat_ctortab1.*)
> +               *(.ukplat_ctortab2)
> +               *(.ukplat_ctortab2.*)
> +               *(.ukplat_ctortab3)
> +               *(.ukplat_ctortab3.*)
> +               *(.ukplat_ctortab4)
> +               *(.ukplat_ctortab4.*)
> +               *(.ukplat_ctortab5)
> +               *(.ukplat_ctortab5.*)
> +               *(.ukplat_ctortab6)
> +               *(.ukplat_ctortab6.*)
> +               *(.ukplat_ctortab7)
> +               *(.ukplat_ctortab7.*)
> +               LONG(0);
> +       }
> +       ukplat_ectortab = .;
> +
>         . = ALIGN(0x1000);
>         _erodata = .;
>   
So you put the constructor before the end symbol signifying the end of 
read-only data. That works as expected:
$ nm -n build/build/unikraft_linuxu-x86_64
[...]
0000000000405000 R _rodata
00000000004052c0 r hex2ascii_data
0000000000405300 R _nolibc_ctype
0000000000405c60 R ukplat_ctortab
0000000000405c64 D ukplat_ectortab
0000000000406000 D _data
0000000000406000 D _erodata
[...]

However, when I look at the sections, ctortab is writable:

$ readelf -S build/unikraft_linuxu-x86_64
There are 10 section headers, starting at offset 0x74e8:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
[...]
  [ 2] .rodata           PROGBITS         0000000000405000  00005000
       0000000000000c60  0000000000000000   A       0     0     32
  [ 3] .ctortab          PROGBITS         0000000000405c60  00005c60
       0000000000000004  0000000000000000  WA       0     0     1
  [ 4] .data             PROGBITS         0000000000406000  00006000
       00000000000000bc  0000000000000000  WA       0     0     32
[...]

That leaves me with two questions: First, is .ctortab supposed to be 
writable? My gut feeling is no, because it is completely populated at 
compile/link time, so no dynamicity there. Second, is _erodata designed 
to point at the end of .ctortab instead of .rodata? Even if .ctortab is 
supposed to be non-writable, I'm not sure about this one. Also, third: 
is the alignment of 1 correct?
Oh, and one more minor question unrelated to this: why did you not 
follow the example of the other pointers and started the name with an 
underscore (and name it the same as the section)?

Cheers,
Florian

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