Re: [Xen-devel] [PATCH v2 3/5] xen/livepatch/ARM32: Don't load and crash on livepatches loaded with wrong alignment.

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Tue, Sep 19, 2017 at 09:04:44AM -0600, Jan Beulich wrote:
> >>> On 18.09.17 at 21:37, <konrad.wilk@xxxxxxxxxx> wrote:
> > On Tue, Sep 12, 2017 at 02:57:04AM -0600, Jan Beulich wrote:
> >> >>> On 12.09.17 at 02:22, <konrad@xxxxxxxxxx> wrote:
> >> > If I compile the test-case under ARM32 it works OK (as the
> >> > .livepatch.depends ends up being aligned to four bytes).
> >> 
> >> So why is that? What entity is creating this section (or the
> >> directive(s) to create it)?
> > 
> > gcc
> > 
> > Looking at the xen_bye_world.o produced by cross-compiler:
> > 
> > xen_bye_world.o:     file format elf32-littlearm
> > 
> > Contents of section .rodata:
> >  0000 78656e5f 65787472 615f7665 7273696f  xen_extra_versio
> >  0010 6e00                                 n. 
> > 
> > And native:
> > 
> > xen_bye_world.o:     file format elf32-littlearm
> > 
> > Contents of section .rodata:
> >  0000 78656e5f 65787472 615f7665 7273696f  xen_extra_versio
> >  0010 6e000000                             n...      
> 
> This may rather be a gas than a gcc behavioral difference. What's
> the alignment of .rodata in both cases?

Cross:

* on the livepatch:
..snip..
  [ 4] .rodata           PROGBITS        00000000 000074 000012 00   A  0   0  4
  [ 5] .rodata.str1.4    PROGBITS        00000000 000088 00000b 01 AMS  0   0  4
  [ 6] .livepatch.depend PROGBITS        00000000 000093 000024 00   A  0   0  1

* on the .o file:
Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
.. snip..
  [ 1] .text             PROGBITS        00000000 000034 000000 00  AX  0   0  1
  [ 2] .data             PROGBITS        00000000 000034 000000 00  WA  0   0  1
  [ 3] .bss              NOBITS          00000000 000034 000000 00  WA  0   0  1
  [ 4] .rodata           PROGBITS        00000000 000034 000014 00   A  0   0  4
  [ 5] .livepatch.funcs  PROGBITS        00000000 000048 000034 00  WA  0   0  4

Native:

 * on the livepatch:
..snip..
  [ 4] .rodata           PROGBITS        00000000 000074 000014 00   A  0   0  4
  [ 5] .rodata.str1.4    PROGBITS        00000000 000088 00000c 01 AMS  0   0  4
  [ 6] .livepatch.depend PROGBITS        00000000 000094 000024 00   A  0   0  1

* on the .o file:
..snip..
  [ 1] .text             PROGBITS        00000000 000034 000000 00  AX  0   0  1
  [ 2] .data             PROGBITS        00000000 000034 000000 00  WA  0   0  1
  [ 3] .bss              NOBITS          00000000 000034 000000 00  WA  0   0  1
  [ 4] .rodata           PROGBITS        00000000 000034 000012 00   A  0   0  4
  [ 5] .livepatch.funcs  PROGBITS        00000000 000048 000034 00  WA  0   0  4



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