On Wed, May 29, 2019 at 03:20:20PM -0700, Andrew Morton wrote:
> On Thu, 30 May 2019 00:37:08 +0300 Alexey Dobriyan <adobri...@gmail.com>
> wrote:
>
> > AT_RANDOM content is always misaligned on x86_64:
> >
> > $ LD_SHOW_AUXV=1 /bin/true | grep AT_RANDOM
> > AT_RANDOM: 0x7fff02101019
> >
> > glibc copies first few bytes for stack protector stuff, aligned
> > access should be slightly faster.
>
> I just don't understand the implications of this. Is there
> (badly-behaved) userspace out there which makes assumptions about the
> current alignment?
I don't think so: glibc has getauxval(AT_RANDOM) and userspace should
use whatever it returns as "char[16]" base pointer;
> How much faster, anyway? How frequently is the AT_RANDOM record
> accessed?
I don't think it is measureable :-\
It is accessed twice per execve: first by the kernel putting data there,
second by glibc fetching first sizeof(uintptr_t) bytes for stack canary.
Here is stack layout at the beginning of execution:
....10 e8 03 00 00 00 00 00 00 17 00 00 00 00 00 00 00 |................|
....20 00 00 00 00 00 00 00 00 19 00 00 00 00 00 00 00 |................|
AT_RANDOM=25
....30 79 dd ff ff ff 7f 00 00 1a 00 00 00 00 00 00 00 |y...............|
AT_RANDOM pointer
....40 00 00 00 00 00 00 00 00 1f 00 00 00 00 00 00 00 |................|
....50 e2 ef ff ff ff 7f 00 00 0f 00 00 00 00 00 00 00 |................|
....60 89 dd ff ff ff 7f 00 00 00 00 00 00 00 00 00 00 |................|
AT_RANDOM bytes (misaligned)
....70 00 00 00 00 00 00 00 00 00|a2 ef 76 37 0c 0c 69 |...........v7..i|
....80 04 32 68 e4 68 2d 53 cf a5|78 38 36 5f 36 34 00 |.2h.h-S..x86_64.|
AT_RANDOM------------------|"x86_64" (misaligned)
argv[0], envp[0]
....90 00 00 00|2f 68 6f 6d 65 2f 61 64 2f 73 2d 74 65 |.../home/ad/s-te|
....a0 73 74 2f 61 2e 6f 75 74 00 47 53 5f 4c 49 42 3d |st/a.out.GS_LIB=|
> I often have questions such as these about your performance/space
> tweaks :(. Please try to address them as a matter of course when
> preparing changelogs?
OK.
