On Wed, May 29, 2019 at 03:20:20PM -0700, Andrew Morton wrote: > On Thu, 30 May 2019 00:37:08 +0300 Alexey Dobriyan <adobriyan@xxxxxxxxx> 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.
