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?
How much faster, anyway? How frequently is the AT_RANDOM record
accessed?
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?
And let's Cc Kees, who wrote the thing.
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -144,11 +144,15 @@ static int padzero(unsigned long elf_bss)
> #define STACK_ALLOC(sp, len) ({ \
> elf_addr_t __user *old_sp = (elf_addr_t __user *)sp; sp += len; \
> old_sp; })
> +#define STACK_ALIGN(sp, align) \
> + ((typeof(sp))(((unsigned long)sp + (int)align - 1) & ~((int)align - 1)))
I suspect plain old ALIGN() could be used here.
> #else
> #define STACK_ADD(sp, items) ((elf_addr_t __user *)(sp) - (items))
> #define STACK_ROUND(sp, items) \
> (((unsigned long) (sp - items)) &~ 15UL)
> #define STACK_ALLOC(sp, len) ({ sp -= len ; sp; })
> +#define STACK_ALIGN(sp, align) \
> + ((typeof(sp))((unsigned long)sp & ~((int)align - 1)))
And maybe there's a helper which does this, dunno.
> #endif
>
> #ifndef ELF_BASE_PLATFORM
> @@ -217,6 +221,12 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
> return -EFAULT;
> }
>
> + /*
> + * glibc copies first bytes for stack protector purposes
> + * which are misaligned on x86_64 because strlen("x86_64") + 1 == 7.
> + */
> + p = STACK_ALIGN(p, sizeof(long));
> +
> /*
> * Generate 16 random bytes for userspace PRNG seeding.
> */
