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?
>
> 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));
> > +
I have no objection to eating some bytes here. Though perhaps things could just
be reordered to leave all the aligned things together and put all the
strings later?
-Kees
> > /*
> > * Generate 16 random bytes for userspace PRNG seeding.
> > */
>
--
Kees Cook
