Messages in this thread | | | Date | Wed, 29 May 2019 15:20:20 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] elf: align AT_RANDOM bytes |
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On Thu, 30 May 2019 00:37:08 +0300 Alexey Dobriyan <adobriyan@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?
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. > */
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