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Date: Wed, 29 May 2019 16:00:23 -0700 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Alexey Dobriyan <adobriyan@...il.com>, linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org Subject: Re: [PATCH] elf: align AT_RANDOM bytes 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@...il.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)); > > + 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
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