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SubjectRe: [PATCH -mm] elf: init pt_regs pointer later
On Fri, Apr 19, 2019 at 01:08:26PM -0700, Andrew Morton wrote:
> On Fri, 19 Apr 2019 23:03:43 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:
>
> > Get "current_pt_regs" pointer right before usage.
> >
> > Space savings on x86_64:
> >
> > add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-180 (-180)
> > Function old new delta
> > load_elf_binary 5806 5626 -180 !!!
>
> -256 bytes with my setup.
>
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -704,12 +704,12 @@ static int load_elf_binary(struct linux_binprm *bprm)
> > unsigned long start_code, end_code, start_data, end_data;
> > unsigned long reloc_func_desc __maybe_unused = 0;
> > int executable_stack = EXSTACK_DEFAULT;
> > - struct pt_regs *regs = current_pt_regs();
> > struct {
> > struct elfhdr elf_ex;
> > struct elfhdr interp_elf_ex;
> > } *loc;
> > struct arch_elf_state arch_state = INIT_ARCH_ELF_STATE;
> > + struct pt_regs *regs;
> >
> > loc = kmalloc(sizeof(*loc), GFP_KERNEL);
> > if (!loc) {
> > @@ -1159,6 +1159,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
> > MAP_FIXED | MAP_PRIVATE, 0);
> > }
> >
> > + regs = current_pt_regs();
> > #ifdef ELF_PLAT_INIT
> > /*
> > * The ABI may specify that certain registers be set up in special
>
> Why the heck does this make such a difference?

Good question. Looks like compiler doesn't know that "current_pt_regs" is
stable pointer (because it doesn't know ->stack isn't) even though it knows
that "current" is stable pointer. So it saves it in the very beginning and
then tries to carry it through a lot of code.

Here is what happens here:

load_elf_binary()
...
mov rax,QWORD PTR gs:0x14c00
mov r13,QWORD PTR [rax+0x18] r13 = current->stack
call kmem_cache_alloc # first kmalloc

[980 bytes later!]

# let's spill that sucker because we need a register
# for "load_bias" calculations at
#
# if (interpreter) {
# load_bias = ELF_ET_DYN_BASE;
# if (current->flags & PF_RANDOMIZE)
# load_bias += arch_mmap_rnd();
# elf_flags |= elf_fixed;
# }
mov QWORD PTR [rsp+0x68],r13

If this is not _the_ root cause it is still eeeeh.

After the patch things become much simpler:

mov rax, QWORD PTR gs:0x14c00 # current
mov rdx, QWORD PTR [rax+0x18] # current->stack
movq [rdx+0x3fb8], 0 # fill pt_regs
...
call finalize_exec

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