Messages in this thread | | | Date | Sat, 20 Apr 2019 21:07:03 +0300 | From | Alexey Dobriyan <> | Subject | Re: [PATCH -mm] elf: init pt_regs pointer later |
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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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