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elf: init pt_regs pointer later

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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 !!!

Looks like the 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

Link: http://lkml.kernel.org/r/20190419200343.GA19788@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Tested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Alexey Dobriyan 2019-05-14 15:43:54 -07:00 committed by Linus Torvalds
parent d8e7cb39ac
commit 249b08e4e5
1 changed files with 2 additions and 1 deletions
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3
fs/binfmt_elf.c
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@ -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) {
@ -1150,6 +1150,7 @@ out_free_interp:
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