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| author | Alexey Dobriyan <adobriyan@gmail.com> | 2019-05-14 15:43:54 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2019-05-14 19:52:50 -0700 |
| commit | 249b08e4e504d4c54eda3453c9c97edbafa51401 (patch) | |
| tree | ec52500abd913e84b8f5072975e09dc472dd244f /fs/binfmt_elf.c | |
| parent | d8e7cb39acc66316106ef8bda9b76f9f3cbbcad0 (diff) | |
| download | lwn-249b08e4e504d4c54eda3453c9c97edbafa51401.tar.gz lwn-249b08e4e504d4c54eda3453c9c97edbafa51401.zip | |
elf: init pt_regs pointer later
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>
Diffstat (limited to 'fs/binfmt_elf.c')
| -rw-r--r-- | fs/binfmt_elf.c | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index 31f264bd5126..1a66b6215c80 100644 --- 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) { @@ -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 |