Re: Unnecessary overhead with stack protector.

[Eric Sandeen <sandeen@redhat.com>]

Ingo Molnar wrote:
> (Cc:-ed Arjan too.)
> 
> * Dave Jones <davej@redhat.com> wrote:
> 
>> 113c5413cf9051cc50b88befdc42e3402bb92115 introduced a change that made 
>> CC_STACKPROTECTOR_ALL not-selectable if someone enables 
>> CC_STACKPROTECTOR.
>>
>> We've noticed in Fedora that this has introduced noticable overhead on 
>> some functions, including those which don't even have any on-stack 
>> variables.
>>
>> According to the gcc manpage, -fstack-protector will protect functions 
>> with as little as 8 bytes of stack usage. So we're introducing a huge 
>> amount of overhead, to close a small amount of vulnerability (the >0 
>> && <8 case).
>>
>> The overhead as it stands right now means this whole option is 
>> unusable for a distro kernel without reverting the above commit.
> 
> Exactly what workload showed overhead, and how much?
> 
> 	Ingo

I had xfs blowing up pretty nicely; granted, xfs is not svelte but it
was never this bad before.

-Eric


         Depth    Size   Location    (65 entries)
         -----    ----   --------
   0)     7280      80   check_object+0x6c/0x1d3
   1)     7200     112   __slab_alloc+0x332/0x3f0
   2)     7088      16   kmem_cache_alloc+0xcb/0x18a
   3)     7072     112   mempool_alloc_slab+0x28/0x3e
   4)     6960     128   mempool_alloc+0x71/0x13c
   5)     6832      32   scsi_sg_alloc+0x5d/0x73
   6)     6800     128   __sg_alloc_table+0x6f/0x134
   7)     6672      64   scsi_alloc_sgtable+0x3b/0x74
   8)     6608      48   scsi_init_sgtable+0x34/0x8c
   9)     6560      80   scsi_init_io+0x3e/0x177
  10)     6480      48   scsi_setup_fs_cmnd+0x9c/0xb9
  11)     6432     160   sd_prep_fn+0x69/0x8bd
  12)     6272      64   blk_peek_request+0xf0/0x1c8
  13)     6208     112   scsi_request_fn+0x92/0x4c4
  14)     6096      48   __blk_run_queue+0x54/0x9a
  15)     6048      80   elv_insert+0xbd/0x1e0
  16)     5968      64   __elv_add_request+0xa7/0xc2
  17)     5904      64   blk_insert_cloned_request+0x90/0xc8
  18)     5840      48   dm_dispatch_request+0x4f/0x8b
  19)     5792      96   dm_request_fn+0x141/0x1ca
  20)     5696      48   __blk_run_queue+0x54/0x9a
  21)     5648      80   cfq_insert_request+0x39d/0x3d4
  22)     5568      80   elv_insert+0x120/0x1e0
  23)     5488      64   __elv_add_request+0xa7/0xc2
  24)     5424      96   __make_request+0x35e/0x3f1
  25)     5328      64   dm_request+0x55/0x234
  26)     5264     128   generic_make_request+0x29e/0x2fc
  27)     5136      80   submit_bio+0xe3/0x100
  28)     5056     112   _xfs_buf_ioapply+0x21d/0x25c [xfs]
  29)     4944      48   xfs_buf_iorequest+0x58/0x9f [xfs]
  30)     4896      48   _xfs_buf_read+0x45/0x74 [xfs]
  31)     4848      48   xfs_buf_read_flags+0x67/0xb5 [xfs]
  32)     4800     112   xfs_trans_read_buf+0x1be/0x2c2 [xfs]
  33)     4688     112   xfs_btree_read_buf_block+0x64/0xbc [xfs]
  34)     4576      96   xfs_btree_lookup_get_block+0x9c/0xd8 [xfs]
  35)     4480     192   xfs_btree_lookup+0x14a/0x408 [xfs]
  36)     4288      32   xfs_alloc_lookup_eq+0x2c/0x42 [xfs]
  37)     4256     112   xfs_alloc_fixup_trees+0x85/0x2b4 [xfs]
  38)     4144     176   xfs_alloc_ag_vextent_near+0x339/0x8e8 [xfs]
  39)     3968      48   xfs_alloc_ag_vextent+0x44/0x126 [xfs]
  40)     3920     128   xfs_alloc_vextent+0x2b1/0x403 [xfs]
  41)     3792     272   xfs_bmap_btalloc+0x4fc/0x6d4 [xfs]
  42)     3520      32   xfs_bmap_alloc+0x21/0x37 [xfs]
  43)     3488     464   xfs_bmapi+0x70b/0xde1 [xfs]
  44)     3024     256   xfs_iomap_write_allocate+0x21d/0x35d [xfs]
  45)     2768     192   xfs_iomap+0x208/0x28a [xfs]
  46)     2576      48   xfs_map_blocks+0x3d/0x5a [xfs]
  47)     2528     256   xfs_page_state_convert+0x2b8/0x589 [xfs]
  48)     2272      96   xfs_vm_writepage+0xbf/0x10e [xfs]
  49)     2176      48   __writepage+0x29/0x5f
  50)     2128     320   write_cache_pages+0x27b/0x415
  51)     1808      32   generic_writepages+0x38/0x4e
  52)     1776      80   xfs_vm_writepages+0x60/0x7f [xfs]
  53)     1696      48   do_writepages+0x3d/0x63
  54)     1648     144   writeback_single_inode+0x169/0x29d
  55)     1504     112   generic_sync_sb_inodes+0x21d/0x37f
  56)     1392      64   writeback_inodes+0xb6/0x125
  57)     1328     192   balance_dirty_pages_ratelimited_nr+0x172/0x2b0
  58)     1136     240   generic_file_buffered_write+0x240/0x33c
  59)      896     256   xfs_write+0x4d4/0x723 [xfs]
  60)      640      32   xfs_file_aio_write+0x79/0x8f [xfs]
  61)      608     320   do_sync_write+0xfa/0x14b
  62)      288      80   vfs_write+0xbd/0x12e
  63)      208      80   sys_write+0x59/0x91
  64)      128     128   system_call_fastpath+0x16/0x1b