linux-kernel - Re: Unnecessary overhead with stack protector.

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Date:	Wed, 21 Oct 2009 14:09:31 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, esandeen@...hat.com,
	cebbert@...hat.com, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: Unnecessary overhead with stack protector.

Eric Sandeen wrote:
> Arjan van de Ven wrote:

...

>> do you have any indication that SP actually increases the stack
>> footprint by that much? it's only a few bytes....
>>
>>
> 
> Here's a sample of some of the largest xfs stack users,
> and the effect stack-protector had on them.  This was just
> done with objdump -d xfs.ko | scripts/checkstack.pl; I don't
> know if there's extra runtime stack overhead w/ stackprotector?
> 
> -Eric
> 
> function                  nostack stackprot delta delta %
> xfs_bmapi                      376      408    32  9%
> xfs_bulkstat                   328      344    16  5%
> _xfs_trans_commit              296      312    16  5%
> xfs_iomap_write_delay          264      280    16  6%
> xfs_file_ioctl                 248      312    64 26%
> xfs_symlink                    248      264    16  6%
> xfs_bunmapi                    232      280    48 21%
> xlog_do_recovery_pass          232      248    16  7%
> xfs_trans_unreserve_and_mod_sb 224      240    16  7%
> xfs_bmap_del_extent            216      248    32 15%
> xfs_cluster_write              216      232    16  7%
> xfs_file_compat_ioctl          216      296    80 37%
> xfs_attr_set_int               200      216    16  8%
> xfs_bmap_add_extent_delay_real 200      248    48 24%
> 

but maybe more to Dave's original point, xfs on x86_64 in my tree had
243 functions with minimal stack usage of 8 bytes.  w/
CC_STACKPROTECTOR_ALL in force, I end up with these sizes for those
functions:

  count bytes
      3 16
    236 24
      1 32
      5 40

8->24 bytes is pretty significant too, w/ a 200% increase, if you add a
few up...

-Eric
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