[RFC] security: smack: add hash table for smack for quick labelsearching

[Tomasz Stanislawski]

Hi everyone,
I am a developer working on optimization of the TIZEN system.
Recently, I've discovered a performance issue in SMACK subsystem.
I used the PERF tool to find performance bottlenecks.

The test scenario was simple. Run multiple applications and
see what the system does using the following command:

 perf record -a -g

Next, see the results with the command:

 perf report -s symbol -g graph,0.5

Among the many lines, the following ones are especially interesting:

     5.96%  [k] smk_find_entry                                                                         
            |          
            |--5.06%-- smk_access
            |          |          
            |           --4.99%-- smk_curacc
            |                     |          
            |                     |--3.79%-- smack_ptrace_access_check
            |                     |          security_ptrace_access_check
            |                     |          __ptrace_may_access
            |                     |          ptrace_may_access
            |                     |          |          
            |                     |           --3.78%-- mm_access
            |                     |                     mm_for_maps
            |                     |                     m_start
            |                     |                     seq_read
            |                     |                     vfs_read
            |                     |                     sys_read
            |                     |                     ret_fast_syscall
            |                     |                     |          
            |                     |                      --3.19%-- (nil)
            |                     |          
            |                      --0.71%-- smack_inode_permission
            |                                security_inode_permission
            |                                inode_permission
            |          
             --0.89%-- smack_to_secid
                       smack_socket_getpeersec_dgram
                       security_socket_getpeersec_dgram
                       |          
                        --0.54%-- unix_stream_sendmsg

     4.63%  [k] strcmp                                                                                 
            |          
            |--2.16%-- smk_find_entry
            |          |          
            |           --1.92%-- smk_access
            |                     |          
            |                      --1.85%-- smk_curacc
            |                                |          
            |                                 --1.20%-- smack_ptrace_access_check
            |                                           security_ptrace_access_check
            |                                           __ptrace_may_access
            |                                           ptrace_may_access
            |                                           mm_access
            |                                           mm_for_maps
            |                                           m_start
            |                                           seq_read
            |                                           vfs_read
            |                                           sys_read
            |                                           ret_fast_syscall
            |                                           |          
            |                                            --0.99%-- (nil)
            |          
             --2.14%-- smk_access
                       |          
                        --2.11%-- smk_curacc
                                  |          
                                   --1.75%-- smack_ptrace_access_check
                                             security_ptrace_access_check
                                             __ptrace_may_access
                                             ptrace_may_access
                                             |          
                                              --1.73%-- mm_access
                                                        mm_for_maps
                                                        m_start
                                                        seq_read
                                                        vfs_read
                                                        sys_read
                                                        ret_fast_syscall
                                                        |          
                                                         --1.40%-- (nil)

To sum up, the result indicates that the CPU spents circa 8% (2.16% + 5.96%)
of cycles searching for a SMACK label in the smk_find_entry function.
The function iterates through smack_known_list to find an entry.
The further analysis showed that the size of the list can reach even 600.
I measured that it takes circa 200 tries to find an entry on average.
The value was computed as a total number iterations in the smk_find_entry's
loop divided by the times smk_find_entry was called in a time-window of
the length of 10 seconds.

IMO, this is a serious performance issue which scales badly with
a complexity of the system.

I implemented a patch that makes a use of a hash table to quicken searching
for SMACK's labels.  The patch is rebased onto the latest v3.9-rc6 kernel.
The code is thread-safe (I hope) because it shares the RCU mechanism
and locks with smack_known_list.

There is still some place for improvements like:
a) using struct hlist_head instead of struct list_head to reduce
   the memory size of the hash table.

OR

b) use smack_known::list instead of introducing smack_known::htab_list
   and modify all smack_known_list related code to iterate over
   the hash table.

I decided to postpone the mentioned improvements for a sake of simplicity
of this RFC. After applying the patch, the smk_find_entry overhead was
reduced to mere 0.05% of CPU cycles.

I hope you find the measurement and the patch useful.
All comments are welcome.

Regards,
Tomasz Stanislawski


Tomasz Stanislawski (1):
  security: smack: add hash table for smack for quick label searching

 security/smack/smack.h        |    5 +++++
 security/smack/smack_access.c |   33 +++++++++++++++++++++++++++++++--
 security/smack/smack_lsm.c    |   21 +++++++++++++++------
 3 files changed, 51 insertions(+), 8 deletions(-)

-- 
1.7.9.5

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