sysctl: Improve the sysctl sanity checks

- Stop validating subdirectories now that we only register leaf tables

- Cleanup and improve the duplicate filename check.
  * Run the duplicate filename check under the sysctl_lock to guarantee
    we never add duplicate names.
  * Reduce the duplicate filename check to nearly O(M*N) where M is the
    number of entries in tthe table we are registering and N is the
    number of entries in the directory before we got there.

- Move the duplicate filename check into it's own function and call
  it directtly from __register_sysctl_table

- Kill the config option as the sanity checks are now cheap enough
  the config option is unnecessary. The original reason for the config
  option was because we had a huge table used to verify the proc filename
  to binary sysctl mapping.  That table has now evolved into the binary_sysctl
  translation layer and is no longer part of the sysctl_check code.

- Tighten up the permission checks.  Guarnateeing that files only have read
  or write permissions.

- Removed redudant check for parents having a procname as now everything has
  a procname.

- Generalize the backtrace logic so that we print a backtrace from
  any failure of __register_sysctl_table that was not caused by
  a memmory allocation failure.  The backtrace allows us to track
  down who erroneously registered a sysctl table.

Bechmark before (CONFIG_SYSCTL_CHECK=y):
    make-dummies 0 999 -> 12s
    rmmod dummy        -> 0.08s

Bechmark before (CONFIG_SYSCTL_CHECK=n):
    make-dummies 0 999 -> 0.7s
    rmmod dummy        -> 0.06s
    make-dummies 0 99999 -> 1m13s
    rmmod dummy          -> 0.38s

Benchmark after:
    make-dummies 0 999 -> 0.65s
    rmmod dummy        -> 0.055s
    make-dummies 0 9999 -> 1m10s
    rmmod dummy         -> 0.39s

The sysctl sanity checks now impose no measurable cost.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

1 files changed, 0 insertions, 8 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8745ac7d1f75..943a6182cdf2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1113,14 +1113,6 @@ config LATENCYTOP
           Enable this option if you want to use the LatencyTOP tool
           to find out which userspace is blocking on what kernel operations.
 
-config SYSCTL_SYSCALL_CHECK
-        bool "Sysctl checks"
-        depends on SYSCTL
-        ---help---
-          sys_sysctl uses binary paths that have been found challenging
-          to properly maintain and use. This enables checks that help
-          you to keep things correct.
-
 source mm/Kconfig.debug
 source kernel/trace/Kconfig