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SYSCTL: Print binary sysctl warnings (nearly) only once 

When printing legacy sysctls print the warning message
for each of them only once.  This way there is a guarantee
the syslog won't be flooded for any sane program.

The original attempt at this made the tables non const and stored
the flag inline.

Linus suggested using a separate hash table for this, this is based on a
code snippet from him.

The hash implies this is not exact and can sometimes not print a
new sysctl due to a hash collision, but in practice this should not
be a problem

I used a FNV32 hash over the binary string with a 32byte bitmap. This
gives relatively little collisions when all the predefined binary sysctls
are hashed:

size 256
bucket
length      number
0:          [25]
1:          [67]
2:          [88]
3:          [47]
4:          [22]
5:          [6]
6:          [1]

The worst case is a single collision of 6 hash values.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
wifi-calibration
Andi Kleen 2009-12-23 21:00:20 +01:00
parent f42ecb2808
commit 4440095c82
1 changed files with 30 additions and 1 deletions
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31
kernel/sysctl_binary.c
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@ -1417,6 +1417,35 @@ static void deprecated_sysctl_warning(const int *name, int nlen)
return;
}
#define WARN_ONCE_HASH_BITS 8
#define WARN_ONCE_HASH_SIZE (1<<WARN_ONCE_HASH_BITS)
static DECLARE_BITMAP(warn_once_bitmap, WARN_ONCE_HASH_SIZE);
#define FNV32_OFFSET 2166136261U
#define FNV32_PRIME 0x01000193
/*
* Print each legacy sysctl (approximately) only once.
* To avoid making the tables non-const use a external
* hash-table instead.
* Worst case hash collision: 6, but very rarely.
* NOTE! We don't use the SMP-safe bit tests. We simply
* don't care enough.
*/
static void warn_on_bintable(const int *name, int nlen)
{
int i;
u32 hash = FNV32_OFFSET;
for (i = 0; i < nlen; i++)
hash = (hash ^ name[i]) * FNV32_PRIME;
hash %= WARN_ONCE_HASH_SIZE;
if (__test_and_set_bit(hash, warn_once_bitmap))
return;
deprecated_sysctl_warning(name, nlen);
}
static ssize_t do_sysctl(int __user *args_name, int nlen,
void __user *oldval, size_t oldlen, void __user *newval, size_t newlen)
{
@ -1431,7 +1460,7 @@ static ssize_t do_sysctl(int __user *args_name, int nlen,
if (get_user(name[i], args_name + i))
return -EFAULT;
deprecated_sysctl_warning(name, nlen);
warn_on_bintable(name, nlen);
return binary_sysctl(name, nlen, oldval, oldlen, newval, newlen);
}