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Commit 4440095c authored by Andi Kleen's avatar Andi Kleen
Browse files

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: default avatarAndi Kleen <ak@linux.intel.com>
parent f42ecb28

kernel/sysctl_binary.c

+30 −1
Original line number Diff line number Diff line
@@ -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);

}