SYSCTL: Print binary sysctl warnings (nearly) only once (4440095c) · Commits · 戴 / test

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>

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);
		}

Admin message