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.

Andi Kleen committed 16 years ago

Commit 44400951 parent f42ecb21 file · +30 −1Browse files

Showing the changes to kernel/sysctl_binary.c (1 of 1 file). Show all

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kernel/sysctl_binary.c+30−1

@@ -1417,6 +1417,35 @@ static void deprecated_sysctl_warning(const int *name, int nlen) · 1416 unmodified lines
1417	1417	return;
1418	1418	}
1419	1419
	1420	+#define WARN_ONCE_HASH_BITS 8
	1421	+#define WARN_ONCE_HASH_SIZE (1<<WARN_ONCE_HASH_BITS)
	1422	+
	1423	+static DECLARE_BITMAP(warn_once_bitmap, WARN_ONCE_HASH_SIZE);
	1424	+
	1425	+#define FNV32_OFFSET 2166136261U
	1426	+#define FNV32_PRIME 0x01000193
	1427	+
	1428	+/*
	1429	+ * Print each legacy sysctl (approximately) only once.
	1430	+ * To avoid making the tables non-const use a external
	1431	+ * hash-table instead.
	1432	+ * Worst case hash collision: 6, but very rarely.
	1433	+ * NOTE! We don't use the SMP-safe bit tests. We simply
	1434	+ * don't care enough.
	1435	+ */
	1436	+static void warn_on_bintable(const int *name, int nlen)
	1437	+{
	1438	+ int i;
	1439	+ u32 hash = FNV32_OFFSET;
	1440	+
	1441	+ for (i = 0; i < nlen; i++)
	1442	+ hash = (hash ^ name[i]) * FNV32_PRIME;
	1443	+ hash %= WARN_ONCE_HASH_SIZE;
	1444	+ if (__test_and_set_bit(hash, warn_once_bitmap))
	1445	+ return;
	1446	+ deprecated_sysctl_warning(name, nlen);
	1447	+}
	1448	+
1420	1449	static ssize_t do_sysctl(int __user *args_name, int nlen,
1421	1450	void __user oldval, size_t oldlen, void __user newval, size_t newlen)
1422	1451	{
@@ -1431,7 +1460,7 @@ static ssize_t do_sysctl(int __user *args_name, int nlen, · 8 unmodified lines
1431	1460	if (get_user(name[i], args_name + i))
1432	1461	return -EFAULT;
1433	1462
1434		− deprecated_sysctl_warning(name, nlen);
	1463	+ warn_on_bintable(name, nlen);
1435	1464
1436	1465	return binary_sysctl(name, nlen, oldval, oldlen, newval, newlen);
1437	1466	}