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bpf: do not use reciprocal divide

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At first Jakub Zawadzki noticed that some divisions by reciprocal_divide
were not correct. (off by one in some cases)
http://www.wireshark.org/~darkjames/reciprocal-buggy.c

He could also show this with BPF:
http://www.wireshark.org/~darkjames/set-and-dump-filter-k-bug.c

The reciprocal divide in linux kernel is not generic enough,
lets remove its use in BPF, as it is not worth the pain with
current cpus.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Cc: Mircea Gherzan <mgherzan@gmail.com>
Cc: Daniel Borkmann <dxchgb@gmail.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Matt Evans <matt@ozlabs.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet
2014-01-15 06:50:07 -08:00
committed by David S. Miller
parent ba42fad096
commit aee636c480
6 changed files with 45 additions and 46 deletions
Download Patch File Download Diff File Expand all files Collapse all files
net/core/filter.c
+2 -28
View File
@@ -36,7 +36,6 @@
#include <asm/uaccess.h>
#include <asm/unaligned.h>
#include <linux/filter.h>
#include <linux/reciprocal_div.h>
#include <linux/ratelimit.h>
#include <linux/seccomp.h>
#include <linux/if_vlan.h>
@@ -166,7 +165,7 @@ unsigned int sk_run_filter(const struct sk_buff *skb,
A /= X;
continue;
case BPF_S_ALU_DIV_K:
A = reciprocal_divide(A, K);
A /= K;
continue;
case BPF_S_ALU_MOD_X:
if (X == 0)
@@ -553,11 +552,6 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
/* Some instructions need special checks */
switch (code) {
case BPF_S_ALU_DIV_K:
/* check for division by zero */
if (ftest->k == 0)
return -EINVAL;
ftest->k = reciprocal_value(ftest->k);
break;
case BPF_S_ALU_MOD_K:
/* check for division by zero */
if (ftest->k == 0)
@@ -853,27 +847,7 @@ void sk_decode_filter(struct sock_filter *filt, struct sock_filter *to)
to->code = decodes[code];
to->jt = filt->jt;
to->jf = filt->jf;
if (code == BPF_S_ALU_DIV_K) {
/*
* When loaded this rule user gave us X, which was
* translated into R = r(X). Now we calculate the
* RR = r(R) and report it back. If next time this
* value is loaded and RRR = r(RR) is calculated
* then the R == RRR will be true.
*
* One exception. X == 1 translates into R == 0 and
* we can't calculate RR out of it with r().
*/
if (filt->k == 0)
to->k = 1;
else
to->k = reciprocal_value(filt->k);
BUG_ON(reciprocal_value(to->k) != filt->k);
} else
to->k = filt->k;
to->k = filt->k;
}
int sk_get_filter(struct sock *sk, struct sock_filter __user *ubuf, unsigned int len)