On Tuesday 23 June 2009, Linus Torvalds wrote:
> You might need to make 'result', 'carry', and 'w' be 'unsigned int' too.
Yes, you're right.
> Now, it's possible (even likely) that even with a 64-bit word, we'll never
> actually do large enough areas that 'result' would ever have very many
> bits set in the 32+ bit region, and since we do end up folding to 16 bits
> twice (once after the loop and once at the end), it probably gets things
> right in most cases. But I doubt "probably" is strong enough. Somebody
> should check.
I think it would overrun only if we have more than 65536 u32 words of
0xffffffff in a single IP packet, on a 64 bit machine. A more obvious
reason to change it is that it relies on from32to16() actually behaving
like a from47to16() function on 64-bit. Changing it to use unsigned int
throughout makes it both more obvious and more consistent between
32 and 64 bit unsigned long types.
> Or just see arch/alpha/lib/checksum.c, which does the whole 64-bit case.
> Maybe lib/checksum.c should be lib/checksum_{32,64}.c.
Mike Frysinger earlier suggested just making the do_csum function optional
in this file because this is the one that most architectures would override.
The alpha code is the only 64-bit platform implementing do_csum in C, so
if Richard wants to use the generic code in its current form, he could simply
override the do_csum implementation.
I've now added these two patches:
commit 217a8c7b6af924379a2083439b4bb606f332e7b1
Author: Arnd Bergmann <arnd@xxxxxxxx>
Date: Tue Jun 23 21:37:26 2009 +0200
lib/checksum.c: make do_csum optional
Mike Frysinger suggested that do_csum should be optional
so that an architecture can use the generic checksum code
but still provide an optimized fast-path for the most
critical function.
This can mean an implementation using inline assembly,
or in case of Alpha one using 64-bit arithmetic in C.
Cc: Mike Frysinger <vapier@xxxxxxxxxx>
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
diff --git a/lib/checksum.c b/lib/checksum.c
index 886b48d..b08c2d0 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -37,6 +37,7 @@
#include <asm/byteorder.h>
+#ifndef do_csum
static inline unsigned short from32to16(unsigned int x)
{
/* add up 16-bit and 16-bit for 16+c bit */
@@ -102,6 +103,7 @@ static unsigned int do_csum(const unsigned char *buff, int len)
out:
return result;
}
+#endif
/*
* This is a version of ip_compute_csum() optimized for IP headers,
commit 5cb59758c3e2170b24e9c0d659eb6c03872155c0
Author: Arnd Bergmann <arnd@xxxxxxxx>
Date: Tue Jun 23 21:22:58 2009 +0200
lib/checksum.c: use 32-bit arithmetic consistently
The use of 'unsigned long' variables in the 32-bit part of do_csum()
is confusing at best, and potentially broken for long input on 64-bit
machines.
This changes the code to use 'unsigned int' instead, which makes
the code behave in the same (correct) way on both 32 and 64 bit
machines.
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
diff --git a/lib/checksum.c b/lib/checksum.c
index b2e2fd4..886b48d 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -37,7 +37,7 @@
#include <asm/byteorder.h>
-static inline unsigned short from32to16(unsigned long x)
+static inline unsigned short from32to16(unsigned int x)
{
/* add up 16-bit and 16-bit for 16+c bit */
x = (x & 0xffff) + (x >> 16);
@@ -49,7 +49,7 @@ static inline unsigned short from32to16(unsigned long x)
static unsigned int do_csum(const unsigned char *buff, int len)
{
int odd, count;
- unsigned long result = 0;
+ unsigned int result = 0;
if (len <= 0)
goto out;
@@ -73,9 +73,9 @@ static unsigned int do_csum(const unsigned char *buff, int len)
}
count >>= 1; /* nr of 32-bit words.. */
if (count) {
- unsigned long carry = 0;
+ unsigned int carry = 0;
do {
- unsigned long w = *(unsigned int *) buff;
+ unsigned int w = *(unsigned int *) buff;
count--;
buff += 4;
result += carry;
--
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