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SubjectRe: [PATCH] random.c bugfix
On Oct 29, 2001  18:23 -0600, Oliver Xymoron wrote:
> On Mon, 29 Oct 2001, Andreas Dilger wrote:
> > Having an accumulator would only handle a rarely-used corner case. We
> > could just as easily fix any user-space entropy daemon to write at least
> > 4 bytes at a time. Alternately, we could "pad" with enough bytes from
> > the random pool, and not accumulate at all.
>
> Padding -from the pool- is acceptable (and simple enough a slow path to
> add to the low-level function). Padding with constants is bad and padding
> with zeros tends to be really bad.

I would rather fix this at the caller and not in the low-level code, because
a large majority of the callers (esp. on the fast path) only use full word
inputs. Even for the random_write() case, we would only need this at most
a single time per write, instead of once per "hunk" (=64 bytes). What I
have done is to accumulate bytes in random_write() until we get a full word.

> > In any case, this is in the noise compared to not using the input data
> > at all (which I fixed in the other patch).
>
> Any of this made it into recent kernels yet? Backport to 2.2 might be a
> good idea too..

Well, I just saw that the "in++" and "nwords * 4" patches went into -pre4.
These are the only real non-cosmetic parts of what has been sent. The
other patches were not officially submitted to Linus yet (using bytes
as parameters, and removing poolwords from the struct). I have reverted
those patches in my tree, and gone back to using words as units for
add_entropy(), since it doesn't make sense to take bytes as a parameter
and then require a multiple of 4 bytes for input sizes.

The following patch changes random_write() to accumulate partial words.
It should apply against any recent kernel, and assumes the two "byte"
patches I previously sent out are not applied. Haven't had a chance
to test it yet, but it compiles.

Cheers, Andreas
===========================================================================
--- linux.orig/drivers/char/random.c Thu Oct 25 03:04:34 2001
+++ linux/drivers/char/random.c Mon Oct 29 20:31:42 2001
@@ -1576,34 +1576,56 @@
}

static ssize_t
-random_write(struct file * file, const char * buffer,
- size_t count, loff_t *ppos)
+random_write(struct file *file, const char *buffer, size_t count, loff_t *ppos)
{
+ static char save[4];
+ static int nsave = 0;
int ret = 0;
- size_t bytes;
- __u32 buf[16];
- const char *p = buffer;
- size_t c = count;
+ const __u32 *p = (__u32 *)buffer;
+ size_t w = count / 4;
+ int tail = count & 3;
+ int extra = 0;
+
+ while (w > 0) {
+ size_t words;
+ __u32 buf[16];

- while (c > 0) {
- bytes = MIN(c, sizeof(buf));
+ words = min(w, sizeof(buf) / 4);

- bytes -= copy_from_user(&buf, p, bytes);
- if (!bytes) {
+ if (copy_from_user(buf, p, words * 4)) {
ret = -EFAULT;
- break;
+ goto out;
}
- c -= bytes;
- p += bytes;
+ w -= words;
+ p += words;

- add_entropy_words(random_state, buf, (bytes + 3) / 4);
+ add_entropy_words(random_state, p, words);
+ }
+
+ /* Accumulate partial words until we get a full word for addition */
+ while (tail) {
+ int bytes = min(tail, 4 - nsave);
+
+ if (copy_from_user(save + nsave, (char *)p + extra, bytes)) {
+ ret = -EFAULT;
+ goto out;
+ }
+ nsave += bytes;
+ extra += bytes;
+ tail -= bytes;
+
+ if (nsave >= 4) {
+ add_entropy_words(random_state, (__u32 *)save, 1);
+ nsave = 0;
+ }
}
- if (p == buffer) {
+out:
+ if (p == (__u32 *)buffer && extra == 0) {
return (ssize_t)ret;
} else {
file->f_dentry->d_inode->i_mtime = CURRENT_TIME;
mark_inode_dirty(file->f_dentry->d_inode);
- return (ssize_t)(p - buffer);
+ return (ssize_t)((p - (__u32 *)buffer) * 4 + extra);
}
}

--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/
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