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SubjectRe: [PATCH] bitmap parsing/printing routines, version 4
Joe Korty wrote:
> This release of the bitmap parser/printer continues the cycle of making
> ever-smaller changes to the base code.
>
> The most significant change this time around is to be sure every
> character in the string is scanned for correctness. In the previous
> version, the scan would stop on the first whitespace character following
> a non-whitespace character. There was no determination made if that was
> the start of either embedded or trailing whitspace -- it was assumed to
> be trailing.
>
> I gradually came to feel that examining the whole string was important, as
> the parser does not return a pointer to where it stopped as the string(3)
> family does, so it is not possible for the caller to easily determine
> if the condition stopping the scan was reasonable or not.
>
> ChangeLog:
> o remove trailing whitespace early termination.
> o add leading / embedded / trailing whitespace tests.
> o 'extern' not needed on prototypes (Bill Irwin)
> o made terse variable names readable.
> o continued the reduction of the algorithm to simpler forms.
>
> This release concludes my contributions to this patch, other than changes
> driven by user requests and experience.
>
> Against 2.6.1-mm4.
>
> Joe

Joe,
I've attatched a small patch with some *small* changes, and the
addition of a whole lotta comments. I'd like to see what you think.

Changes:
1) Added a missing '"' in the comment for the bitmap_parse function
2) Renamed 'oc' to 'old_c' for readability
3) Remove "totaldigits == 0" check at the end of bitmap_parse. I
believe this check is redundant. The only way that totaldigits could be
0 at the end of the function is if ndigits is also 0 (because they're
both incremented at the same time), and this condition is already
checked for at the end of each chunk parsed. Is this correct?

Additions:
4) A whole bunch of comments. Are these all correct?

None of the things in my patch (with the possible exception of #3)
change the functionality of the code, which looks great.

Andrew, I agree with Paul's "thumbs-up" of Joe's patch. My patch is
solely meant to increase the readability of the bitmap_parse function.

Cheers!

-Matt
--- linux-2.6.1-joe_korty-bitmap/lib/bitmap.c.orig Mon Jan 19 11:45:32 2004
+++ linux-2.6.1-joe_korty-bitmap/lib/bitmap.c Mon Jan 19 13:11:24 2004
@@ -209,13 +209,13 @@ EXPORT_SYMBOL(bitmap_snprintf);
* bits of the resultant bitmask. No chunk may specify a value larger
* than 32 bits (-EOVERFLOW), and if a chunk specifies a smaller value
* then leading 0-bits are prepended. -EINVAL is returned for illegal
- * characters and for grouping errors such as "1,,5", ,44", "," and "".
+ * characters and for grouping errors such as "1,,5", ",44", "," and "".
* Leading and trailing whitespace accepted, but not embedded whitespace.
*/
int bitmap_parse(const char __user *ubuf, unsigned int ubuflen,
unsigned long *maskp, int nmaskbits)
{
- int i, c, oc, ndigits, totaldigits, nchunks, nbits;
+ int i, c, old_c, totaldigits, ndigits, nchunks, nbits;
u32 chunk;

bitmap_clear(maskp, nmaskbits);
@@ -223,40 +223,73 @@ int bitmap_parse(const char __user *ubuf
nchunks = nbits = totaldigits = c = 0;
do {
chunk = ndigits = 0;
+
+ /* Get the next chunk of the bitmap */
while (ubuflen) {
- oc = c;
+ /* Remember the last char & get the next char */
+ old_c = c;
if (get_user(c, ubuf++))
return -EFAULT;
ubuflen--;
+
+ /* Ignore spaces */
if (isspace(c))
continue;
- if (totaldigits && c && isspace(oc))
+
+ /*
+ * If the last character was a space and the current
+ * character isn't '\0' we've got embedded whitespace.
+ * This is a no-no, so throw an error.
+ */
+ if (totaldigits && c && isspace(old_c))
return -EINVAL;
+
+ /* A '\0' or a ',' signal the end of the chunk */
if (!c || c == ',')
break;
+
+ /* A non-hexdigit is also a no-no, so throw an error */
if (!isxdigit(c))
return -EINVAL;
+
+ /*
+ * Make sure there are at least 4 free bits in 'chunk'.
+ * If not, this hexdigit will overflow 'chunk', so
+ * throw an error.
+ */
if (chunk & ~((1UL << (CHUNKSZ - 4)) - 1))
return -EOVERFLOW;
+
+ /*
+ * Add this expanded hexdigit to 'chunk' and increment
+ * both the current chunk & total digit counts.
+ */
chunk = (chunk << 4) | unhex(c);
ndigits++; totaldigits++;
}
+ /* Empty chunks are another no-no. Throw an error. */
if (ndigits == 0)
return -EINVAL;
+
+ /* If the first chunk is all 0's, just move to the next one */
if (nchunks == 0 && chunk == 0)
continue;
+
+ /* Shift the bitmap right to make room for this chunk */
bitmap_shift_right(maskp, maskp, CHUNKSZ, nmaskbits);
+
+ /* Copy the chunk into the bitmap, and increment chunk count */
for (i = 0; i < CHUNKSZ; i++)
if (chunk & (1 << i))
set_bit(i, maskp);
nchunks++;
+
+ /* Increment the bit count & make sure we didn't overflow */
nbits += (nchunks == 1) ? nbits_to_hold_value(chunk) : CHUNKSZ;
if (nbits > nmaskbits)
return -EOVERFLOW;
} while (ubuflen && c == ',');

- if (totaldigits == 0)
- return -EINVAL;
return 0;
}
EXPORT_SYMBOL(bitmap_parse);
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