[Bug 600893] [NEW] buffer size rounding for >2G fails

[Stuart R Balfour]

Public bug reported:

Binary package hint: dvd+rw-tools

dvd+rw-tools-7.1-6

Inspection of the following code in growisofs.c
    /*
     * Ensure the_buffer_size is degree of 2
     */

    { unsigned int shift=0,sz=the_buffer_size;

        while (sz>>=1) shift++;
        if (shift<20) shift=20; /* 1MB is minumum */
        sz = 1<<shift;
        if (sz < the_buffer_size) sz<<=1;
        the_buffer_size = sz;
    }

Example: for input the_buffer_size  2G+1  yields the_buffer_size = 0.  The while loop
will shift 31 times before sz is 0, so shift is 31, and sz=1<<31 or 2G.  But 2G < 2G+1
below, so sz gets shifted up again, the bit falls off, and sz is now zero.

The buffer size needs a maximum of 2G as well as some minimum, i.e. 1M(?).  Even 
at 1X, we run a high risk of buffer underruns at 1M.  Maybe we ought to adjust this
as part of the bug fix.

It is apparent that the algorithm intends to round the_buffer_size up if it is not an even
power of two.   The algorithm labors. There is a simple and elegant  way to round up
to a power of 2 without all the tricky bit shifting.  Find it and fix it.  Better yet, define
or find something generally useful like log2(n), then use it here. I.e.,

unsigned int t=log2(the_buffer_size);     // index of high bit
if (1<<t!=the_buffer_size || t<10)          // not 2^n or <1M
     the_buffer_size=1 <<( (t<10)?10
                                        :(t==31)?31
                                        :t+1   );  // round up to 2^n

Of course, unless there is hardware/asm support (on i386, it's a BSR
instruction), computing log2(n) isn't especially cheap or elegant.  But
from a modularity and readability point of view, the code is impeccable
and therefore substantially more likely to be correct.

nearest and employ it here.

** Affects: dvd+rw-tools (Ubuntu)
     Importance: Undecided
         Status: New

-- 
buffer size rounding for >2G fails
https://bugs.launchpad.net/bugs/600893
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