Re: bin/37656: fdisk segfault on invalid MBR entries on non-x86 geometry detection

[Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>]

jnemeth%victoria.tc.ca@localhost wrote:

> } my understanding is that on non-x86 arch you check the MBR to guess the
> 
>      It does on x86 as well (see below and note that none of the head
> numbers makes sense).

On x86 machines intuit_translated_geometry() is not called directly
but get_geometry() is called first.

> } geometry ? what about brand new harddrives, MBR must always be null no ?
> 
>      Not sure how this is handled, but yes the MBR would be all 0s.

The original investigation by Eric is:
---
>> So I guess it's failing in this subpart of intuit_translated_geometry()
:
>> 
>> [...]
>> 1459     /* Try to deduce the number of heads from two different mappings. */
>> 1460     for (i = 0; i < MBR_PART_COUNT * 2 - 1; i++) {
>> 1461         if (get_mapping(i, &c1, &h1, &s1, &a1) < 0)
>> 1462             continue;
>> 1463         a1 -= s1;
>> 1464         for (j = i + 1; j < MBR_PART_COUNT * 2; j++) {
>> 1465             if (get_mapping(j, &c2, &h2, &s2, &a2) < 0)
>> 1466                 continue;
>> 1467             a2 -= s2;
>> 1468             num = (uint64_t)h1 * a2 - (uint64_t)h2 * a1;
>> 1469             denom = (uint64_t)c2 * a1 - (uint64_t)c1 * a2;
>> 1470             if (denom != 0 && num % denom == 0) {
>> 1471                 xheads = num / denom;
>> 1472                 xsectors = a1 / (c1 * xheads + h1);
>> 1473                 break;
>> 1474             }
>> 1475         }
>> 1476         if (xheads != -1)
>> 1477             break;
>> 1478     }
>> [...]
>> 
>> But I'm not sure what it is doing... :/
>> 
>> I keep investigating..
>
> the crash was due by a divide by 0 on xsectors line 1472, as xheads == 0,
> because num == 0, num == 0 because h1 == 0 and h2 == 0
> returned by get_mappings()...
---

Maybe we have to change this line
>> 1470             if (denom != 0 && num % denom == 0) {
to
>> 1470             if (denom != 0 && num != 0 && num % denom == 0) {
to avoid a possible division by zero?
---
Izumi Tsutsui