Re: xfs_bmap_add_extent_delay_real: Uninited r[3] corrupts startoff

[Lachlan McIlroy <lachlan@sgi.com>]

I considered that approach but wasn't keen on setting br_startblock to 0
when it should be NULLSTARTBLOCK.  Subsequent calls to xfs_bmbt_set_all()
handle NULLSTARTBLOCK differently but the net result ends up being the
same and the startblock eventually gets overridden anyway.  I'll go with
your suggestion.

Shailendra Tripathi wrote:
> Hi Lachlan,
>        I would prefer manual assignment here than struct assignment. 
> r[1].br_startoff and r[1].br_blockcount will be
> modified immediately, so it is not worth assigning via ( r[1] = PREV) as 
> it does extra instructions.
> Compiler would most likely eliminate the extra assignment but, why to 
> leave on the wit of the compiler.
> 
> It should be like
> r[1].br_state = PREV.br_state;
> r[1].br_startblock = 0 ; /* No fancy stuff required here as the aim here 
> is that br_startoff does not get any thing random */
> 
> Regards,
> Shailendra
> 
> Lachlan McIlroy wrote:
> 
>> This should be all that's needed.  This code handles the case where 
>> the middle
>> portion of a delayed allocation is being converted and splits the 
>> extent into
>> three.  The r[1] extent is the rightmost extent that will remain a 
>> delayed
>> allocation.  Both br_startblock and br_state need to be setup and they 
>> will be
>> the same as the original delayed allocation (PREV) so we just inherit 
>> those
>> values.  Comments?
>>
>> --- fs/xfs/xfs_bmap.c_1.358     2006-11-01 14:44:38.000000000 +0000
>> +++ fs/xfs/xfs_bmap.c   2006-11-02 13:22:41.000000000 +0000
>> @@ -1171,6 +1171,7 @@
>>                 xfs_bmap_trace_pre_update(fname, "0", ip, idx, 
>> XFS_DATA_FORK);
>>                 xfs_bmbt_set_blockcount(ep, temp);
>>                 r[0] = *new;
>> +               r[1] = PREV;
>>                 r[1].br_startoff = new_endoff;
>>                 temp2 = PREV.br_startoff + PREV.br_blockcount - 
>> new_endoff;
>>                 r[1].br_blockcount = temp2;
>>
>> Lachlan
>>
>> Vlad Apostolov wrote:
>>
>>> Hi Shailendra,
>>>
>>> Shailendra Tripathi wrote:
>>>
>>>> Hi,
>>>>     It appears that uninitialized r[3] in 
>>>> xfs_bmap_add_extent_delay_real can potentially corrupt the startoff 
>>>> for a particular case.
>>>>
>>>> This sequence is below:
>>>>
>>>>        xfs_bmap_add_extent_delay_real (
>>>>        ...
>>>>        xfs_bmbt_irec_t         r[3];   /* neighbor extent entries */
>>>>
>>>> case 0:
>>>>          /*
>>>>           * Filling in the middle part of a previous delayed 
>>>> allocation.
>>>>           * Contiguity is impossible here.
>>>>           * This case is avoided almost all the time.
>>>>           */
>>>> temp = new->br_startoff - PREV.br_startoff;
>>>> xfs_bmbt_set_blockcount(ep, temp);
>>>> r[0] = *new;
>>>> r[1].br_startoff = new_endoff;
>>>> temp2 = PREV.br_startoff + PREV.br_blockcount - new_endoff;
>>>> r[1].br_blockcount = temp2;
>>>> xfs_bmap_insert_exlist(ip, idx + 1, 2, &r[0], XFS_DATA_FORK);
>>>> ip->i_df.if_lastex = idx + 1;
>>>> ip->i_d.di_nextents++;
>>>>
>>>> Look at extent r[1]. It does not set br_startblock. That is, it is 
>>>> any random value. Now, look at the xfs_bmbt_set_all. Though, it sets 
>>>> the blockcount later, the startoff does not get changed.
>>>>
>>>> #if XFS_BIG_BLKNOS
>>>>         ASSERT((s->br_startblock & XFS_MASK64HI(12)) == 0);
>>>>         r->l0 = ((xfs_bmbt_rec_base_t)extent_flag << 63) |
>>>>                  ((xfs_bmbt_rec_base_t)s->br_startoff << 9) |
>>>>                  ((xfs_bmbt_rec_base_t)s->br_startblock >> 43);
>>>> Top 21 bits are taken as it is. However, only 9 bit should be taken. 
>>>> So, for random values, it corrupts the startoff which from 9-63 bits.
>>>
>>>
>>>  From the code inspection I agree with you that br_startblock doesn't 
>>> appear
>>> to be initialized in this scenario. Otherwise I think the code looks 
>>> good.
>>> If the br_startblock is initialized  it should be a value that fits
>>> in 52 bits out of 64 (this is what the ASSERT is for) and the top 12 
>>> bits will be 0.
>>> The r->l0 gets the top 21 bits of br_startblock, the most significant 
>>> 12 bits of
>>> which are 0 and least significant 9 could be non 0. The r->l1 gets the
>>> rest 43 (= 52-9 = 64-21) bits of br_startblock.
>>>
>>> I will open a bug report for the uninitialized br_startblock.
>>>
>>> Thank you for finding this problem.
>>>
>>> Regards,
>>> Vlad
>>>
>>>>
>>>>         r->l1 = ((xfs_bmbt_rec_base_t)s->br_startblock << 21) |
>>>>                  ((xfs_bmbt_rec_base_t)s->br_blockcount &
>>>>                  (xfs_bmbt_rec_base_t)XFS_MASK64LO(21));
>>>>
>>>> I have attached a small program which does the same thing as it is 
>>>> being done here. I would appreciate if someone can verify that 
>>>> assertion is correct.
>>>>
>>>>
>>>> Regards,
>>>> Shailendra
>>>> ------------------------------------------------------------------------ 
>>>>
>>>>
>>>> #include <stdio.h>
>>>> typedef unsigned long __uint64_t;
>>>> typedef struct xfs_bmbt_rec_64
>>>> {
>>>>         __uint64_t              l0, l1;
>>>> } xfs_bmbt_rec_64_t;
>>>>
>>>> typedef __uint64_t      xfs_bmbt_rec_base_t;    typedef 
>>>> xfs_bmbt_rec_64_t xfs_bmbt_rec_t, xfs_bmdr_rec_t;
>>>>
>>>> typedef enum {
>>>>        XFS_EXT_NORM, XFS_EXT_UNWRITTEN,
>>>>        XFS_EXT_DMAPI_OFFLINE
>>>> } xfs_exntst_t;
>>>>
>>>> typedef struct xfs_bmbt_irec
>>>> {
>>>>     __uint64_t   br_startoff;    /* starting file offset */
>>>>     __uint64_t   br_startblock;  /* starting block number */
>>>>     __uint64_t   br_blockcount;  /* number of blocks */
>>>>     xfs_exntst_t    br_state;       /* extent state */
>>>> } xfs_bmbt_irec_t;
>>>>
>>>> #define XFS_MASK64LO(n)         (((__uint64_t)1 << (n)) - 1)
>>>> #define XFS_MASK64HI(n)         ((__uint64_t)-1 << (64 - (n)))
>>>>
>>>> int main(void) {
>>>>     xfs_bmbt_irec_t s;
>>>>     xfs_bmbt_rec_t  r;
>>>>     int extent_flag;
>>>>
>>>>     s.br_startoff = 0;
>>>>     s.br_blockcount = 5;
>>>>     s.br_startblock = 0xfffffffffffffff0;
>>>>     extent_flag = (s.br_state == XFS_EXT_NORM) ? 0 : 1;
>>>>
>>>>     printf("blockcount = 0x%llx\n", s.br_startblock);
>>>>     r.l0 = ((xfs_bmbt_rec_base_t)extent_flag << 63) |
>>>>               ((xfs_bmbt_rec_base_t)s.br_startoff << 9) |
>>>>               ((xfs_bmbt_rec_base_t)s.br_startblock >> 43);
>>>>     r.l1 = ((xfs_bmbt_rec_base_t)s.br_startblock << 21) |
>>>>              ((xfs_bmbt_rec_base_t)s.br_blockcount &
>>>>              (xfs_bmbt_rec_base_t)XFS_MASK64LO(21));
>>>>
>>>>     printf("l0 = 0x%llx l1 = 0x%llx\n", r.l0, r.l1);
>>>>
>>>>     r.l0 = (r.l0 & (xfs_bmbt_rec_base_t)XFS_MASK64HI(55)) |
>>>>             (xfs_bmbt_rec_base_t)((__uint64_t)100 >> 43);
>>>>     r.l1 = (r.l1 & (xfs_bmbt_rec_base_t)XFS_MASK64LO(21)) |
>>>>             (xfs_bmbt_rec_base_t)((__uint64_t)100 << 21);
>>>>
>>>>     printf("l0 = 0x%llx l1 = 0x%llx\n", r.l0, r.l1);
>>>>     return 0;
>>>> }
>>>>   
>>>
>>>
>>>
>>>
> 
>