Re: xfs_bmap_add_extent_delay_real: Uninited r[3] corrupts startoff

[Shailendra Tripathi <stripathi@agami.com>]

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;
>>> }
>>>   
>>
>>
>>