Re: [PATCH v2 1/2] ext4: Include forgotten start block on fallocate insert range

Roman Penyaev
2017-01-09T12:36:34Z

On Sat, Jan 7, 2017 at 10:22 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> On Fri, Jan 06, 2017 at 09:26:00PM +0100, Roman Pen wrote:
>> While doing 'insert range' start block should be also shifted right.
>> The bug can be easily reproduced by the following test:
>>
>> ptr = malloc(4096);
>> assert(ptr);
>>
>> fd = open("./ext4.file", O_CREAT | O_TRUNC | O_RDWR, 0600);
>> assert(fd >= 0);
>>
>> rc = fallocate(fd, 0, 0, 8192);
>> assert(rc == 0);
>> for (i = 0; i < 2048; i++)
>> *((unsigned short *)ptr + i) = 0xbeef;
>> rc = pwrite(fd, ptr, 4096, 0);
>> assert(rc == 4096);
>> rc = pwrite(fd, ptr, 4096, 4096);
>> assert(rc == 4096);
>>
>> for (block = 2; block < 1000; block++) {
>> rc = fallocate(fd, FALLOC_FL_INSERT_RANGE, 4096, 4096);
>> assert(rc == 0);
>>
>> for (i = 0; i < 2048; i++)
>> *((unsigned short *)ptr + i) = block;
>>
>> rc = pwrite(fd, ptr, 4096, 4096);
>> assert(rc == 4096);
>> }
>>
>> Because start block is not included in the range the hole appears at
>> the wrong offset (just after the desired offset) and the following
>> pwrite() overwrites already existent block, keeping hole untouched.
>>
>> Simple way to verify wrong behaviour is to check zeroed blocks after
>> the test:
>>
>> $ hexdump ./ext4.file | grep '0000 0000'
>>
>> The root cause of the bug is a wrong range (start, stop], where start
>> should be inclusive, i.e. [start, stop].
>>
>> This patch fixes the problem by including start into the range. But
>> not to break left shift (range collapse) stop points to the beginning
>> of the a block, not to the end.
>>
>> The other not obvious change is an iterator check on validness in a
>> main loop. Because iterator is unsigned the following corner case
>> should be considered with care: insert a block at 0 offset, when stop
>> variables overflows and never becomes less than start, which is 0.
>> To handle this special case iterator is set to NULL to indicate that
>> end of the loop is reached.
>>
>> Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
>
> Thanks, applied.
>

Could you please provide with the SHA1 of the patch in your branch?
I want to make an exact reference in a new test of the xfstest which
covers that bug.

--
Roman