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

Theodore Ts'o
2017-01-07T21:22:10Z

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.

- Ted