On Sun, Nov 15, 2015 at 03:08:13PM +0200, Avi Kivity wrote:
> Due to a bug in my program, I initiated a read beyond eof. Specifically, the
> file size is 13002 bytes and the read offset is 13312 (0x3400).
>
> I would expect such a read to return 0 bytes read, but io_getevents returns
> -310, which is suspiciously equal to (13002 - 13312).
>
> I attach a reproducer.
>
> 4.2.5-201.fc22.x86_64
>
> Are my expectations incorrect, or is this a bug in aio or xfs?
FWIW, I added some printk()'s and reproduced. This looks like a dio
issue to me (a sync dio read reproduces just the same as aio). See the
short read check in dio_complete():
...
/* Check for short read case */
if ((dio->rw == READ) && ((offset + transferred) > dio->i_size))
transferred = dio->i_size - offset;
...
So if offset starts beyond i_size, we set transferred to a negative
value. I suppose we could check for offset >= i_size here independently,
but I'm not necessarily sure something earlier doesn't need to change
(e.g., I see dio->result = 3072 in this case and I'm not familiar enough
with core dio to know if that's expected).
Brian
> #define _GNU_SOURCE
>
> #include <libaio.h>
> #include <assert.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <sys/stat.h>
> #include <fcntl.h>
>
> int main(int ac, char** av) {
> int fd;
> char* buf;
> io_context_t ioc = NULL;
> int r;
> struct iocb iocb;
> struct iocb *iocbp[1];
> struct io_event ioev;
>
> buf = aligned_alloc(4096, 4096*4);
> assert(buf);
> r = io_setup(1, &ioc);
> assert(r == 0);
> fd = open("tmp.tmp", O_RDWR | O_CREAT | O_DIRECT, 0600);
> assert(fd >= 0);
> io_prep_pwrite(&iocb, fd, buf, 4096*4, 0);
> iocbp[0] = &iocb;
> r = io_submit(ioc, 1, iocbp);
> assert(r == 1);
> r = io_getevents(ioc, 1, 1, &ioev, NULL);
> assert(r == 1);
> assert(ioev.res == 4*4096);
> ftruncate(fd, 13002);
> io_prep_pread(&iocb, fd, buf, 8192, 13312);
> r = io_submit(ioc, 1, iocbp);
> assert(r == 1);
> r = io_getevents(ioc, 1, 1, &ioev, NULL);
> assert(r == 1);
> printf("read result: %d\n", (int)ioev.res);
> return 0;
> }
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