On 11/19/2015 05:30 PM, Jan Kara wrote:
On Tue 17-11-15 14:52:19, Avi Kivity wrote:
On 11/16/2015 09:27 PM, Jeff Moyer wrote:
Hi Avi,
Avi Kivity <avi@xxxxxxxxxxxx> writes:
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?
Your expectations are correct. The bug was introduced by commit
9fe55eea7e4b4 (Fix race when checking i_size on direct i/o read). I've
CC'd the patch author and linux-fsdevel. I'm not sure what the right
fix is, given that the size checks were removed from the vfs to fix some
race condition. Unfortunately, the commit message doesn't really do a
good job of explaining the race. In order to save others time, here is
a good explanation of the problem that commit is meant to fix, along
with a reproducer:
http://marc.info/?l=linux-fsdevel&m=138641356614458&w=2
Thanks for the great bug report, and sorry I have no solution to
proffer.
Thanks. I will await a fix with interest.
Can you please post the reproduce here as well? I couldn't easily find it
with google.
Attached.
I am told that a simple synchronous O_DIRECT read beyond unaligned eof
suffices as well.
#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;
}
