AIO read returns negative number for bytes read

[Avi Kivity <avi@scylladb.com>]

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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?

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#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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