Commit 73ba05d9 authored by Richard W.M. Jones's avatar Richard W.M. Jones Committed by Kevin Wolf
Browse files

block/raw-posix: Open file descriptor O_RDWR to work around glibc posix_fallocate emulation issue. 

  https://bugzilla.redhat.com/show_bug.cgi?id=1265196

The following command fails on an NFS mountpoint:

  $ qemu-img create -f qcow2 -o preallocation=falloc disk.img 262144
  Formatting 'disk.img', fmt=qcow2 size=262144 encryption=off cluster_size=65536 preallocation='falloc' lazy_refcounts=off
  qemu-img: disk.img: Could not preallocate data for the new file: Bad file descriptor

The reason turns out to be because NFS doesn't support the
posix_fallocate call.  glibc emulates it instead.  However glibc's
emulation involves using the pread(2) syscall.  The pread syscall
fails with EBADF if the file descriptor is opened without the read
open-flag (ie. open (..., O_WRONLY)).

I contacted glibc upstream about this, and their response is here:

  https://bugzilla.redhat.com/show_bug.cgi?id=1265196#c9



There are two possible fixes: Use Linux fallocate directly, or (this
fix) work around the problem in qemu by opening the file with O_RDWR
instead of O_WRONLY.

Signed-off-by: default avatarRichard W.M. Jones <rjones@redhat.com>
BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1265196


Reviewed-by: default avatarJeff Cody <jcody@redhat.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent 99b7e775

block/raw-posix.c

+1 −1
Original line number Diff line number Diff line
@@ -1648,7 +1648,7 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp) 
        goto out;

    }



    fd = qemu_open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY,

    fd = qemu_open(filename, O_RDWR | O_CREAT | O_TRUNC | O_BINARY,

                   0644);

    if (fd < 0) {

        result = -errno;