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

Browse Source 
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: Richard W.M. Jones <rjones@redhat.com>
BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1265196
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Richard W.M. Jones 2015-09-29 16:54:10 +01:00 committed by Kevin Wolf
parent 99b7e77567
commit 73ba05d936
1 changed files with 1 additions and 1 deletions
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2
block/raw-posix.c
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@ -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;