[Qemu-devel] [PATCH] block/raw-posix: Open file descriptor O_RDWR to work around glibc posix_fallocate emulation issue.

["Richard W.M. Jones" <rjones@redhat.com>]

  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
---
 block/raw-posix.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index 30df8ad..86f8562 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -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;
-- 
2.5.0