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

[Jeff Cody <jcody@redhat.com>]

On Tue, Sep 29, 2015 at 04:54:10PM +0100, Richard W.M. Jones wrote:
>   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
> 
>

Reviewed-by: Jeff Cody <jcody@redhat.com>