Re: [Qemu-devel] [PATCH 3/4] raw-posix: Introduce hdev_is_sg()

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/05/2015 13:38, Dimitris Aragiorgis wrote:
> Until now, an SG device was identified only by checking if its path
> started with "/dev/sg". Then, hdev_open() set bs->sg accordingly.
> This is very fragile, e.g. it fails with symlinks or relative paths.
> We should rely on the actual properties of the device instead of the
> specified file path.
> 
> Test for an SG device (e.g. /dev/sg0) by ensuring that all of the
> following holds:
> 
>  - The device supports the SG_GET_VERSION_NUM ioctl
>  - The device supports the SG_GET_SCSI_ID ioctl
>  - The specified file name corresponds to a character device
> 
> Signed-off-by: Dimitris Aragiorgis <dimara@arrikto.com>
> ---
>  block/raw-posix.c |   39 ++++++++++++++++++++++++++++-----------
>  1 file changed, 28 insertions(+), 11 deletions(-)
> 
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index 24b061f..d35e5ac 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -57,6 +57,7 @@
>  #include <linux/fd.h>
>  #include <linux/fs.h>
>  #include <linux/hdreg.h>
> +#include <scsi/sg.h>
>  #ifdef __s390__
>  #include <asm/dasd.h>
>  #endif
> @@ -2073,15 +2074,38 @@ static void hdev_parse_filename(const char *filename, QDict *options,
>      qdict_put_obj(options, "filename", QOBJECT(qstring_from_str(filename)));
>  }
>  
> +static int hdev_is_sg(BlockDriverState *bs)
> +{
> +
> +#if defined(__linux__)
> +
> +    struct stat st;
> +    struct sg_scsi_id scsiid;
> +    int sg_version;
> +
> +    if (!bdrv_ioctl(bs, SG_GET_VERSION_NUM, &sg_version) &&
> +        !bdrv_ioctl(bs, SG_GET_SCSI_ID, &scsiid) &&
> +        stat(bs->filename, &st) >= 0 && S_ISCHR(st.st_mode)) {
> +        DEBUG_BLOCK_PRINT("SG device found: type=%d, version=%d\n",
> +            scsiid.scsi_type, sg_version);

Please replace the DEBUG_BLOCK_PRINT with a tracepoint (see the
trace-events file).  With this change, the patch is okay and definitely
an improvement.

Paolo

> +        return 1;
> +    }
> +
> +#endif
> +
> +    return 0;
> +}
> +
>  static int hdev_open(BlockDriverState *bs, QDict *options, int flags,
>                       Error **errp)
>  {
>      BDRVRawState *s = bs->opaque;
>      Error *local_err = NULL;
>      int ret;
> -    const char *filename = qdict_get_str(options, "filename");
>  
>  #if defined(__APPLE__) && defined(__MACH__)
> +    const char *filename = qdict_get_str(options, "filename");
> +
>      if (strstart(filename, "/dev/cdrom", NULL)) {
>          kern_return_t kernResult;
>          io_iterator_t mediaIterator;
> @@ -2110,16 +2134,6 @@ static int hdev_open(BlockDriverState *bs, QDict *options, int flags,
>  #endif
>  
>      s->type = FTYPE_FILE;
> -#if defined(__linux__)
> -    {
> -        char resolved_path[ MAXPATHLEN ], *temp;
> -
> -        temp = realpath(filename, resolved_path);
> -        if (temp && strstart(temp, "/dev/sg", NULL)) {
> -            bs->sg = 1;
> -        }
> -    }
> -#endif
>  
>      ret = raw_open_common(bs, options, flags, 0, &local_err);
>      if (ret < 0) {
> @@ -2129,6 +2143,9 @@ static int hdev_open(BlockDriverState *bs, QDict *options, int flags,
>          return ret;
>      }
>  
> +    /* Since this does ioctl the device must be already opened */
> +    bs->sg = hdev_is_sg(bs);
> +
>      if (flags & BDRV_O_RDWR) {
>          ret = check_hdev_writable(s);
>          if (ret < 0) {
>