Re: [Xen-devel] [PATCH v2] libxl: don't segfault when creating domain with invalid pvusb device

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Feb 08, 2017 at 08:04:04AM +0100, Juergen Gross wrote:
> Creating a domain with an invalid controller specification for a pvusb
> device will currently segfault.
> 
> Avoid this by bailing out early in case of a mandatory xenstore path
> not existing.
> 
> Signed-of-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> This patch is a backport candidate for 4.8
> 
> V2: introduce libxl__xs_read_mandatory() as suggested by Ian Jackson
> ---
>  tools/libxl/libxl_internal.h |  6 ++++++
>  tools/libxl/libxl_usb.c      |  6 +++---
>  tools/libxl/libxl_xshelp.c   | 12 ++++++++++++
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index 5f46578..d591b79 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -738,6 +738,12 @@ int libxl__xs_mknod(libxl__gc *gc, xs_transaction_t t,
>  
>  /* On success, *result_out came from the gc.
>   * On error, *result_out is undefined.
> + */

Please spell out the distinction between this function and the _checked
variant: This doesn't tolerate ENOENT.

Other than this, the code looks good.

> +int libxl__xs_read_mandatory(libxl__gc *gc, xs_transaction_t t,
> +                             const char *path, const char **result_out);
> +
> +/* On success, *result_out came from the gc.
> + * On error, *result_out is undefined.
>   * ENOENT counts as success but sets *result_out=0
>   */
>  int libxl__xs_read_checked(libxl__gc *gc, xs_transaction_t t,
> diff --git a/tools/libxl/libxl_usb.c b/tools/libxl/libxl_usb.c
> index ea7a2ab..d8948d5 100644
> --- a/tools/libxl/libxl_usb.c
> +++ b/tools/libxl/libxl_usb.c
> @@ -652,9 +652,9 @@ int libxl_device_usbctrl_getinfo(libxl_ctx *ctx, uint32_t 
> domid,
>      usbctrlinfo->devid = usbctrl->devid;
>  
>  #define READ_SUBPATH(path, subpath) ({                                  \
> -        rc = libxl__xs_read_checked(gc, XBT_NULL,                       \
> -                                    GCSPRINTF("%s/" subpath, path),     \
> -                                    &tmp);                              \
> +        rc = libxl__xs_read_mandatory(gc, XBT_NULL,                     \
> +                                      GCSPRINTF("%s/" subpath, path),   \
> +                                      &tmp);                            \
>          if (rc) goto out;                                               \
>          (char *)tmp;                                                    \
>      })
> diff --git a/tools/libxl/libxl_xshelp.c b/tools/libxl/libxl_xshelp.c
> index a50805f..c4a18df 100644
> --- a/tools/libxl/libxl_xshelp.c
> +++ b/tools/libxl/libxl_xshelp.c
> @@ -193,6 +193,18 @@ char *libxl__xs_libxl_path(libxl__gc *gc, uint32_t domid)
>      return s;
>  }
>  
> +int libxl__xs_read_mandatory(libxl__gc *gc, xs_transaction_t t,
> +                             const char *path, const char **result_out)
> +{
> +    char *result = libxl__xs_read(gc, t, path);
> +    if (!result) {
> +        LOGE(ERROR, "xenstore read failed: `%s'", path);
> +        return ERROR_FAIL;
> +    }
> +    *result_out = result;
> +    return 0;
> +}
> +
>  int libxl__xs_read_checked(libxl__gc *gc, xs_transaction_t t,
>                             const char *path, const char **result_out)
>  {
> -- 
> 2.10.2
> 

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