[FFmpeg-devel] [PATCH] kmsgrab: Drop DRM master if running as root

[Anton Khirnov]

Quoting Mark Thompson (2020-04-12 16:46:25)
> If we have both root and DRM master then drop the latter because we
> don't need both and holding onto it can interfere with running other
> programs.
> 
> From a discussion with deltasquared on IRC.
> ---
> It would be marginally better to check whether we have CAP_SYS_ADMIN rather than just whether we are effectively root, but the capabilities interface is a lot more complex to query and this is already sufficient to cover the reported case.
> 
>  libavdevice/kmsgrab.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/libavdevice/kmsgrab.c b/libavdevice/kmsgrab.c
> index d0de774871..53280da0f5 100644
> --- a/libavdevice/kmsgrab.c
> +++ b/libavdevice/kmsgrab.c
> @@ -263,6 +263,13 @@ static av_cold int kmsgrab_read_header(AVFormatContext *avctx)
>      ctx->device = (AVHWDeviceContext*) ctx->device_ref->data;
>      ctx->hwctx  = (AVDRMDeviceContext*)ctx->device->hwctx;
>  
> +    if (geteuid() == 0 && drmIsMaster(ctx->hwctx->fd)) {
> +        // If we have both root and DRM master then drop the latter
> +        // because we don't need both and holding onto it can interfere
> +        // with running other programs.
> +        drmDropMaster(ctx->hwctx->fd);
> +    }

Any idea how would this interact with user namespaces?

-- 
Anton Khirnov