Re: [PATCH v7 03/11] drm/i915: return EACCES for check_cmd() failures

[Matthew Auld <matthew.william.auld@gmail.com>]

On 25 October 2016 at 00:19, Robert Bragg <robert@sixbynine.org> wrote:
> check_cmd() is checking whether a command adheres to certain
> restrictions that ensure it's safe to execute within a privileged batch
> buffer. Returning false implies a privilege problem, not that the
> command is invalid.
>
> The distinction makes the difference between allowing the buffer to be
> executed as an unprivileged batch buffer or returning an EINVAL error to
> userspace without executing anything.
>
> In a case where userspace may want to test whether it can successfully
> write to a register that needs privileges the distinction may be
> important and an EINVAL error may be considered fatal.
>
> In particular this is currently true for Mesa, which includes a test for
> whether OACONTROL can be written too, but Mesa treats any error when
> flushing a batch buffer as fatal, calling exit(1).
>
> As it is currently Mesa can gracefully handle a failure to write to
> OACONTROL if the command parser is disabled, but if we were to remove
> OACONTROL from the parser's whitelist then the returned EINVAL would
> break Mesa applications as they attempt an OACONTROL write.
>
> This bumps the command parser version from 7 to 8, as the change is
> visible to userspace.
>
> Signed-off-by: Robert Bragg <robert@sixbynine.org>
Seems reasonable.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
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