[PATCH RESEND v11 7/8] open: openat2(2) syscall

[Daniel Colascione]

On Mon, Aug 19, 2019 at 8:37 PM Aleksa Sarai <cyphar at cyphar.com> wrote:
>
> The most obvious syscall to add support for the new LOOKUP_* scoping
> flags would be openat(2). However, there are a few reasons why this is
> not the best course of action:
>
>  * The new LOOKUP_* flags are intended to be security features, and
>    openat(2) will silently ignore all unknown flags. This means that
>    users would need to avoid foot-gunning themselves constantly when
>    using this interface if it were part of openat(2). This can be fixed
>    by having userspace libraries handle this for users[1], but should be
>    avoided if possible.
>
>  * Resolution scoping feels like a different operation to the existing
>    O_* flags. And since openat(2) has limited flag space, it seems to be
>    quite wasteful to clutter it with 5 flags that are all
>    resolution-related. Arguably O_NOFOLLOW is also a resolution flag but
>    its entire purpose is to error out if you encounter a trailing
>    symlink -- not to scope resolution.
>
>  * Other systems would be able to reimplement this syscall allowing for
>    cross-OS standardisation rather than being hidden amongst O_* flags
>    which may result in it not being used by all the parties that might
>    want to use it (file servers, web servers, container runtimes, etc).
>
>  * It gives us the opportunity to iterate on the O_PATH interface. In
>    particular, the new @how->upgrade_mask field for fd re-opening is
>    only possible because we have a clean slate without needing to re-use
>    the ACC_MODE flag design nor the existing openat(2) @mode semantics.
>
> To this end, we introduce the openat2(2) syscall. It provides all of the
> features of openat(2) through the @how->flags argument, but also
> also provides a new @how->resolve argument which exposes RESOLVE_* flags
> that map to our new LOOKUP_* flags. It also eliminates the long-standing
> ugliness of variadic-open(2) by embedding it in a struct.
>
> In order to allow for userspace to lock down their usage of file
> descriptor re-opening, openat2(2) has the ability for users to disallow
> certain re-opening modes through @how->upgrade_mask. At the moment,
> there is no UPGRADE_NOEXEC. The open_how struct is padded to 64 bytes
> for future extensions (all of the reserved bits must be zeroed).

Why pad the structure when new functionality (perhaps accommodated via
a larger structure) could be signaled by passing a new flag? Adding
reserved fields to a structure with a size embedded in the ABI makes a
lot of sense --- e.g., pthread_mutex_t can't grow. But this structure
can grow, so the reservation seems needless to me.