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

From: Jeff Layton
Date: Wed Aug 28 2019 - 11:55:51 EST

On Mon, 2019-08-26 at 19:50 +0000, sbaugh@xxxxxxxxxx wrote:
> Aleksa Sarai <cyphar@xxxxxxxxxx> writes:
> > 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.
> I don't like this usage of a structure in memory to pass arguments that
> would fit in registers. This would be quite inconvenient for me as a
> userspace developer.
> Others have brought up issues with this: the issue of seccomp, and the
> issue of mismatch between the userspace interface and the kernel
> interface, are the most important for me. I want to add another,
> admittedly somewhat niche, concern.
> This interfaces requires a program to allocate memory (even on the
> stack) just to pass arguments to the kernel which could be passed
> without allocating that memory. That makes it more difficult and less
> efficient to use this syscall in any case where memory is not so easily
> allocatable: such as early program startup or assembly, where the stack
> may be limited in size or not even available yet, or when injecting a
> syscall while ptracing.
> A struct-passing interface was needed for clone, since we ran out of
> registers; but we have not run out of registers yet for openat, so it
> would be nice to avoid this if we can. We can always expand later...

We can't really expand later like you suggest.

Suppose in a couple of years that we need to add some new argument to
openat2 that isn't just a new flag. If all these values are passed by
individual arguments, you can't add one later without adding yet another

Using a struct for this allows this to be extended later, OTOH. You can
extend it, and add a flag that tells the kernel that it can access the
new field. No new syscall required.
Jeff Layton <jlayton@xxxxxxxxxx>