Re: [PATCH 18/41] regset: new method and helpers for it

[Al Viro]

On Tue, Jun 30, 2020 at 09:53:12AM -0700, Linus Torvalds wrote:
> On Tue, Jun 30, 2020 at 6:25 AM Al Viro <v...@zeniv.linux.org.uk> wrote:
> >
> > How about ->regset_get()?
> 
> Sounds good to me. And if you ever do something similar for 'set', you
> have a natural name to pick.

Umm...  Something similar for 'set' would, AFAICS, work reasonably well
with the following primitives.
membuf_read(&from, data, size) -- copy min(size, amount left)
membuf_fetch(&from, data) -- copy min(sizeof(data), amount left) into data
                        (obviously a macro)
membuf_skip(&from, size) -- obvious
membuf_pick(&from, size), along the lines of
        if (unlikely(size > p->left)
                return ERR_PTR(-EINVAL);
        res = p->p;
        p->p += size;
        p->left -= size;
        return res;

So it would be something like

int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
                   struct membuf from)
{
        const struct user_desc *info;
        int n = from.left / sizeof(*info);
        int i;

        info = membuf_pick(&from, n * sizeof(*info)); // can't fail here
        for (i = 0; i < n; i++)
                if (!tls_desc_okay(info + i))
                        return -EINVAL;

        set_tls_desc(target, GDT_ENTRY_TLS_MIN, info, n);
        return 0;
}

and

static int genregs32_set(struct task_struct *target,
                         const struct user_regset *regset,
                         struct membuf from)
{
        int err;
        unsigned pos, v;

        for (pos = 0; from.left; pos += sizeof(u32)) {
                membuf_fetch(&from, v);
                err = putreg32(target, pos, v);
                if (err)
                        return err;
        }
        return 0;
}

or

/*
 * Copy the supplied 64-bit NT_MIPS_DSP buffer to the DSP context.
 */
static int dsp64_set(struct task_struct *target,
                     const struct user_regset *regset,
                     struct membuf from)
{
        if (!cpu_has_dsp)
                return -EIO;

        membuf_read(&from, target->thread.dsp.dspr, NUM_DSP_REGS * sizeof(u64));
        return membuf_read(&from, target->thread.dsp.dspcontrol, sizeof(u64));
}

etc.