Re: [PATCH 3/5] uaccess: add copy_struct_{from,to}_bounce_buffer() helpers

[Stefan Metzmacher]

Hi David,

> On Tue,  7 Apr 2026 18:03:15 +0200
> Stefan Metzmacher <metze@xxxxxxxxx> wrote:
>
> > These are similar to copy_struct_{from,to}_user() but operate
> > on kernel buffers instead of user buffers.
> >
> > They can be used when there is a temporary bounce buffer used,
> > e.g. in msg_control or similar places.
> >
> > It allows us to have the same logic to handle old vs. current
> > and current vs. new structures in the same compatible way.
> >
> > copy_struct_from_sockptr() will also be able to
> > use copy_struct_from_bounce_buffer() for the kernel
> > case as follow us patch.
> >
> > I'll use this in my IPPROTO_SMBDIRECT work,
> > but maybe it will also be useful for others...
> > IPPROTO_QUIC will likely also use it.
> >
> > Cc: Dmitry Safonov <0x7f454c46@xxxxxxxxx>
> > Cc: Dmitry Safonov <dima@xxxxxxxxxx>
> > Cc: Francesco Ruggeri <fruggeri@xxxxxxxxxx>
> > Cc: Salam Noureddine <noureddine@xxxxxxxxxx>
> > Cc: David Ahern <dsahern@xxxxxxxxxx>
> > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > Cc: Michal Luczaj <mhal@xxxxxxx>
> > Cc: David Wei <dw@xxxxxxxxxxx>
> > Cc: Luiz Augusto von Dentz <luiz.von.dentz@xxxxxxxxx>
> > Cc: Luiz Augusto von Dentz <luiz.dentz@xxxxxxxxx>
> > Cc: Marcel Holtmann <marcel@xxxxxxxxxxxx>
> > Cc: Xin Long <lucien.xin@xxxxxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> > Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> > Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
> > Cc: Neal Cardwell <ncardwell@xxxxxxxxxx>
> > Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> > Cc: Simon Horman <horms@xxxxxxxxxx>
> > Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
> > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > CC: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: netdev@xxxxxxxxxxxxxxx
> > Cc: linux-bluetooth@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Stefan Metzmacher <metze@xxxxxxxxx>
> > ---
> >   include/linux/uaccess.h | 63 +++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 63 insertions(+)
> >
> > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> > index 1234b5fa4761..a6cd4f48bb99 100644
> > --- a/include/linux/uaccess.h
> > +++ b/include/linux/uaccess.h
> > @@ -513,6 +513,69 @@ copy_struct_to_user(void __user *dst, size_t usize, const void *src,
> >   	return 0;
> >   }
> >   
> > +static __always_inline void
> > +__copy_struct_generic_bounce_buffer(void *dst, size_t dstsize,
> > +				    const void *src, size_t srcsize,
> > +				    bool *ignored_trailing)
> > +{
> > +	size_t size = min(dstsize, srcsize);
> > +	size_t rest = max(dstsize, srcsize) - size;
> > +
> > +	/* Deal with trailing bytes. */
> > +	if (dstsize > srcsize)
> > +		memset(dst + size, 0, rest);
> > +	if (ignored_trailing)
> > +		*ignored_trailing = dstsize < srcsize &&
> > +			memchr_inv(src + size, 0, rest) != NULL;
> > +	/* Copy the interoperable parts of the struct. */
> > +	memcpy(dst, src, size);
> > +}
>
> Return 'ignored_trailing' rather than pass by reference.

I also thought about that but it makes
the copy_struct_to_ case more complex.

I'm not sure but my guess would be that
the compiler would have the chance to skip the
ignore_trailing logic if (as all current callers do)
NULL is passed.

> And this is probably too big to inline.

In the next patch this replace open coded logic in
copy_struct_from_sockptr. So as all of copy_struct_*
consists of inline functions I thought it would be good to
keep it that way.

So unless there a real good reason to change it
I'd like to keep it as is.

Thanks!
metze