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

[Stefan Metzmacher <metze@samba.org>]

Hi David,

> On Tue,  7 Apr 2026 18:03:15 +0200
> Stefan Metzmacher <metze@samba.org> 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@gmail.com>
>> Cc: Dmitry Safonov <dima@arista.com>
>> Cc: Francesco Ruggeri <fruggeri@arista.com>
>> Cc: Salam Noureddine <noureddine@arista.com>
>> Cc: David Ahern <dsahern@kernel.org>
>> Cc: David S. Miller <davem@davemloft.net>
>> Cc: Michal Luczaj <mhal@rbox.co>
>> Cc: David Wei <dw@davidwei.uk>
>> Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
>> Cc: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
>> Cc: Marcel Holtmann <marcel@holtmann.org>
>> Cc: Xin Long <lucien.xin@gmail.com>
>> Cc: Eric Dumazet <edumazet@google.com>
>> Cc: Kuniyuki Iwashima <kuniyu@google.com>
>> Cc: Paolo Abeni <pabeni@redhat.com>
>> Cc: Willem de Bruijn <willemb@google.com>
>> Cc: Neal Cardwell <ncardwell@google.com>
>> Cc: Jakub Kicinski <kuba@kernel.org>
>> Cc: Simon Horman <horms@kernel.org>
>> Cc: Aleksa Sarai <cyphar@cyphar.com>
>> Cc: Christian Brauner <brauner@kernel.org>
>> CC: Kees Cook <keescook@chromium.org>
>> Cc: netdev@vger.kernel.org
>> Cc: linux-bluetooth@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Stefan Metzmacher <metze@samba.org>
>> ---
>>   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