Re: [PATCH v2] ptrace: Clarify PTRACE_GETREGSET/PTRACE_SETREGSET, documentation in uapi header

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

On 05/14/2014 04:24 PM, Pedro Alves wrote:
> On 05/14/14 08:10, Anshuman Khandual wrote:
>> On 05/13/2014 11:39 PM, Pedro Alves wrote:
>>> On 05/05/14 05:10, Anshuman Khandual wrote:
>>>> On 05/01/2014 07:43 PM, Pedro Alves wrote:
>>> OK, then this is what I suggest instead:
> ...
>>>> Shall I resend the patch with the your proposed changes and your "Signed-off-by" and
>>>> moving myself as "Reported-by" ?
>>>
>>> No idea of the actual policy to follow.  Feel free to do that if that's the
>>> standard procedure.
>>
>> Even I am not sure about this, so to preserve the correct authorship, would you
>> mind sending this patch ?
> 
> Here you go.  This is against current Linus'.  Please take it from
> here if necessary.
> 
> 8<------------------------------------------
> From 1237f5ac5896f3910f66df83a5093bb548006188 Mon Sep 17 00:00:00 2001
> From: Pedro Alves <palves@redhat.com>
> Date: Wed, 14 May 2014 11:05:07 +0100
> Subject: [PATCH] ptrace: Clarify PTRACE_GETREGSET/PTRACE_SETREGSET
>  documentation in uapi header
> 
> The current comments don't explicitly state in plain words that
> iov.len must be set to the buffer's length prior to the ptrace call.
> A user might get confused and leave that uninitialized.
> 
> In the ptrace_regset function (snippet below) we see that the buffer
> length has to be a multiple of the slot/register size for the given
> NT_XXX_TYPE:
> 
>         if (!regset || (kiov->iov_len % regset->size) != 0)
>                 return -EINVAL;
> 
> Note regset->size is the size of each slot/register in the set, not
> the size of the whole set.
> 
> And then, we see here:
> 
>  kiov->iov_len = min(kiov->iov_len,
>                             (__kernel_size_t) (regset->n * regset->size));
> 
> that the kernel takes care of capping the requested length to the size
> of the whole regset.
> 
> Signed-off-by: Pedro Alves <palves@redhat.com>
> Reported-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
> ---
>  include/uapi/linux/ptrace.h | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index cf1019e..30836b9 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -39,12 +39,17 @@
>   * payload are exactly the same layout.
>   *
>   * This interface usage is as follows:
> - *	struct iovec iov = { buf, len};
> + *	struct iovec iov = { buf, len };
>   *
>   *	ret = ptrace(PTRACE_GETREGSET/PTRACE_SETREGSET, pid, NT_XXX_TYPE, &iov);
>   *
> - * On the successful completion, iov.len will be updated by the kernel,
> - * specifying how much the kernel has written/read to/from the user's iov.buf.
> + * On entry, iov describes the buffer's address and length.  The buffer's length
> + * must be a multiple of the size of a single register in the register set.  The
> + * kernel never reads or writes more than iov.len, and caps the buffer length to
> + * the register set's size.  In other words, the kernel reads or writes
> + * min(iov.len, regset size).  On successful completion, iov.len is updated by
> + * the kernel, specifying how much the kernel has read from / written to the
> + * user's iov.buf.
>   */
>  #define PTRACE_GETREGSET	0x4204
>  #define PTRACE_SETREGSET	0x4205

Hey Peter/Oleg,

The above patch is a documentation fix which we discussed sometime back. Could you please
kindly review and consider merging. Thank you.