Re: IOSQE_BUFFER_SELECT buffer returned even in case of failure?

[Olivier Langlois <olivier@trillion01.com>]

On Wed, 2021-06-09 at 15:19 -0400, Olivier Langlois wrote:
> the man page says the following:
> 
>     If succesful, the resulting CQE will have IORING_CQE_F_BUFFER set
> in the flags part of the struct, and the upper
> IORING_CQE_BUFFER_SHIFT
> bits will contain the ID of the selected buffers.
> 
> based on my understanding of the kernel code (it could wrong. testing
> is still ongoing to be 100% sure), the buffer will be returned even
> if
> the underlying syscall fails. (I have only checked io_read() for
> IORING_OP_READ).
> 
> At the minimum, the man page should be clarified to better reflect
> the
> code behavior. (and there is a missing 's' in succesful)
> 
> ideally, imho, I believe the code should be modified to do what the
> man
> page says because:
> 
> 1. doing otherwise is counter-intuitive and error-prone (I cannot
> think
> of a single example of a syscall failing and still require the user
> to
> free the allocated resources)
> 
> 2. it is inefficient because the buffer is unneeded since there is no
> data to transfer back to the user and the buffer will need to be
> returned back to io_uring to avoid a leak.
> 
I have confirmed that a buffer was returned and needed to be provided
back io_uring even when read() syscall was failing.

It failed with ECONNRESET with obviously no data and I still had to
return back the buffer to avoid a leak (which my app was suffering of).

Unless someone says that it works as designed, I'll figure out a way to
fix that with a patch. It should be trivial to do.

Greetings,
Olivier