Re: [BUG io_uring] Failed RECVSEND_BUNDLE can persistently shrink non-INC pbuf ring len and affect later READ operations

[Jens Axboe]

On 6/7/26 3:22 PM, Nyakundi Emmanuel wrote:
> On Sun, 7 Jun 2026, Federico Brasili wrote:
>> I found a reproducible io_uring provided-buffer ring issue on Ubuntu
>> kernel 7.0.0-22-generic.
>>
>> A failed IORING_RECVSEND_BUNDLE receive on a non-INC provided-buffer
>> ring can persistently shrink the user-visible buffer descriptor length.
> 
> Confirmed reproducible on:
> 
>   Linux archlinux 7.0.11-arch1-1 #1 SMP PREEMPT_DYNAMIC
>   Tue, 02 Jun 2026 18:26:58 +0000 x86_64
>   Arch Linux (rolling)
> 
> Output from your reproducer, run unprivileged:
> 
>   [INIT] entry0 len=4096 bid=0 entry1 len=4096 bid=1 tail=2
>   [STEP1] poison empty socket: BUNDLE len=1 expect -EAGAIN but entry0 len may truncate
>   [CQE1] res=-11 flags=0x0 user=0x1111
>   [AFTER1] entry0 len=1 entry1 len=4096 tail=2 changed_buf0=0 changed_buf1=0 guard_before=0 guard_after=0
>   [STEP2] wrote pipe bytes=4096, now IORING_OP_READ len=4096 after recv-BUNDLE poisoning
>   [CQE_READ] res=1 flags=0x1 user=0x6666
>   [AFTER_READ] entry0 len=1 entry1 len=4096 tail=2 changed_buf0=1 changed_buf1=0 guard_before=0 guard_after=0
>   [STEP3] wrote second pipe chunk bytes=4096, second IORING_OP_READ len=4096 without republish
>   [CQE_READ2] res=4096 flags=0x10001 user=0x7777
>   [AFTER_READ2] entry0 len=1 entry1 len=4096 tail=2 changed_buf0=1 changed_buf1=4096 guard_before=0 guard_after=0
> 
> entry0.len persistently corrupted 4096 -> 1 after -EAGAIN RECV_BUNDLE.
> Subsequent IORING_OP_READ consumed the poisoned length as reported.
> 
> This confirms the issue is not Ubuntu-specific and reproduces on a
> stock upstream-tracking kernel.

Which is entirely expected, it's just a generic kernel bug and I doubt
that ubuntu is shipping any specific patches here that aren't already
in stable or upstream.

-- 
Jens Axboe