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Date:   Fri, 28 May 2021 17:33:50 -0700
From:   Andres Freund <andres@anarazel.de>
To:     Pavel Begunkov <asml.silence@gmail.com>,
        Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Memory uninitialized after "io_uring: keep table of pointers to
 ubufs"
Message-ID: <20210529003350.m3bqhb3rnug7yby7@alap3.anarazel.de>
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Hi,

I started to see buffer registration randomly failing with ENOMEM on
5.13. Registering buffer or two often succeeds, but more than that
rarely. Running the same program as root succeeds - but the user has a high
rlimit.

The issue is that io_sqe_buffer_register() doesn't initialize
imu. io_buffer_account_pin() does imu->acct_pages++, before calling
io_account_mem(ctx, imu->acct_pages);

Which means that a random amount of memory is being accounted for. On the first
few allocations this sometimes fails to fail because the memory is zero, but
after a bit of reuse...

It only doesn't fail as root because the rlimit doesn't apply.

This is caused by

commit 41edf1a5ec967bf4bddedb83c48e02dfea8315b4
Author: Pavel Begunkov <asml.silence@gmail.com>
Date:   2021-04-25 14:32:23 +0100

    io_uring: keep table of pointers to ubufs

Greetings,

Andres Freund