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Subject: Re: [PATCH v4 1/2] procfs: Allow reading fdinfo with PTRACE_MODE_READ
To:     Kalesh Singh <kaleshsingh@google.com>
Cc:     jannh@google.com, jeffv@google.com, keescook@chromium.org,
        surenb@google.com, minchan@kernel.org, hridya@google.com,
        christian.koenig@amd.com, kernel-team@android.com,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Alexey Gladkov <gladkov.alexey@gmail.com>,
        NeilBrown <neilb@suse.de>, Szabolcs Nagy <szabolcs.nagy@arm.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Michel Lespinasse <walken@google.com>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Andrei Vagin <avagin@gmail.com>,
        Yafang Shao <laoar.shao@gmail.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-doc@vger.kernel.org
References: <20210205213353.669122-1-kaleshsingh@google.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <fe8780a1-364e-ec8f-48ee-192438d52c01@infradead.org>
Date:   Fri, 5 Feb 2021 16:39:29 -0800
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On 2/5/21 1:33 PM, Kalesh Singh wrote:
> Android captures per-process system memory state when certain low memory
> events (e.g a foreground app kill) occur, to identify potential memory
> hoggers. In order to measure how much memory a process actually consumes,
> it is necessary to include the DMA buffer sizes for that process in the
> memory accounting. Since the handle to DMA buffers are raw FDs, it is
> important to be able to identify which processes have FD references to
> a DMA buffer.
> 
> Currently, DMA buffer FDs can be accounted using /proc/<pid>/fd/* and
> /proc/<pid>/fdinfo -- both are only readable by the process owner,
> as follows:
>   1. Do a readlink on each FD.
>   2. If the target path begins with "/dmabuf", then the FD is a dmabuf FD.
>   3. stat the file to get the dmabuf inode number.
>   4. Read/ proc/<pid>/fdinfo/<fd>, to get the DMA buffer size.
> 
> Accessing other processes’ fdinfo requires root privileges. This limits

Tangential:
Please just use ASCII "'" -- it's good enough.

> the use of the interface to debugging environments and is not suitable
> for production builds.  Granting root privileges even to a system process
> increases the attack surface and is highly undesirable.
> 
> Since fdinfo doesn't permit reading process memory and manipulating
> process state, allow accessing fdinfo under PTRACE_MODE_READ_FSCRED.
> 
> Suggested-by: Jann Horn <jannh@google.com>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> ---
> Changes in v2:
>   - Update patch description
> 
>  fs/proc/base.c |  4 ++--
>  fs/proc/fd.c   | 15 ++++++++++++++-
>  2 files changed, 16 insertions(+), 3 deletions(-)


-- 
~Randy