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Date:   Fri, 2 Jul 2021 16:54:03 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     adobriyan@gmail.com, avagin@gmail.com, bernd.edlinger@hotmail.de,
        christian.koenig@amd.com, corbet@lwn.net, deller@gmx.de,
        ebiederm@xmission.com, gladkov.alexey@gmail.com, hridya@google.com,
        jamorris@linux.microsoft.com, jannh@google.com, jeffv@google.com,
        kaleshsingh@google.com, keescook@chromium.org, linux-mm@kvack.org,
        mchehab+huawei@kernel.org, mhocko@suse.com, minchan@kernel.org,
        mm-commits@vger.kernel.org, rdunlap@infradead.org,
        surenb@google.com, szabolcs.nagy@arm.com,
        torvalds@linux-foundation.org, viro@zeniv.linux.org.uk,
        walken@google.com, willy@infradead.org
Subject: Re: [patch 142/192] procfs: allow reading fdinfo with
 PTRACE_MODE_READ
Message-ID: <20210702145403.asuwl5mmvyxhzs5o@wittgenstein>
References: <20210630184624.9ca1937310b0dd5ce66b30e7@linux-foundation.org>
 <20210701015444.ZOZaFPX0b%akpm@linux-foundation.org>
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On Wed, Jun 30, 2021 at 06:54:44PM -0700, Andrew Morton wrote:
> From: Kalesh Singh <kaleshsingh@google.com>
> Subject: procfs: allow reading fdinfo with PTRACE_MODE_READ
> 
> 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
> 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.
> 
> Link: https://lkml.kernel.org/r/20210308170651.919148-1-kaleshsingh@google.com
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> Suggested-by: Jann Horn <jannh@google.com>
> Acked-by: Christian König <christian.koenig@amd.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: Alexey Gladkov <gladkov.alexey@gmail.com>
> Cc: Andrei Vagin <avagin@gmail.com>
> Cc: Bernd Edlinger <bernd.edlinger@hotmail.de>
> Cc: Christian Brauner <christian.brauner@ubuntu.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Helge Deller <deller@gmx.de>
> Cc: Hridya Valsaraju <hridya@google.com>
> Cc: James Morris <jamorris@linux.microsoft.com>
> Cc: Jeff Vander Stoep <jeffv@google.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Michel Lespinasse <walken@google.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---

Rather useful (also for CRIU and others).
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>