linux-kernel - Re: [PATCH v2] Document /proc/pid PID reuse behavior

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Message-ID: <20181106130524.GC2453@dhcp22.suse.cz>
Date:   Tue, 6 Nov 2018 14:05:24 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     linux-kernel@...r.kernel.org, rppt@...ux.ibm.com,
        timmurray@...gle.com, joelaf@...gle.com, surenb@...gle.com,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior

On Mon 05-11-18 13:22:05, Daniel Colascione wrote:
> State explicitly that holding a /proc/pid file descriptor open does
> not reserve the PID. Also note that in the event of PID reuse, these
> open file descriptors refer to the old, now-dead process, and not the
> new one that happens to be named the same numeric PID.

This sounds quite obvious otherwise anybody could simply DoS the system
by consuming all available pids. But I do not see any strong reason
against having that stated explicitly.

> Signed-off-by: Daniel Colascione <dancol@...gle.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  Documentation/filesystems/proc.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> Moved paragraphed to start of /proc/pid section; added signed-off-by.
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 12a5e6e693b6..0b14460f721d 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -125,6 +125,13 @@ process running on the system, which is named after the process ID (PID).
>  The link  self  points  to  the  process reading the file system. Each process
>  subdirectory has the entries listed in Table 1-1.
>  
> +Note that an open a file descriptor to /proc/<pid> or to any of its
> +contained files or subdirectories does not prevent <pid> being reused
> +for some other process in the event that <pid> exits. Operations on
> +open /proc/<pid> file descriptors corresponding to dead processes
> +never act on any new process that the kernel may, through chance, have
> +also assigned the process ID <pid>. Instead, operations on these FDs
> +usually fail with ESRCH.
>  
>  Table 1-1: Process specific entries in /proc
>  ..............................................................................
> -- 
> 2.19.1.930.g4563a0d9d0-goog

-- 
Michal Hocko
SUSE Labs