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

[Michal Hocko]

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@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  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