plucky (5) proc.5.gz

NAME

       proc - process information, system information, and sysctl pseudo-filesystem

DESCRIPTION

       The  proc filesystem is a pseudo-filesystem which provides an interface to kernel data structures.  It is
       commonly mounted at /proc.  Typically, it is mounted automatically by the system,  but  it  can  also  be
       mounted manually using a command such as:

           mount -t proc proc /proc

       Most  of  the  files  in  the proc filesystem are read-only, but some files are writable, allowing kernel
       variables to be changed.

   Mount options
       The proc filesystem supports the following mount options:

       hidepid=n (since Linux 3.3)
              This option controls who can access the information in /proc/pid directories.  The argument, n, is
              one of the following values:

              0   Everybody  may  access  all  /proc/pid directories.  This is the traditional behavior, and the
                  default if this mount option is not specified.

              1   Users may not access files and subdirectories inside any /proc/pid directories but  their  own
                  (the   /proc/pid   directories   themselves   remain   visible).    Sensitive  files  such  as
                  /proc/pid/cmdline and /proc/pid/status are now protected against other users.  This  makes  it
                  impossible  to  learn  whether  any user is running a specific program (so long as the program
                  doesn't otherwise reveal itself by its behavior).

              2   As for mode 1, but in addition the /proc/pid  directories  belonging  to  other  users  become
                  invisible.   This  means  that /proc/pid entries can no longer be used to discover the PIDs on
                  the system.  This doesn't hide the fact that a process with a specific PID  value  exists  (it
                  can  be  learned by other means, for example, by "kill -0 $PID"), but it hides a process's UID
                  and GID, which could otherwise be learned by employing stat(2) on a /proc/pid directory.  This
                  greatly complicates an attacker's task of gathering information about running processes (e.g.,
                  discovering whether some daemon is running with elevated privileges, whether another  user  is
                  running  some  sensitive  program,  whether other users are running any program at all, and so
                  on).

              gid=gid (since Linux 3.3)
                  Specifies the ID of a  group  whose  members  are  authorized  to  learn  process  information
                  otherwise  prohibited by hidepid (i.e., users in this group behave as though /proc was mounted
                  with hidepid=0).  This group should be used instead of  approaches  such  as  putting  nonroot
                  users into the sudoers(5) file.

       subset=pid (since Linux 5.8)
              Show only the specified subset of procfs, hiding all top level files and directories in the procfs
              that are not related to tasks.

   Overview
       Underneath /proc, there are the following general groups of files and subdirectories:

       /proc/pid subdirectories
              Each one of these subdirectories contains files and subdirectories exposing information about  the
              process with the corresponding process ID.

              Underneath  each  of the /proc/pid directories, a task subdirectory contains subdirectories of the
              form task/tid, which contain corresponding information about each of the threads in  the  process,
              where tid is the kernel thread ID of the thread.

              The  /proc/pid  subdirectories are visible when iterating through /proc with getdents(2) (and thus
              are visible when one uses ls(1) to view the contents of /proc).

       /proc/tid subdirectories
              Each one of these subdirectories contains files and subdirectories exposing information about  the
              thread  with  the  corresponding thread ID.  The contents of these directories are the same as the
              corresponding /proc/pid/task/tid directories.

              The /proc/tid subdirectories are not visible when iterating through /proc  with  getdents(2)  (and
              thus are not visible when one uses ls(1) to view the contents of /proc).

       /proc/self
              When  a  process  accesses  this  magic  symbolic link, it resolves to the process's own /proc/pid
              directory.

       /proc/thread-self
              When  a  thread  accesses  this  magic  symbolic  link,  it  resolves   to   the   process's   own
              /proc/self/task/tid directory.

       /proc/[a-z]*
              Various other files and subdirectories under /proc expose system-wide information.

       All of the above are described in more detail in separate manpages whose names start with proc_.

NOTES

       Many files contain strings (e.g., the environment and command line) that are in the internal format, with
       subfields terminated by null bytes ('\0').  When inspecting such files, you may find that the results are
       more readable if you use a command of the following form to display them:

           $ cat file | tr '\000' '\n'

SEE ALSO

       cat(1),  dmesg(1),  find(1),  free(1),  htop(1),  init(1), ps(1), pstree(1), tr(1), uptime(1), chroot(2),
       mmap(2),  readlink(2),  syslog(2),  slabinfo(5),  sysfs(5),  hier(7),  namespaces(7),  time(7),   arp(8),
       hdparm(8), ifconfig(8), lsmod(8), lspci(8), mount(8), netstat(8), procinfo(8), route(8), sysctl(8)

       The         Linux         kernel         source         files:        Documentation/filesystems/proc.rst,
       Documentation/admin-guide/sysctl/fs.rst,                     Documentation/admin-guide/sysctl/kernel.rst,
       Documentation/admin-guide/sysctl/net.rst, and Documentation/admin-guide/sysctl/vm.rst.