[v3,0/3] Add pidfd_spawn, pidfd_spawnp, pidfd_fork, and pidfd_getpid

The glibc 2.36 added wrappers for Linux syscall pidfd_open,
pidfd_getfd, and pidfd_send_signal, and exported the P_PIDFD to use
along with waitid.  However, although the pidfd is a race free
interface, the pidfd_open is subject to TOCTOU if the file descriptor
is not obtained directly from the clone or clone3 syscall (there is
still a small window between the clone return and the pidfd_getfd
where the process can be reaped and the process ID reused).

A fully race free interface with posix_spawn interface is being
discussed by GNOME [1] [2], and Qt already uses on its QtProcess
implementation [3].  The Qt implementation has some pitfalls:

  - It calls clone through the syscall symbol, which does not run the
    pthread_atfork handlers even though it really intends to use the
    clone semantic for fork (by only using CLONE_PIDFD | SIGCHLD).

  - It also does not reset any internal state, such as internal IO,
    malloc, loader, etc. locks.

  - It does not set the TCB tid field nor the robust list, used by
    pthread code.

  - It does not optimize process creation by using CLONE_VM and
    CLONE_VFORK.

This patchset adds new interfaces that take care of this potential
issues.  The pidfd_spawn and pidfd_spawnp handles all these cases by
using the same internal implementation used by posix_spawn:

  int pidfd_spawn (int *restrict pidfd,
 		   const char *restrict file,
  		   const posix_spawn_file_actions_t *restrict facts,
  		   const posix_spawnattr_t *restrict attrp,
  		   char *const argv[restrict],
  		   char *const envp[restrict])

  int pidfd_spawnp (int *restrict pidfd,
 		    const char *restrict path,
  		    const posix_spawn_file_actions_t *restrict facts,
  		    const posix_spawnattr_t *restrict attrp,
  		    char *const argv[restrict_arr],
  		    char *const envp[restrict_arr]);

The implementation makes sure that kernel must support the complete
pidfd interface, meaning that waitid (P_PIDFD) should be supported.  It
ensure that non racy workaround is required (such as reading procfs
fdinfo pid to use along with old wait interfaces).  If kernel does not
have the required support the interface returns ENOSYS.

A new symbol is used instead of a posix_spawn extension to avoid
possible issue with language bindings that might track the argument
lifetime.

Both symbols reuse the posix_spawn posix_spawn_file_actions_t and
posix_spawnattr_t, to either avoid rehash posix_spawn API or add a new
one.  It also mean that both interfaces support the same attribute and
file actions, and a new flag or file actions on posix_spawn is also
added automatically for pidfd_spawn.

Along with the spawn interface, a fork like one is also provided:

  int pidfd_fork (unsigned int flags)

The kernel already sets O_CLOEXEC as default and it follow fork/_Fork
convention on returning a positive or negative value to the parent
(with negative indicating an error) and zero to the child.

Similar to fork, pidfd_fork also run the handler setup by pthread_atfork
and reset some internal libc state.  It can be changed by using the
PIDFDFORK_ASYNCSAFE, where pidfd_fork acts as _Fork.

To have a way to interop between process IDs and process file
descriptors, the pidfd_getpid is also provided.  It just read the procps
fdinfo entry from the file descriptor to get the process ID.

--

Changes from v2:
- Added pidfd_fork and pidfd_getpid manual entries
- Change pidfd_fork to act as fork as default, instead as _Fork.
- Changed PIDFD_FORK_RUNATFORK flag to PIDFDFORK_ASYNCSAFE.
- Added pidfd_getpid test for EREMOTE.

Changes from v1:
- Extended pidfd_getpid error codes to return EBADF if fdinfo does not
  have Pid entry or if the value is invalid, EREMOTE is pid is in a 
  separate namespace, and ESRCH if is already terminated.
- Extended tst-pidfd_getpid.
- Rename PIDFD_FORK_RUNATFORK to PIDFDFORK_RUNATFORK to avoid clash
  with possible kernel extensions.

[1] https://gitlab.gnome.org/GNOME/glib/-/issues/1866
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=30349
[3]
https://codebrowser.dev/qt6/qtbase/src/3rdparty/forkfd/forkfd_linux.c.html

Adhemerval Zanella (3):
  posix: Add pidfd_spawn and pidfd_spawnp (BZ# 30349)
  posix: Add pidfd_fork
  linux: Add pidfd_getpid

 NEWS                                          |  16 ++
 bits/spawn_ext.h                              |  21 +++
 include/clone_internal.h                      |   4 +
 manual/process.texi                           |  81 ++++++--
 posix/Makefile                                |   5 +-
 posix/fork-internal.c                         | 125 +++++++++++++
 posix/fork-internal.h                         |  29 +++
 posix/fork.c                                  |  98 +---------
 posix/spawn.h                                 |   2 +
 posix/spawn_int.h                             |   3 +-
 posix/tst-posix_spawn-setsid.c                | 168 ++++++++++++-----
 posix/tst-spawn-chdir.c                       |  15 +-
 posix/tst-spawn.c                             |  24 +--
 posix/tst-spawn.h                             |  36 ++++
 posix/tst-spawn2.c                            |  17 +-
 posix/tst-spawn3.c                            | 100 +++++-----
 posix/tst-spawn4.c                            |   7 +-
 posix/tst-spawn5.c                            |  14 +-
 posix/tst-spawn6.c                            |  15 +-
 posix/tst-spawn7.c                            |  13 +-
 sysdeps/nptl/_Fork.c                          |   2 +-
 sysdeps/unix/sysv/linux/Makefile              |  26 +++
 sysdeps/unix/sysv/linux/Versions              |   6 +
 sysdeps/unix/sysv/linux/aarch64/libc.abilist  |   4 +
 sysdeps/unix/sysv/linux/alpha/libc.abilist    |   4 +
 sysdeps/unix/sysv/linux/arc/libc.abilist      |   4 +
 sysdeps/unix/sysv/linux/arch-fork.h           |  16 +-
 sysdeps/unix/sysv/linux/arm/be/libc.abilist   |   4 +
 sysdeps/unix/sysv/linux/arm/le/libc.abilist   |   4 +
 sysdeps/unix/sysv/linux/bits/spawn_ext.h      |  45 +++++
 sysdeps/unix/sysv/linux/clone-pidfd-support.c |  58 ++++++
 sysdeps/unix/sysv/linux/csky/libc.abilist     |   4 +
 sysdeps/unix/sysv/linux/hppa/libc.abilist     |   4 +
 sysdeps/unix/sysv/linux/i386/libc.abilist     |   4 +
 sysdeps/unix/sysv/linux/ia64/libc.abilist     |   4 +
 .../sysv/linux/loongarch/lp64/libc.abilist    |   4 +
 .../sysv/linux/m68k/coldfire/libc.abilist     |   4 +
 .../unix/sysv/linux/m68k/m680x0/libc.abilist  |   4 +
 .../sysv/linux/microblaze/be/libc.abilist     |   4 +
 .../sysv/linux/microblaze/le/libc.abilist     |   4 +
 .../sysv/linux/mips/mips32/fpu/libc.abilist   |   4 +
 .../sysv/linux/mips/mips32/nofpu/libc.abilist |   4 +
 .../sysv/linux/mips/mips64/n32/libc.abilist   |   4 +
 .../sysv/linux/mips/mips64/n64/libc.abilist   |   4 +
 sysdeps/unix/sysv/linux/nios2/libc.abilist    |   4 +
 sysdeps/unix/sysv/linux/or1k/libc.abilist     |   4 +
 sysdeps/unix/sysv/linux/pidfd_fork.c          |  76 ++++++++
 sysdeps/unix/sysv/linux/pidfd_getpid.c        |  94 ++++++++++
 sysdeps/unix/sysv/linux/pidfd_spawn.c         |  30 +++
 sysdeps/unix/sysv/linux/pidfd_spawnp.c        |  30 +++
 .../linux/powerpc/powerpc32/fpu/libc.abilist  |   4 +
 .../powerpc/powerpc32/nofpu/libc.abilist      |   4 +
 .../linux/powerpc/powerpc64/be/libc.abilist   |   4 +
 .../linux/powerpc/powerpc64/le/libc.abilist   |   4 +
 sysdeps/unix/sysv/linux/procutils.c           | 104 ++++++++++
 sysdeps/unix/sysv/linux/procutils.h           |  35 ++++
 .../unix/sysv/linux/riscv/rv32/libc.abilist   |   4 +
 .../unix/sysv/linux/riscv/rv64/libc.abilist   |   4 +
 .../unix/sysv/linux/s390/s390-32/libc.abilist |   4 +
 .../unix/sysv/linux/s390/s390-64/libc.abilist |   4 +
 sysdeps/unix/sysv/linux/sh/be/libc.abilist    |   4 +
 sysdeps/unix/sysv/linux/sh/le/libc.abilist    |   4 +
 .../sysv/linux/sparc/sparc32/libc.abilist     |   4 +
 .../sysv/linux/sparc/sparc64/libc.abilist     |   4 +
 sysdeps/unix/sysv/linux/spawni.c              |  20 +-
 sysdeps/unix/sysv/linux/sys/pidfd.h           |  17 ++
 sysdeps/unix/sysv/linux/tst-pidfd.c           |  45 +++++
 sysdeps/unix/sysv/linux/tst-pidfd_fork.c      | 150 +++++++++++++++
 sysdeps/unix/sysv/linux/tst-pidfd_getpid.c    | 177 ++++++++++++++++++
 .../sysv/linux/tst-posix_spawn-setsid-pidfd.c |  20 ++
 .../unix/sysv/linux/tst-spawn-chdir-pidfd.c   |  20 ++
 sysdeps/unix/sysv/linux/tst-spawn-pidfd.c     |  20 ++
 sysdeps/unix/sysv/linux/tst-spawn-pidfd.h     |  63 +++++++
 sysdeps/unix/sysv/linux/tst-spawn2-pidfd.c    |  20 ++
 sysdeps/unix/sysv/linux/tst-spawn3-pidfd.c    |  20 ++
 sysdeps/unix/sysv/linux/tst-spawn4-pidfd.c    |  20 ++
 sysdeps/unix/sysv/linux/tst-spawn5-pidfd.c    |  20 ++
 sysdeps/unix/sysv/linux/tst-spawn6-pidfd.c    |  20 ++
 sysdeps/unix/sysv/linux/tst-spawn7-pidfd.c    |  20 ++
 .../unix/sysv/linux/x86_64/64/libc.abilist    |   4 +
 .../unix/sysv/linux/x86_64/x32/libc.abilist   |   4 +
 81 files changed, 1844 insertions(+), 259 deletions(-)
 create mode 100644 bits/spawn_ext.h
 create mode 100644 posix/fork-internal.c
 create mode 100644 posix/fork-internal.h
 create mode 100644 posix/tst-spawn.h
 create mode 100644 sysdeps/unix/sysv/linux/bits/spawn_ext.h
 create mode 100644 sysdeps/unix/sysv/linux/clone-pidfd-support.c
 create mode 100644 sysdeps/unix/sysv/linux/pidfd_fork.c
 create mode 100644 sysdeps/unix/sysv/linux/pidfd_getpid.c
 create mode 100644 sysdeps/unix/sysv/linux/pidfd_spawn.c
 create mode 100644 sysdeps/unix/sysv/linux/pidfd_spawnp.c
 create mode 100644 sysdeps/unix/sysv/linux/procutils.c
 create mode 100644 sysdeps/unix/sysv/linux/procutils.h
 create mode 100644 sysdeps/unix/sysv/linux/tst-pidfd_fork.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-pidfd_getpid.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-posix_spawn-setsid-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn-chdir-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn-pidfd.h
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn2-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn3-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn4-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn5-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn6-pidfd.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-spawn7-pidfd.c