[PATCH v4 1/3] posix: Add pidfd_spawn and pidfd_spawnp (BZ# 30349)

[Florian Weimer]

* Adhemerval Zanella:

> Returning a pidfd allows a process to keep a race-free handle to a child
> process, otherwise the caller will need to either use pidfd_open (which
> still might be subject to TOCTOU) or keep using the old racy interface.
>
> The implementation makes sure that kernel must support the complete
> pidfd interface, meaning that waitid (P_PIDFD) should be supported
> (added on Linux 5.4).  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.

“It avoids workarounds with race conditions”?

> These interfaces are similar to the posix_spawn and posix_spawnp, with
> the only different diferent being it returns a process file descriptor
> (int) instead of process ID (pid_t).  Their prototypes are:
>
>   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]);

Should we return the pid_t value as we..

> A new symbol is used instead of a posix_spawn extension to avoid possible
> issue with language bindings that might track the return argument
> lifetime.  Although, on Linux pid_t and int are interchangeable, POSIX
> only state that pid_t should be a signed interger.
>
> Both symbols reuse the posix_spawn posix_spawn_file_actions_t and
> posix_spawnattr_t, to void 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.

Same question as with pidfd_fork: how does this interact with SIGCHLD,
and can the PID be used with waitpid?

> diff --git a/posix/spawn.h b/posix/spawn.h
> index 0a6a070523..2e20991627 100644
> --- a/posix/spawn.h
> +++ b/posix/spawn.h
> @@ -229,6 +229,8 @@ posix_spawn_file_actions_addtcsetpgrp_np (posix_spawn_file_actions_t *,
>  
>  #endif /* __USE_MISC */
>  
> +#include <bits/spawn_ext.h>
> +
>  __END_DECLS

Nested __BEGIN_DECLS.  You should probably move the #include after
__END_DECLS.

> diff --git a/sysdeps/unix/sysv/linux/clone-pidfd-support.c b/sysdeps/unix/sysv/linux/clone-pidfd-support.c
> new file mode 100644
> index 0000000000..4bf2317c17
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/clone-pidfd-support.c

> +         Also tThe waitid is a cancellation entrypoint, so issue the syscall
> +	 directly.  */

Typo: “tThe”

Thanks,
Florian