Add a pdvfork system call.

[theraven]

Summary

Currently, if you wish to make use of process descriptors then you must
use fork and incur the full CoW overhead for the child process, even if
it intends to call execve immediately.

This patch refactors the fork / vfork / pdfork implementations (which
included a lot of copied and pasted logic) into a single function and
implements wrappers for all four combinations of {fork, vfork} behaviour
combined with {fork, pdfork} output types.

[theraven]

@brooks, this should also be useful for combining CHERI coprocesses with Capsicum (coprocesses are currently created with vfork + coexecve).

[kib]

pdfork(2) already takes flags, adding PD_VFORK is much better IMO than adding yet another syscall. I think it should go by the route of duplicating needed rfork(2) flags.

Can you split addition of do_pdvfork() vs. pdvfork/PD_VFORK introduction?

I had another note, to not include generated files into review and commit, but if you do PD_VFORK, this is resolved automatically.

[jilles]

This functionality clearly fills a gap, but the provided interface is too low level for applications to call. The code in lib/libc/gen/posix_spawn.c uses various non-standard facilities to handle things like signals correctly. Depending on how this is planned to be used, additional libc code may be useful, but it need not be part of this review.

Of the non-standard facilities used by posix_spawn(), RFSPAWN is a flag for the rfork syscall. It prevents running any signal handler function inherited from the parent and equivalent functionality should probably be added to pdvfork.

sys/sys/procdesc.h

125

This needs __returns_twice like vfork/rfork/setjmp. If a PD_VFORK flag is added to pdfork() instead of adding pdvfork(), then pdfork() will need to be annotated.

125

The standard syscall stub will not work properly on x86 for this function. The child process pops the return address off the stack and if it is subsequently overwritten, the parent process will jump to nowhere. See for example lib/libc/amd64/sys/vfork.S.

[kib]

sys/sys/procdesc.h

125

Note that this patch does not even add the proposed syscall to libc/sys/Symbol.ver.

[theraven]

Abandoning: This is probably not the right approach and @jhb is working on something similar so I'll wait for his version.