Date: Sun, 12 Jul 2026 14:25:42 +0000 From: Konstantin Belousov <kib@FreeBSD.org> To: doc-committers@FreeBSD.org, dev-commits-doc-all@FreeBSD.org Subject: git: 2ac8002351 - main - pdopenpid(2)/pddupfd(2)/execblock report Message-ID: <6a53a3e6.38e5e.6278b543@gitrepo.freebsd.org>
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The branch main has been updated by kib: URL: https://cgit.FreeBSD.org/doc/commit/?id=2ac80023518f95ac2869935ecb3ca3bb90c678d4 commit 2ac80023518f95ac2869935ecb3ca3bb90c678d4 Author: Konstantin Belousov <kib@FreeBSD.org> AuthorDate: 2026-06-09 01:14:36 +0000 Commit: Konstantin Belousov <kib@FreeBSD.org> CommitDate: 2026-07-12 14:25:17 +0000 pdopenpid(2)/pddupfd(2)/execblock report Reviewed by: salvadore Sponsored by: The FreeBSD Foundation Differential revision: https://reviews.freebsd.org/D57935 --- .../status/report-2026-04-2026-06/pdopenpid.adoc | 33 ++++++++++++++++++++++ 1 file changed, 33 insertions(+) diff --git a/website/content/en/status/report-2026-04-2026-06/pdopenpid.adoc b/website/content/en/status/report-2026-04-2026-06/pdopenpid.adoc new file mode 100644 index 0000000000..8439605cf7 --- /dev/null +++ b/website/content/en/status/report-2026-04-2026-06/pdopenpid.adoc @@ -0,0 +1,33 @@ +=== Process Descriptor API completion 2 + +Contact: Konstantin Belousov <kib@FreeBSD.org> + +Links: + +link:https://reviews.freebsd.org/D57124[sys: add pdopenpid(2)] URL: link:https://reviews.freebsd.org/D57124[] + +link https://reviews.freebsd.org/D57163[pddupfd(2)] https://reviews.freebsd.org/D57163[] + +Following the addition of man:pdwait[2] and man:pdrfork[2] system calls, I looked at the other missing features that are requested by application programmers. +The largest feature asked for seems to be an ability to obtain a process descriptor for an existing process. + +Currently only one file descriptor may reference an internal struct procdesc in kernel, which in part explains why the interface to open the process by pid is not yet done. +The work added the ability for procdescs to be pointed to by more than one file. +Attributes controlling the procdesc behavior, e.g. the daemon (closing the file does not terminate the process) were moved from procdesc to the file. + +This change allowed to extend the procdesc internal interfaces to implement the man:pdopenid(2) system call. + +A facility implemented e.g. by the Linux pidfd descriptors is the pidfd_getfd() system call, that duplicates specified file descriptor from the target process, into the caller. +The implementation of it for FreeBSD, named man:pddupfd(2) is relatively straightforward once we get the fget_remote(9) helper. + +A problem with it started when the algorithm to check for privileges required to allow the operation was developed. +The kernel checks the result of p_candebug() to see if something should be allowed that directly accesses programming resources of the remote process. +But p_candebug() result is only valid while the target process lock is owned. +Besides, p_candebug() denies actions if the target process changes the executing program with man:execve(2), which might change the privileges of the process if the image is set-uid or set-gid. +Overall, this makes the plain check with p_candebug() inadequate, because we cannot own the process lock over fget_remote(). + +For that, a facility was developed that provides mutual exclusion for execing in the target process vs. the caller, called execblock. +Also, helpers to reference the current vmspace for a process were added, to avoid using execblock when only a consistent target address space is required for the action, like reading of the process strings or copying the process memory. +Existing places in system that are affected by the race were identified, and the usage of execblock or vmspace referencing interfaces applied as needed. + +Then, the man:pddupfd(2) was implemented with the help of fget_remote() and execblock. + +Sponsor: The FreeBSD Foundationhome | help
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