PR/59037 CVS commit: src/sys

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

The following reply was made to PR kern/59037; it has been noted by GNATS.

From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/59037 CVS commit: src/sys
Date: Sun, 27 Apr 2025 17:40:56 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Sun Apr 27 17:40:56 UTC 2025
 
 Modified Files:
 	src/sys/kern: kern_exec.c
 	src/sys/uvm: uvm_extern.h uvm_map.c
 
 Log Message:
 posix_spawn(2): Allocate a new vmspace at process creation time.
 
 This allocates a new vmspace for the process at the time the new
 process is created, rather than sharing some other vmspace temporarily.
 This eliminates any risk of anything bad happening due to temporary
 sharing, since there isn't any sharing.
 
 Resolves a race to where:
 
 1. we set up the child to share proc0.p_vmspace at first,
 
 2. another process tries to read the new child's psstrings via
    kern.proc_args.<childpid>.argv or similar with the child's
    p_reflock held and gets stuck in a uvm fault loop because
    proc0.p_vmspace doesn't have the child's psstrings address
    (inherited from the parent) mapped,
 
 3. the child is waiting for p_reflock before it can replace its
    p_vmspace or psstrings.
 
 By allocating the vmspace up front, with no mappings in it, we avoid
 exposing the child in this scenario.  Minor possible downside is that
 sysctl kern.proc_args.<childpid>.argv might spuriously fail with
 EFAULT during this time (rather than fail with EBUSY as it does if
 p_reflock is held concurrently) but that's not a particularly big
 deal.
 
 Patch and first paragraph of commit message written by chs@; minor
 tweaks to comments -- and any mistakes in the analysis -- by me.
 
 PR kern/59037: deadlock in posix_spawn
 PR kern/59175: posix_spawn hang, hanging other process too
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.527 -r1.528 src/sys/kern/kern_exec.c
 cvs rdiff -u -r1.233 -r1.234 src/sys/uvm/uvm_extern.h
 cvs rdiff -u -r1.426 -r1.427 src/sys/uvm/uvm_map.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.