[Bug 297426] linuxulator: linprocfs /proc/self/exe host path breaks execve under Linux ABI

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=297426

            Bug ID: 297426
           Summary: linuxulator: linprocfs /proc/self/exe host path breaks
                    execve under Linux ABI
           Product: Base System
           Version: 16.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: [email protected]
          Reporter: [email protected]

Observed behavior
-----------------

Linux programs that re-execute themselves via /proc/self/exe fail under
the Linuxulator even though the same binary runs when invoked by its
real path.

A minimal check (Linux python under linux_base-rl9):

  readlink(/proc/self/exe)  succeeds and returns a path
  stat(/proc/self/exe)      fails with ENOENT
  access(/proc/self/exe, X) fails
  execve(/proc/self/exe)    fails with ENOENT
  execve(<same binary by its normal path>) succeeds

Chromium-family browsers under the Linuxulator (for example
www/linux-brave) hit this immediately: helper processes are started with
execvp("/proc/self/exe"), which fails with:

  LaunchProcess: failed to execvp:
  /proc/self/exe

The GPU process then exits (status 32512), and the browser aborts
(SIGTRAP / "GPU process isn't usable").  Sites and DRM are beside the
point; the process cannot spawn helpers at all.  Native FreeBSD
www/chromium is not this bug.

Mechanism
---------

linprocfs exposes /proc/<pid>/exe as a symlink.  The fill routine uses
proc_get_binpath(), which returns a FreeBSD host path.  For a binary
installed under the emulator root that looks like:

  /compat/linux/usr/bin/python3.9

Linux ABI absolute lookups start at compat.linux.emul_path (default
/compat/linux).  Following the symlink therefore searches for:

  /compat/linux/compat/linux/usr/bin/python3.9

which does not exist -- hence ENOENT on stat/access/execve of
/proc/self/exe, while execve of the Linux-visible path
(/usr/bin/python3.9) works.

On Linux, /proc/self/exe readlink returns a path already meaningful
inside that namespace (e.g. /usr/bin/python3.9), so follow/exec succeed.

Proposed direction
------------------

Patch forthcoming on Phabricator: when filling the linprocfs exe
symlink, report a Linux-visible path (strip the emul_path prefix when
present) so readlink, namei follow, and execve of /proc/self/exe behave
as on Linux.  Native FreeBSD procfs should keep reporting the host path
for FreeBSD processes.

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