Re: libc/libsys split coming soon

On 2/2/24, Brooks Davis <brooks@freebsd.org> wrote:
> TL;DR: The implementation of system calls is moving to a seperate
> library (libsys).  No changes are required to existing software (except
> to ensure that libsys is present when building custom disk images).
>
> Code: https://github.com/freebsd/freebsd-src/pull/908
>
> After nearly a decade of intermittent work, I'm about to land a series
> of patches which moves system calls, vdso support, and libc's parsing of
> the ELF auxiliary argument vector into a separate library (libsys).  I
> plan to do this early next week (February 5th).
>
> This change serves three primary purposes:
>   1. It's easier to completely replace system call implementations for
>      tracing or compartmentalization purposes.
>   2. It simplifies the implementation of restrictions on system calls such
>      as those implemented by OpenBSD's msyscall(2)
>      (https://man.openbsd.org/msyscall.2).
>   3. It allows language runtimes to link with libsys for system call
>      implementations without requiring libc.
>
> libsys is an auxiliary filter for libc.  This means that for any symbol
> defined by both, the libsys version takes precedence at runtime.  For
> system call implementations, libc contains empty stubs.  For others it
> contains copies of the functions (this could be further refined at a
> later date).  The statically linked libc contains the full
> implementations so linking libsys is not required.
>

Do I read it correctly that everything dynamically linked will also be
linked to libsys, as in executing such a binary will now require
loading one extra .so?

Binary startup is very slow, for example execve of a hello world
binary in a Linux-based chroot on FreeBSD is faster by a factor of 2
compared to a native one. As such perf-wise this looks like a step in
the wrong direction.

Is there a problem making it so that libc ends up unchanged, but all
the bits are available separately in libsys if one does not want libc?

-- 
Mateusz Guzik <mjguzik gmail.com>