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Sharing ELF definitions between NetBSD userspace and kernel
Hello tech-kern and tech-userlevel,
I would like to discuss ways to implement the following two goals:
Goal 1: To wean userland off the use of <sys/exec_elf.h> (a kernel
header).
Goal 2: To have a single set of definitions for 'generic' ELF symbols,
for consistency across NetBSD's userspace and kernel.
Where we are currently:
- Currently, userland code in NetBSD expects the kernel header
<sys/exec_elf.h> to implement ELF-related symbols.
However, from the POV of the kernel, almost all of the content of
this file is clutter -- only a small portion of the content in
<sys/exec_elf.h> is actually relevant to the kernel.
- The contents of <sys/exec_elf.h> could be grouped into the following
categories:
A) Symbols and definitions that need to be consistent across OSes
for interoperability - the 'generic' set of definitions, further
subdivided into:
A1) Symbols that are 'generic' but are NOT used by NetBSD kernel
code.
A2) Symbols and definitions that are 'generic', and are used by
the kernel.
B) NetBSD-local symbols used by userland only.
C) NetBSD-local symbols needed by both userland and the kernel
(e.g. the definitions needed for implementing execve(2)).
D) Kernel-only symbols which are in an #ifdef _KERNEL block. This
set is not relevant to this discussion.
For goal 1, we could offer a user header <elf.h>.
Implementing <elf.h> would be straightforward:
- The definitions for the 'generic' sets [A1, A2] are available in
<sys/elfdefinitions.h> (via external/bsd/elftoolchain), which
<elf.h> could #include.
- The symbols in set [B] can be moved over from <sys/exec_elf.h>.
Pros:
- Eases porting of third-party code that expects to use an
<elf.h>.
- Removes the dependency the use of a kernel header.
Cons: (I can't think of a serious con.)
For goal 2, we have the following options (at least):
Option 1: Have <sys/exec_elf.h> include <sys/elfdefinitions.h>
for 'generic' symbols.
Something like:
Index: exec_elf.h
===================================================================
RCS file: /cvsroot/src/sys/sys/exec_elf.h,v
retrieving revision 1.183
diff -u -r1.183 exec_elf.h
--- exec_elf.h 18 Nov 2025 13:02:13 -0000 1.183
+++ exec_elf.h 25 Nov 2025 13:58:30 -0000
@@ -53,6 +53,8 @@
#include <stddef.h>
#endif /* _KERNEL || _STANDALONE */
+#include <sys/elfdefinitions.h>
+
#if HAVE_NBTOOL_CONFIG_H
#include <nbinclude/machine/elf_machdep.h>
#else
This is how some OSes keep userland and the kernel consistent:
e.g. FreeBSD's userland header <elf.h> includes <sys/elf*.h>, and
OpenSolaris's <elf.h> brings in <sys/elf.h>.
The 'generic' definitions currently in <sys/exec_elf.h> can
then be removed from the file.
Pros:
- <sys/exec_elf.h> can become smaller and narrower in focus.
- Existing third-party code that uses <sys/exec_elf.h> for accessing
'generic' ELF symbols will continue to compile; such code can
gradually be adapted to use <elf.h>.
Cons:
- This approach requires the Elftoolchain header
<sys/elfdefinitions.h> to be made available to the kernel build.
Option 2: Manually or semi-automatically maintain all of the 'generic'
ELF symbols currently present in <sys/exec_elf.h>, say by using a
script that keeps these symbols in sync with userland.
Pros:
- Does not need the kernel build process to change.
- Application code that uses <sys/exec_elf.h> for 'generic'
symbols will continue to compile.
Cons:
- Non-kernel clutter remains in <sys/exec_elf.h>.
- Manual steps would be needed to keep the file up to date.
- Potential for subtle bugs in the future.
Option 3: Like option 2 above but in this option <sys/exec_elf.h> is
trimmed down to *only* contain the symbols used by the kernel (i.e.
only the sets [A2], [C] and [D])
This forces userland code to use <elf.h>.
Pros:
- Keeps the kernel header clean and minimal.
- <sys/elfdefinitions.h> does not need to be available at kernel
build time.
- Fewer 'generic' symbols to keep up to date.
Cons:
- Breaks third-party code that expects generic ELF symbols to
be provided by <sys/exec_elf.h>.
- Extra manual steps to maintain the file (like in option 2).
- A small amount of symbol duplication remains.
Thoughts/comments?
Regards,
Joseph Koshy | jkoshy%netbsd.org@localhost
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