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[PATCH 14/14] klp-build: Support cross-compilation

Josh Poimboeuf posted 14 patches 1 month ago
There is a newer version of this series
[PATCH 14/14] klp-build: Support cross-compilation
Posted by Josh Poimboeuf 1 month ago 
Add support for cross-compilation.  The user must export ARCH, and
either CROSS_COMPILE or LLVM.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 scripts/livepatch/klp-build | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/scripts/livepatch/klp-build b/scripts/livepatch/klp-build
index 809e198a561d..b6c057e2120f 100755
--- a/scripts/livepatch/klp-build
+++ b/scripts/livepatch/klp-build
@@ -404,6 +404,14 @@ validate_patches() {
 	revert_patches
 }
 
+cross_compile_init() {
+	if [[ -v LLVM ]]; then
+		OBJCOPY=llvm-objcopy
+	else
+		OBJCOPY="${CROSS_COMPILE:-}objcopy"
+	fi
+}
+
 do_init() {
 	# We're not yet smart enough to handle anything other than in-tree
 	# builds in pwd.
@@ -420,6 +428,7 @@ do_init() {
 	validate_config
 	set_module_name
 	set_kernelversion
+	cross_compile_init
 }
 
 # Refresh the patch hunk headers, specifically the line numbers and counts.
@@ -783,7 +792,7 @@ build_patch_module() {
 	cp -f "$kmod_file" "$kmod_file.orig"
 
 	# Work around issue where slight .config change makes corrupt BTF
-	objcopy --remove-section=.BTF "$kmod_file"
+	"$OBJCOPY" --remove-section=.BTF "$kmod_file"
 
 	# Fix (and work around) linker wreckage for klp syms / relocs
 	"$SRC/tools/objtool/objtool" klp post-link "$kmod_file" || die "objtool klp post-link failed"
-- 
2.53.0
Re: [PATCH 14/14] klp-build: Support cross-compilation
Posted by Song Liu 4 weeks, 1 day ago 
On Wed, Mar 4, 2026 at 7:32 PM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> Add support for cross-compilation.  The user must export ARCH, and
> either CROSS_COMPILE or LLVM.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
>  scripts/livepatch/klp-build | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/livepatch/klp-build b/scripts/livepatch/klp-build
> index 809e198a561d..b6c057e2120f 100755
> --- a/scripts/livepatch/klp-build
> +++ b/scripts/livepatch/klp-build
> @@ -404,6 +404,14 @@ validate_patches() {
>         revert_patches
>  }
>
> +cross_compile_init() {
> +       if [[ -v LLVM ]]; then
> +               OBJCOPY=llvm-objcopy
> +       else
> +               OBJCOPY="${CROSS_COMPILE:-}objcopy"
> +       fi
> +}

Shall we show a specific warning if
  - ARCH is set; and
  - ARCH is not the same as (uname -m); and
  - neither LLVM nor CROSS_COMPILE is set.

Thanks,
Song
Re: [PATCH 14/14] klp-build: Support cross-compilation
Posted by Josh Poimboeuf 3 weeks, 3 days ago 
On Wed, Mar 11, 2026 at 04:18:40PM -0700, Song Liu wrote:
> On Wed, Mar 4, 2026 at 7:32 PM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
> >
> > Add support for cross-compilation.  The user must export ARCH, and
> > either CROSS_COMPILE or LLVM.
> >
> > Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> > ---
> >  scripts/livepatch/klp-build | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/scripts/livepatch/klp-build b/scripts/livepatch/klp-build
> > index 809e198a561d..b6c057e2120f 100755
> > --- a/scripts/livepatch/klp-build
> > +++ b/scripts/livepatch/klp-build
> > @@ -404,6 +404,14 @@ validate_patches() {
> >         revert_patches
> >  }
> >
> > +cross_compile_init() {
> > +       if [[ -v LLVM ]]; then
> > +               OBJCOPY=llvm-objcopy
> > +       else
> > +               OBJCOPY="${CROSS_COMPILE:-}objcopy"
> > +       fi
> > +}
> 
> Shall we show a specific warning if
>   - ARCH is set; and
>   - ARCH is not the same as (uname -m); and
>   - neither LLVM nor CROSS_COMPILE is set.

Yeah, I think that would be a good idea.  Will do that for v2.

-- 
Josh
Re: [PATCH 14/14] klp-build: Support cross-compilation
Posted by Josh Poimboeuf 3 weeks, 2 days ago 
On Mon, Mar 16, 2026 at 12:15:28PM -0700, Josh Poimboeuf wrote:
> On Wed, Mar 11, 2026 at 04:18:40PM -0700, Song Liu wrote:
> > On Wed, Mar 4, 2026 at 7:32 PM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
> > >
> > > Add support for cross-compilation.  The user must export ARCH, and
> > > either CROSS_COMPILE or LLVM.
> > >
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> > > ---
> > >  scripts/livepatch/klp-build | 11 ++++++++++-
> > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/scripts/livepatch/klp-build b/scripts/livepatch/klp-build
> > > index 809e198a561d..b6c057e2120f 100755
> > > --- a/scripts/livepatch/klp-build
> > > +++ b/scripts/livepatch/klp-build
> > > @@ -404,6 +404,14 @@ validate_patches() {
> > >         revert_patches
> > >  }
> > >
> > > +cross_compile_init() {
> > > +       if [[ -v LLVM ]]; then
> > > +               OBJCOPY=llvm-objcopy
> > > +       else
> > > +               OBJCOPY="${CROSS_COMPILE:-}objcopy"
> > > +       fi
> > > +}
> > 
> > Shall we show a specific warning if
> >   - ARCH is set; and
> >   - ARCH is not the same as (uname -m); and
> >   - neither LLVM nor CROSS_COMPILE is set.
> 
> Yeah, I think that would be a good idea.  Will do that for v2.

So, in general ARCH is complicated.  For example:

 - ARCH=arm64 would never match "uname -m" (aarch64)
 - ARCH=i386 would use the same gcc binary (no cross-compiler needed)
 - I'm sure there are many other edge cases...

Instead of a manual error, it may be simpler to just let the build fail
naturally if the user doesn't set the right ARCH.

Though, I think the check can be improved slightly, as ARCH is a
reasonably good indicator that cross-compiling is happening.  So I can
at least add an ARCH check at the beginning like so?

cross_compile_init() {
	if [[ ! -v ARCH ]]; then
		OBJCOPY=objcopy
		return 0
	fi

	if [[ -v LLVM ]]; then
		OBJCOPY=llvm-objcopy
	else
		OBJCOPY="${CROSS_COMPILE:-}objcopy"
	fi
}

-- 
Josh
Re: [PATCH 14/14] klp-build: Support cross-compilation
Posted by Song Liu 3 weeks, 2 days ago 
On Tue, Mar 17, 2026 at 10:52 AM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
[...]
> >
> > Yeah, I think that would be a good idea.  Will do that for v2.
>
> So, in general ARCH is complicated.  For example:
>
>  - ARCH=arm64 would never match "uname -m" (aarch64)
>  - ARCH=i386 would use the same gcc binary (no cross-compiler needed)
>  - I'm sure there are many other edge cases...

Agreed. I haven't worked with i386 for a long time, but I did notice
the arm64 vs. aarch64 difference.

> Instead of a manual error, it may be simpler to just let the build fail
> naturally if the user doesn't set the right ARCH.
>
> Though, I think the check can be improved slightly, as ARCH is a
> reasonably good indicator that cross-compiling is happening.  So I can
> at least add an ARCH check at the beginning like so?
>
> cross_compile_init() {
>         if [[ ! -v ARCH ]]; then
>                 OBJCOPY=objcopy
>                 return 0
>         fi
>
>         if [[ -v LLVM ]]; then
>                 OBJCOPY=llvm-objcopy
>         else
>                 OBJCOPY="${CROSS_COMPILE:-}objcopy"
>         fi
> }

Do we need ARCH when CROSS_COMPILE is set? I was
under the impression that CROSS_COMPILE doesn't require
ARCH.

Thanks,
Song
Re: [PATCH 14/14] klp-build: Support cross-compilation
Posted by Josh Poimboeuf 3 weeks, 2 days ago 
On Tue, Mar 17, 2026 at 11:21:43AM -0700, Song Liu wrote:
> On Tue, Mar 17, 2026 at 10:52 AM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
> [...]
> > >
> > > Yeah, I think that would be a good idea.  Will do that for v2.
> >
> > So, in general ARCH is complicated.  For example:
> >
> >  - ARCH=arm64 would never match "uname -m" (aarch64)
> >  - ARCH=i386 would use the same gcc binary (no cross-compiler needed)
> >  - I'm sure there are many other edge cases...
> 
> Agreed. I haven't worked with i386 for a long time, but I did notice
> the arm64 vs. aarch64 difference.
> 
> > Instead of a manual error, it may be simpler to just let the build fail
> > naturally if the user doesn't set the right ARCH.
> >
> > Though, I think the check can be improved slightly, as ARCH is a
> > reasonably good indicator that cross-compiling is happening.  So I can
> > at least add an ARCH check at the beginning like so?
> >
> > cross_compile_init() {
> >         if [[ ! -v ARCH ]]; then
> >                 OBJCOPY=objcopy
> >                 return 0
> >         fi
> >
> >         if [[ -v LLVM ]]; then
> >                 OBJCOPY=llvm-objcopy
> >         else
> >                 OBJCOPY="${CROSS_COMPILE:-}objcopy"
> >         fi
> > }
> 
> Do we need ARCH when CROSS_COMPILE is set? I was
> under the impression that CROSS_COMPILE doesn't require
> ARCH.

If CROSS_COMPILE is used without ARCH, it will just try to use the host
arch.  I'm not sure if that's considered cross-compiling?   I suppose it
should use the CROSS_COMPILE version of objcopy in that case?  Though in
practice it probably doesn't matter.

I guess the original version of the function is probably fine and we
don't need to complicate matters.

-- 
Josh
Re: [PATCH 14/14] klp-build: Support cross-compilation
Posted by Song Liu 3 weeks, 2 days ago 
On Tue, Mar 17, 2026 at 11:53 AM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
[...]
> >
> > Do we need ARCH when CROSS_COMPILE is set? I was
> > under the impression that CROSS_COMPILE doesn't require
> > ARCH.
>
> If CROSS_COMPILE is used without ARCH, it will just try to use the host
> arch.  I'm not sure if that's considered cross-compiling?   I suppose it
> should use the CROSS_COMPILE version of objcopy in that case?  Though in
> practice it probably doesn't matter.
>
> I guess the original version of the function is probably fine and we
> don't need to complicate matters.

Agreed that the original version works fine. Thanks for bearing with my
nitpick on this.

Song

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