[PATCH 0/5] kbuild: uapi: improvements to header testing

[PATCH 0/5] kbuild: uapi: improvements to header testing

[Thomas Weißschuh]

Also validate that UAPI headers do not depend on libc and remove the
dependency on CC_CAN_LINK.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
Thomas Weißschuh (5):
      kbuild: uapi: validate that headers do not use libc
      hexagon: Drop invalid UAPI header asm/signal.h
      kbuild: uapi: don't compile test bpf_perf_event.h on xtensa
      kbuild: uapi: split out command conditions into variables
      kbuild: uapi: drop dependency on CC_CAN_LINK

 arch/hexagon/include/{uapi => }/asm/signal.h |  0
 init/Kconfig                                 |  2 +-
 usr/include/Makefile                         | 87 +++++++++++++++++++++++++++-
 3 files changed, 87 insertions(+), 2 deletions(-)
---
base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
change-id: 20251203-uapi-nostdinc-64edce85d464

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@linutronix.de>

[PATCH 1/5] kbuild: uapi: validate that headers do not use libc

[Thomas Weißschuh]

The UAPI headers should be self-contained. That means they should not
use other headers from libc. Currently this is not enforced and various
dependencies have crept in.

Add a check to make sure no new ones are added.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

---
This currently depends on a fix for linux/fcntl.h:
https://lore.kernel.org/lkml/20251203-uapi-fcntl-v1-1-490c67bf3425@linutronix.de/
---
 usr/include/Makefile | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/usr/include/Makefile b/usr/include/Makefile
index d8a508042fed..a9a861ec8702 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -68,12 +68,89 @@ endif
 # asm-generic/*.h is used by asm/*.h, and should not be included directly
 no-header-test += asm-generic/%
 
+# The following are using libc header and types.
+#
+# Do not add a new header to the list without legitimate reason.
+# Please consider to fix the header first.
+#
+# Sorted alphabetically.
+uses-libc += linux/a.out.h
+uses-libc += linux/atmbr2684.h
+uses-libc += linux/auto_dev-ioctl.h
+uses-libc += linux/auto_fs.h
+uses-libc += linux/auto_fs4.h
+uses-libc += linux/btrfs_tree.h
+uses-libc += linux/cec-funcs.h
+uses-libc += linux/cec.h
+uses-libc += linux/dvb/dmx.h
+uses-libc += linux/dvb/video.h
+uses-libc += linux/ethtool.h
+uses-libc += linux/ethtool_netlink.h
+uses-libc += linux/fuse.h
+uses-libc += linux/gsmmux.h
+uses-libc += linux/icmp.h
+uses-libc += linux/idxd.h
+uses-libc += linux/if.h
+uses-libc += linux/if_arp.h
+uses-libc += linux/if_bonding.h
+uses-libc += linux/if_pppox.h
+uses-libc += linux/if_tunnel.h
+uses-libc += linux/input.h
+uses-libc += linux/ip6_tunnel.h
+uses-libc += linux/joystick.h
+uses-libc += linux/llc.h
+uses-libc += linux/mctp.h
+uses-libc += linux/mdio.h
+uses-libc += linux/mii.h
+uses-libc += linux/mptcp.h
+uses-libc += linux/netdevice.h
+uses-libc += linux/netfilter/xt_RATEEST.h
+uses-libc += linux/netfilter/xt_hashlimit.h
+uses-libc += linux/netfilter/xt_physdev.h
+uses-libc += linux/netfilter/xt_rateest.h
+uses-libc += linux/netfilter_arp/arp_tables.h
+uses-libc += linux/netfilter_arp/arpt_mangle.h
+uses-libc += linux/netfilter_bridge.h
+uses-libc += linux/netfilter_bridge/ebtables.h
+uses-libc += linux/netfilter_ipv4.h
+uses-libc += linux/netfilter_ipv4/ip_tables.h
+uses-libc += linux/netfilter_ipv6.h
+uses-libc += linux/netfilter_ipv6/ip6_tables.h
+uses-libc += linux/route.h
+uses-libc += linux/shm.h
+uses-libc += linux/soundcard.h
+uses-libc += linux/string.h
+uses-libc += linux/tipc_config.h
+uses-libc += linux/uhid.h
+uses-libc += linux/uinput.h
+uses-libc += linux/vhost.h
+uses-libc += linux/vhost_types.h
+uses-libc += linux/virtio_ring.h
+uses-libc += linux/wireless.h
+uses-libc += regulator/regulator.h
+uses-libc += scsi/fc/fc_els.h
+
+ifeq ($(SRCARCH),hexagon)
+uses-libc += asm/sigcontext.h
+endif
+
+ifeq ($(SRCARCH),nios2)
+uses-libc += asm/ptrace.h
+uses-libc += linux/bpf_perf_event.h
+endif
+
+ifeq ($(SRCARCH),s390)
+uses-libc += asm/chpid.h
+uses-libc += asm/chsc.h
+endif
+
 always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \
 		$(CC) $(c_flags) -fsyntax-only -Werror -x c /dev/null \
+			$(if $(filter-out $(uses-libc), $*.h), -nostdinc) \
 			$(if $(filter-out $(no-header-test), $*.h), -include $< -include $<); \
 		$(PERL) $(src)/headers_check.pl $(obj) $<; \
 		touch $@

-- 
2.52.0

[PATCH 2/5] hexagon: Drop invalid UAPI header asm/signal.h

[Thomas Weißschuh]

This UAPI header contains declarations of internal kernel symbols.
Such declarations are invalid. Normally headers_check.pl would report
an error, but apparently its dependency on CC_CAN_LINK prevent its
execution on hexagon so far.

As the header does not expose any additional UAPI, just make it a
regular internal kernel header. asm-generic/signal.h will be used
for the UAPI automatically, the same as before.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 arch/hexagon/include/{uapi => }/asm/signal.h | 0
 1 file changed, 0 insertions(+), 0 deletions(-)

diff --git a/arch/hexagon/include/uapi/asm/signal.h b/arch/hexagon/include/asm/signal.h
similarity index 100%
rename from arch/hexagon/include/uapi/asm/signal.h
rename to arch/hexagon/include/asm/signal.h

-- 
2.52.0

[PATCH 3/5] kbuild: uapi: don't compile test bpf_perf_event.h on xtensa

[Thomas Weißschuh]

The xtensa UAPI headers do not provide 'struct pt_regs',
triggering an error:

usr/include/linux/bpf_perf_event.h:14:28: error: field 'regs' has incomplete type

Disable the header tests for this file on xtensa.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 usr/include/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/usr/include/Makefile b/usr/include/Makefile
index a9a861ec8702..fd29c11c35cb 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -65,6 +65,10 @@ no-header-test += asm/uctx.h
 no-header-test += asm/fbio.h
 endif
 
+ifeq ($(SRCARCH),xtensa)
+no-header-test += linux/bpf_perf_event.h
+endif
+
 # asm-generic/*.h is used by asm/*.h, and should not be included directly
 no-header-test += asm-generic/%
 

-- 
2.52.0

[PATCH 4/5] kbuild: uapi: split out command conditions into variables

[Thomas Weißschuh]

The condition logic will become a bit more complicated.

Split them out into dedicated variables so they stay readable.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 usr/include/Makefile | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/usr/include/Makefile b/usr/include/Makefile
index fd29c11c35cb..fa01bcda21f5 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -150,12 +150,15 @@ endif
 
 always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
+target-no-libc = $(filter-out $(uses-libc), $*.h)
+target-can-compile = $(filter-out $(no-header-test), $*.h)
+
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \
 		$(CC) $(c_flags) -fsyntax-only -Werror -x c /dev/null \
-			$(if $(filter-out $(uses-libc), $*.h), -nostdinc) \
-			$(if $(filter-out $(no-header-test), $*.h), -include $< -include $<); \
+			$(if $(target-no-libc), -nostdinc) \
+			$(if $(target-can-compile), -include $< -include $<); \
 		$(PERL) $(src)/headers_check.pl $(obj) $<; \
 		touch $@
 

-- 
2.52.0

[PATCH 5/5] kbuild: uapi: drop dependency on CC_CAN_LINK

[Thomas Weißschuh]

The header tests try to compile each header. Some UAPI headers depend on
libc headers so they need a full userspace toolchain to build. This
dependency is expressed in kconfig as a dependency on CC_CAN_LINK.
Many kernel builds do not satisfy CC_CAN_LINK as they only use a
minimal kernel (cross-) compiler. In those configurations the UAPI
headers are not tested at all.

However most UAPI headers do not even depend on any libc headers,
and such dependencies are undesired in any case. Also the static
analysis performed by headers_check.pl does not need CC_CAN_LINK.

Drop the hard dependency on CC_CAN_LINK and instead skip the affected
compilation step for exactly those headers which require libc.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 init/Kconfig         | 2 +-
 usr/include/Makefile | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index fa79feb8fe57..4e7ae65683ee 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -247,7 +247,7 @@ config WERROR
 
 config UAPI_HEADER_TEST
 	bool "Compile test UAPI headers"
-	depends on HEADERS_INSTALL && CC_CAN_LINK
+	depends on HEADERS_INSTALL
 	help
 	  Compile test headers exported to user-space to ensure they are
 	  self-contained, i.e. compilable as standalone units.
diff --git a/usr/include/Makefile b/usr/include/Makefile
index fa01bcda21f5..6d86a53c6f0a 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -151,7 +151,8 @@ endif
 always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/dev/null))
 
 target-no-libc = $(filter-out $(uses-libc), $*.h)
-target-can-compile = $(filter-out $(no-header-test), $*.h)
+target-can-compile = $(and $(filter-out $(no-header-test), $*.h), \
+                           $(or $(CONFIG_CC_CAN_LINK), $(target-no-libc)))
 
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<

-- 
2.52.0

Re: [PATCH 0/5] kbuild: uapi: improvements to header testing

[Nathan Chancellor]

Hi Thomas,

On Tue, Dec 23, 2025 at 08:04:07AM +0100, Thomas Weißschuh wrote:
> Also validate that UAPI headers do not depend on libc and remove the
> dependency on CC_CAN_LINK.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> ---
> Thomas Weißschuh (5):
>       kbuild: uapi: validate that headers do not use libc
>       hexagon: Drop invalid UAPI header asm/signal.h
>       kbuild: uapi: don't compile test bpf_perf_event.h on xtensa
>       kbuild: uapi: split out command conditions into variables
>       kbuild: uapi: drop dependency on CC_CAN_LINK
> 
>  arch/hexagon/include/{uapi => }/asm/signal.h |  0
>  init/Kconfig                                 |  2 +-
>  usr/include/Makefile                         | 87 +++++++++++++++++++++++++++-
>  3 files changed, 87 insertions(+), 2 deletions(-)

This seems like a great improvement to the UAPI header testing. I don't
see any immediate problems and it works for me in my brief local
testing. I will give a week or so for comments before I apply this to
kbuild-next (plus -next should be operating at that point).

Cheers,
Nathan

Re: [PATCH 0/5] kbuild: uapi: improvements to header testing

[Nathan Chancellor]

On Tue, 23 Dec 2025 08:04:07 +0100, Thomas Weißschuh wrote:
> Also validate that UAPI headers do not depend on libc and remove the
> dependency on CC_CAN_LINK.
> 
> 

Applied to

  https://git.kernel.org/pub/scm/linux/kernel/git/kbuild/linux.git kbuild-next-unstable

Thanks!

[1/5] kbuild: uapi: validate that headers do not use libc
      https://git.kernel.org/kbuild/c/6059b880a93c3
[2/5] hexagon: Drop invalid UAPI header asm/signal.h
      https://git.kernel.org/kbuild/c/cc45d2ea5cfb8
[3/5] kbuild: uapi: don't compile test bpf_perf_event.h on xtensa
      https://git.kernel.org/kbuild/c/e2772ba5f43df
[4/5] kbuild: uapi: split out command conditions into variables
      https://git.kernel.org/kbuild/c/4ac85d9bc73ed
[5/5] kbuild: uapi: drop dependency on CC_CAN_LINK
      https://git.kernel.org/kbuild/c/e3970d77ec504

Please look out for regression or issue reports or other follow up
comments, as they may result in the patch/series getting dropped or
reverted. Patches applied to an "unstable" branch are accepted pending
wider testing in -next and any post-commit review; they will generally
be moved to the main branch in a week if no issues are found.

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>