toolchain: print actual version of kernel headers when checking

Since we introduced the _AT_LEAST_XXX for the kernel headers, people
using pre-built custom toolchain now have to specify the version of
the kernel headers their custom toolchain uses.

So, when we detect that there is a mismatch between the selection in
the menuconfig, and the actual version of the headers, we currently
only bail out with a terse message "Incorrect selection of kernel
headers".

This could be confusing some, and getting the version of the headers
used by the toolchain is not trivial (well, it's very easy, but not
trivial.)

This patch changes the way we report the error by moving the message
into the test-code, and by printing the expected and actual versions
of the kernel headers.

BUT! To get this pretty error message, we need to run the
test-program, so we can not use the cross-toolchain, we have to use
the native one.

BUT! The native one has its own linux/version.h header, so we can not
simply include it.

So, we ask the cross-compiler where its default sysroot is, and use
that to then force-feed the cross linux/version.h to the native
toolchain.

[Thomas: augment commit log with a message provided by Yann, fix
coding style to not have spaces after opening parenthesis and before
closing parenthesis, reformatted the message "Incorrect selection..."
to make it fit on one line.]

Reported-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

support/scripts/check-kernel-headers.sh

@@ -8,14 +8,43 @@ HDR_M="${HDR_VER%%.*}"
 HDR_V="${HDR_VER#*.}"
 HDR_m="${HDR_V%%.*}"
 
+EXEC="/tmp/br.check-headers.$(uuidgen)"
+
+# By the time we get here, we do not always have the staging-dir
+# already populated (think external toolchain), so we can not use
+# it.
+# So we just ask the cross compiler what its default sysroot is.
+# For multilib-aware toolchains where we should use a non-default
+# sysroot, it's not really a problem since the version of the kernel
+# headers is the same for all sysroots.
+SYSROOT=$(${CC} -print-sysroot)
+
 # We do not want to account for the patch-level, since headers are
 # not supposed to change for different patchlevels, so we mask it out.
 # This only applies to kernels >= 3.0, but those are the only one
 # we actually care about; we treat all 2.6.x kernels equally.
+${HOSTCC} -imacros "${SYSROOT}/usr/include/linux/version.h" \
+          -x c -o "${EXEC}" - <<_EOF_
+#include <stdio.h>
+#include <stdlib.h>
 
-exec ${CC} -E -x c -o - - >/dev/null 2>&1 <<_EOF_
-#include <linux/version.h>
-#if (LINUX_VERSION_CODE & ~0xFF) != KERNEL_VERSION(${HDR_M},${HDR_m},0)
-#error Incorrect kernel header version.
-#endif
+int main(int argc __attribute__((unused)),
+         char** argv __attribute__((unused)))
+{
+    if((LINUX_VERSION_CODE & ~0xFF)
+        != KERNEL_VERSION(${HDR_M},${HDR_m},0))
+    {
+        printf("Incorrect selection of kernel headers: ");
+        printf("expected %d.%d.x, got %d.%d.x\n", ${HDR_M}, ${HDR_m},
+               ((LINUX_VERSION_CODE>>16) & 0xFF),
+               ((LINUX_VERSION_CODE>>8) & 0xFF));
+        return 1;
+    }
+    return 0;
+}
 _EOF_
+
+"${EXEC}"
+ret=${?}
+rm -f "${EXEC}"
+exit ${ret}

toolchain/helpers.mk

@@ -171,8 +171,7 @@ copy_toolchain_sysroot = \
 #
 check_kernel_headers_version = \
 	if ! support/scripts/check-kernel-headers.sh $(1) $(2); then \
-		echo "Incorrect selection of kernel headers"; \
-	    exit 1; \
+		exit 1; \
 	fi
 
 #