[PATCH v2 2/3] tests: add some tests validating skipped functions due to uncertain arg location

Alexis Lothoré (eBPF Foundation)
2025-07-03T09:02:54Z

Add a small kernel module representing specific cases likely absent from
standard vmlinux files. As a starter, the introduced module exposes a
few functions consuming structs passed by value, some passed by
register, some passed on the stack:

int kmod_test_init(void);
int test_kmod_func_ok(int, void *, char, short int);
int test_kmod_func_struct_ok(int, void *, char, struct kmod_struct);
int test_kmod_func_struct_on_stack_ok(int, void *, char, short int, int, \
void *, char, short int, struct kmod_struct);
int test_kmod_func_struct_on_stack_ko(int, void *, char, short int, int, \
void *, char, short int, struct kmod_struct_packed);

Then enrich btf_functions.sh to make it perform the following steps:
- build the module
- generate BTF info and pfunct listing, both with dwarf and the
generated BTF
- check that any function encoded in BTF is found in DWARF
- check that any function announced as skipped is indeed absent from BTF
- check that any skipped function has been skipped due to uncertain
parameter location

Those new tests are executed only if a kernel directory is provided as
script's second argument, they are otherwise skipped.

Example of the new test execution:
Encoding...Matched 4 functions exactly.
Ok
Validation of skipped function logic...
Skipped encoding 1 functions in BTF.
Ok
Validating skipped functions have uncertain parameter location...
Found 1 legitimately skipped function due to uncertain loc
Ok

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
---
Changes in v2:
- new patch
---
tests/btf_functions.sh | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++
tests/kmod/Makefile | 1 +
tests/kmod/kmod.c | 69 +++++++++++++++++++++++++++++++++++
3 files changed, 169 insertions(+)

diff --git a/tests/btf_functions.sh b/tests/btf_functions.sh
index c92e5ae906f90badfede86eb530108894fbc8c93..64810b7eb51e7f2693929fbf66e0641a9d4e0277 100755
--- a/tests/btf_functions.sh
+++ b/tests/btf_functions.sh
@@ -193,4 +193,103 @@ if [[ -n "$VERBOSE" ]]; then
fi
echo "Ok"

+# Some specific cases can not be tested directly with a standard kernel.
+# We can use the kernel module in kmod/ to test those cases, like packed
+# structs passed on the stack. Run this test only if we have the needed
+# dependencies (eg: some kernel sources directory passed as argument; those
+# must match the used vmlinux file)
+
+KDIR=${KDIR:-$2}
+if [ -z "$KDIR" ] ; then
+ echo "Skipping kmod tests"
+ exit 0
+fi
+
+echo -n "Validation of BTF encoding corner cases with kmod functions; this may take some time: "
+
+test -n "$VERBOSE" && printf "\nBuilding kmod..."
+tests_dir=$(dirname $0)
+make -C ${KDIR} M=${tests_dir}/kmod
+
+test -n "$VERBOSE" && printf "\nEncoding..."
+pahole --btf_features=default --lang_exclude=rust --btf_encode_detached=$outdir/kmod.btf \
+ --verbose ${tests_dir}/kmod/kmod.ko | grep "skipping BTF encoding of function" \
+ > ${outdir}/kmod_skipped_fns
+
+funcs=$(pfunct --format_path=btf $outdir/kmod.btf 2>/dev/null|sort)
+pfunct --all --no_parm_names --format_path=dwarf kmod/kmod.ko | \
+ sort|uniq > $outdir/kmod_dwarf.funcs
+pfunct --all --no_parm_names --format_path=btf $outdir/kmod.btf 2>/dev/null|\
+ awk '{ gsub("^(bpf_kfunc |bpf_fastcall )+",""); print $0}'|sort|uniq > $outdir/kmod_btf.funcs
+
+exact=0
+while IFS= read -r btf ; do
+ # Matching process can be kept simpler as the tested binary is
+ # specifically tailored for tests
+ dwarf=$(grep -F "$btf" $outdir/kmod_dwarf.funcs)
+ if [[ "$btf" != "$dwarf" ]]; then
+ echo "ERROR: mismatch : BTF '$btf' not found; DWARF '$dwarf'"
+ fail
+ else
+ exact=$((exact+1))
+ fi
+done < $outdir/kmod_btf.funcs
+
+if [[ -n "$VERBOSE" ]]; then
+ echo "Matched $exact functions exactly."
+ echo "Ok"
+ echo "Validation of skipped function logic..."
+fi
+
+skipped_cnt=$(wc -l ${outdir}/kmod_skipped_fns | awk '{ print $1}')
+if [[ "$skipped_cnt" == "0" ]]; then
+ echo "No skipped functions. Done."
+ exit 0
+fi
+
+skipped_fns=$(awk '{print $1}' $outdir/kmod_skipped_fns)
+for s in $skipped_fns ; do
+ # Ensure the skipped function are not in BTF
+ inbtf=$(grep " $s(" $outdir/kmod_btf.funcs)
+ if [[ -n "$inbtf" ]]; then
+ echo "ERROR: '${s}()' was added incorrectly to BTF: '$inbtf'"
+ fail
+ fi
+done
+
+if [[ -n "$VERBOSE" ]]; then
+ echo "Skipped encoding $skipped_cnt functions in BTF."
+ echo "Ok"
+ echo "Validating skipped functions have uncertain parameter location..."
+fi
+
+uncertain_loc=$(awk '/due to uncertain parameter location/ { print $1 }' $outdir/kmod_skipped_fns)
+legitimate_skip=0
+
+for f in $uncertain_loc ; do
+ # Extract parameters types
+ raw_params=$(grep ${f} $outdir/kmod_dwarf.funcs|sed -n 's/^[^(]*(\([^)]*\)).*/\1/p')
+ IFS=',' read -ra params <<< "${raw_params}"
+ for param in "${params[@]}"
+ do
+ # Search any param that could be a struct
+ struct_type=$(echo ${param}|grep -E '^struct [^*]' | sed -E 's/^struct //')
+ if [ -n "${struct_type}" ]; then
+ # Check with pahole if the struct is detected as
+ # packed
+ if pahole -C "${struct_type}" kmod/kmod.ko|grep -q __packed__
+ then
+ legitimate_skip=$((legitimate_skip+1))
+ continue 2
+ fi
+ fi
+ done
+ echo "ERROR: '${f}()' should not have been skipped; it has no parameter with uncertain location"
+ fail
+done
+
+if [[ -n "$VERBOSE" ]]; then
+ echo "Found ${legitimate_skip} legitimately skipped function due to uncertain loc"
+fi
+echo "Ok"
exit 0
diff --git a/tests/kmod/Makefile b/tests/kmod/Makefile
new file mode 100644
index 0000000000000000000000000000000000000000..e7c2ed929eaf81e91429f744c3778156ed2be2d2
--- /dev/null
+++ b/tests/kmod/Makefile
@@ -0,0 +1 @@
+obj-m += kmod.o
diff --git a/tests/kmod/kmod.c b/tests/kmod/kmod.c
new file mode 100644
index 0000000000000000000000000000000000000000..5b93614b6156b05925a6cb48809ad63533ccba3e
--- /dev/null
+++ b/tests/kmod/kmod.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/module.h>
+#include <linux/printk.h>
+
+struct kmod_struct {
+ char a;
+ short b;
+ int c;
+ unsigned long long d;
+};
+
+struct kmod_struct_packed {
+ char a;
+ short b;
+ int c;
+ unsigned long long d;
+}__packed;
+
+int test_kmod_func_ok(int a, void *b, char c, short d);
+int test_kmod_func_struct_ok(int a, void *b, char c, struct kmod_struct d);
+int test_kmod_func_struct_on_stack_ok(int a, void *b, char c, short d, int e,
+ void *f, char g, short h,
+ struct kmod_struct i);
+int test_kmod_func_struct_on_stack_ko(int a, void *b, char c, short d, int e,
+ void *f, char g, short h,
+ struct kmod_struct_packed i);
+
+noinline int test_kmod_func_ok(int a, void *b, char c, short d)
+{
+ return a + (long)b + c + d;
+}
+
+noinline int test_kmod_func_struct_ok(int a, void *b, char c,
+ struct kmod_struct d)
+{
+ return a + (long)b + c + d.a + d.b + d.c + d.d;
+}
+
+noinline int test_kmod_func_struct_on_stack_ok(int a, void *b, char c, short d,
+ int e, void *f, char g, short h,
+ struct kmod_struct i)
+{
+ return a + (long)b + c + d + e + (long)f + g + h + i.a + i.b + i.c + i.d;
+}
+
+noinline int test_kmod_func_struct_on_stack_ko(int a, void *b, char c, short d,
+ int e, void *f, char g, short h,
+ struct kmod_struct_packed i)
+{
+ return a + (long)b + c + d + e + (long)f + g + h + i.a + i.b + i.c + i.d;
+}
+
+static int kmod_test_init(void)
+{
+ struct kmod_struct test;
+ struct kmod_struct_packed test_bis;
+
+ test_kmod_func_ok(0, NULL, 0, 0);
+ test_kmod_func_struct_ok(0, NULL, 0, test);
+ test_kmod_func_struct_on_stack_ok(0, NULL, 0, 0, 0, NULL, 0, 0, test);
+ test_kmod_func_struct_on_stack_ko(0, NULL, 0, 0, 0, NULL, 0, 0, test_bis);
+ return 0;
+}
+
+module_init(kmod_test_init);
+
+MODULE_AUTHOR("Alexis Lothoré");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Pahole testing module");

--
2.50.0