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

Ihor Solodrai
2025-07-03T18:31:17Z

On 7/3/25 2:02 AM, Alexis LothorÃ© (eBPF Foundation) wrote:
> 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.

While this shouldn't be a problem for CI, since it checks out a kernel
tree to test vmlinux as input, I wonder if there is a way to do the
same test without this dependency.

We need to generate a binary with DWARF, containing function
prototypes with packed/aligned attributes. Give it to pahole and see
that those functions were skipped.

Any reason it must be a kernel module? Am I missing something?

>
> 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

This part fails for me:

isolodrai@isolodrai-fedora-PC2K40WQ:~/pahole/tests$
KDIR=/home/isolodrai/kernels/bpf-next
vmlinux=/home/isolodrai/kernels/bpf-next/vmlinux ./btf_functions.sh
Validation of BTF encoding of functions; this may take some time: Ok
Validation of BTF encoding corner cases with kmod functions; this may
take some time: make: Entering directory '/home/isolodrai/kernels/bpf-next'
Makefile:199: *** specified external module directory "./kmod" does not
exist. Stop.
make: Leaving directory '/home/isolodrai/kernels/bpf-next'
No skipped functions. Done.

Maybe:

diff --git a/tests/btf_functions.sh b/tests/btf_functions.sh
index 64810b7..fcb1591 100755
--- a/tests/btf_functions.sh
+++ b/tests/btf_functions.sh
@@ -208,7 +208,7 @@ 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 $0)
+tests_dir=$(realpath $(dirname $0))
make -C ${KDIR} M=${tests_dir}/kmod

test -n "$VERBOSE" && printf "\nEncoding..."

Also, in case kernel is built with LLVM, one must set LLVM=1.
Not sure if this is detectable by the test.

> +
> +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");
>