Re: [PATCH v3 2/3] tests: add some tests validating skipped functions due to uncertain arg location

Alexis Lothoré
2025-07-07T14:15:01Z

On Mon Jul 7, 2025 at 4:02 PM CEST, Alexis Lothoré (eBPF Foundation) wrote:
> Add a small binary representing specific cases likely absent from
> standard vmlinux or kernel modules files. As a starter, the introduced
> binary exposes a few functions consuming structs passed by value, some
> passed by register, some passed on the stack:
>
> int main(void);
> int test_bin_func_struct_on_stack_ko(int, void *, char, short int, int, \
> void *, char, short int, struct test_bin_struct_packed);
> int test_bin_func_struct_on_stack_ok(int, void *, char, short int, int, \
> void *, char, short int, struct test_bin_struct);
> int test_bin_func_struct_ok(int, void *, char, struct test_bin_struct);
> int test_bin_func_ok(int, void *, char, short int);
>
> Then enrich btf_functions.sh to make it perform the following steps:
> - build the binary
> - 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
>
> 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...
> pahole: /home/alexis/src/pahole/tests/bin/test_bin: Invalid argument

A word about this specific error: I may have missed it in the previous
iteration, but I systematically get this error when running the following
command:
$ pahole -C test_bin_struct_packed tests/bin/test_bin

I initially thought that it would be something related to the binary being
a userspace program and not a kernel module, but I observe the following:
- the issue is observed even on a .ko file (tested on the previous series
iteration with kmod.ko)
- the issue does not appear if there is no class filtering (ie the `-C`
arg) provided to pahole
- the issue occurs as well with the packaged pahole version on my host (v1.30)
- the struct layout is still displayed correctly despite the error

A quick bisect shows that the error log has started appearing with
59f5409f1357 ("dwarf_loader: Fix termination on BTF encoding error"). This
commit has "enforced" error propagation if dwfl_getmodules returns
something different than 0 (before, it was propagating an error only if the
error code was negative, but dwfl_getmodules seems to be able to return
values > 0 as well). As is sound unrelated to this series, I pushed this
new revision anyway. [1] seems to hint that the issue is known, but in my
case I don't get any additional log about unhandled DWARF operation. The
issue is pretty repeatable on my side, feel free to ask for any additional
detail or manipulation that could help.

[1] https://lore.kernel.org/dwarves/933e199997949c0ac8a71551830f1e6c98d8bff0@linux.dev/
> Found 1 legitimately skipped function due to uncertain loc
> Ok
>
> Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
> ---
> Changes in v3:
> - bring a userspace binary instead of an OoT kernel module
> - remove test dependency to a kernel directory being provided
> - improve test dir detection
>
> Changes in v2:
> - new patch
> ---
> tests/bin/Makefile | 10 ++++++
> tests/bin/test_bin.c | 66 ++++++++++++++++++++++++++++++++++++
> tests/btf_functions.sh | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 167 insertions(+)
>
> diff --git a/tests/bin/Makefile b/tests/bin/Makefile
> new file mode 100644
> index 0000000000000000000000000000000000000000..70bcf57ac4744f30fe03ea12908e42c69390f14a
> --- /dev/null
> +++ b/tests/bin/Makefile
> @@ -0,0 +1,10 @@
> +CC=${CROSS_COMPILE}gcc
> +
> +test_bin: test_bin.c
> + ${CC} $^ -Wall -Wextra -Werror -g -o $@
> +
> +clean:
> + rm -rf test_bin
> +
> +.PHONY: clean
> +
> diff --git a/tests/bin/test_bin.c b/tests/bin/test_bin.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..ca6a4852cc511243925db905e55e040519af9cfd
> --- /dev/null
> +++ b/tests/bin/test_bin.c
> @@ -0,0 +1,66 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <stdio.h>
> +
> +#define noinline __attribute__((noinline))
> +#define __packed __attribute__((__packed__))
> +
> +struct test_bin_struct {
> + char a;
> + short b;
> + int c;
> + unsigned long long d;
> +};
> +
> +struct test_bin_struct_packed {
> + char a;
> + short b;
> + int c;
> + unsigned long long d;
> +}__packed;
> +
> +int test_bin_func_ok(int a, void *b, char c, short d);
> +int test_bin_func_struct_ok(int a, void *b, char c, struct test_bin_struct d);
> +int test_bin_func_struct_on_stack_ok(int a, void *b, char c, short d, int e,
> + void *f, char g, short h,
> + struct test_bin_struct i);
> +int test_bin_func_struct_on_stack_ko(int a, void *b, char c, short d, int e,
> + void *f, char g, short h,
> + struct test_bin_struct_packed i);
> +
> +noinline int test_bin_func_ok(int a, void *b, char c, short d)
> +{
> + return a + (long)b + c + d;
> +}
> +
> +noinline int test_bin_func_struct_ok(int a, void *b, char c,
> + struct test_bin_struct d)
> +{
> + return a + (long)b + c + d.a + d.b + d.c + d.d;
> +}
> +
> +noinline int test_bin_func_struct_on_stack_ok(int a, void *b, char c, short d,
> + int e, void *f, char g, short h,
> + struct test_bin_struct i)
> +{
> + return a + (long)b + c + d + e + (long)f + g + h + i.a + i.b + i.c + i.d;
> +}
> +
> +noinline int test_bin_func_struct_on_stack_ko(int a, void *b, char c, short d,
> + int e, void *f, char g, short h,
> + struct test_bin_struct_packed i)
> +{
> + return a + (long)b + c + d + e + (long)f + g + h + i.a + i.b + i.c + i.d;
> +}
> +
> +int main()
> +{
> + struct test_bin_struct test;
> + struct test_bin_struct_packed test_bis;
> +
> + test_bin_func_ok(0, NULL, 0, 0);
> + test_bin_func_struct_ok(0, NULL, 0, test);
> + test_bin_func_struct_on_stack_ok(0, NULL, 0, 0, 0, NULL, 0, 0, test);
> + test_bin_func_struct_on_stack_ko(0, NULL, 0, 0, 0, NULL, 0, 0, test_bis);
> + return 0;
> +}
> +
> diff --git a/tests/btf_functions.sh b/tests/btf_functions.sh
> index c92e5ae906f90badfede86eb530108894fbc8c93..fb62b0b56662bb2ae58f7adc0a022c400cba5e0f 100755
> --- a/tests/btf_functions.sh
> +++ b/tests/btf_functions.sh
> @@ -193,4 +193,95 @@ if [[ -n "$VERBOSE" ]]; then
> fi
> echo "Ok"
>
> +# Some specific cases can not be tested directly with a standard kernel.
> +# We can use the small binary in bin/ to test those cases, like packed
> +# structs passed on the stack.
> +
> +echo -n "Validation of BTF encoding corner cases with test_bin functions; this may take some time: "
> +
> +test -n "$VERBOSE" && printf "\nBuilding test_bin..."
> +tests_dir=$(realpath $(dirname $0))
> +make -C ${tests_dir}/bin
> +
> +test -n "$VERBOSE" && printf "\nEncoding..."
> +pahole --btf_features=default --lang_exclude=rust --btf_encode_detached=$outdir/test_bin.btf \
> + --verbose ${tests_dir}/bin/test_bin | grep "skipping BTF encoding of function" \
> + > ${outdir}/test_bin_skipped_fns
> +
> +funcs=$(pfunct --format_path=btf $outdir/test_bin.btd 2>/dev/null|sort)
> +pfunct --all --no_parm_names --format_path=dwarf bin/test_bin | \
> + sort|uniq > $outdir/test_bin_dwarf.funcs
> +pfunct --all --no_parm_names --format_path=btf $outdir/test_bin.btf 2>/dev/null|\
> + awk '{ gsub("^(bpf_kfunc |bpf_fastcall )+",""); print $0}'|sort|uniq > $outdir/test_bin_btf.funcs
> +
> +exact=0
> +while IFS= read -r btf ; do
> + # Matching process can be kept simpler as the tested binary is
> + # specifically tailored for tests
> + dwarf=$(grep -F "$btf" $outdir/test_bin_dwarf.funcs)
> + if [[ "$btf" != "$dwarf" ]]; then
> + echo "ERROR: mismatch : BTF '$btf' not found; DWARF '$dwarf'"
> + fail
> + else
> + exact=$((exact+1))
> + fi
> +done < $outdir/test_bin_btf.funcs
> +
> +if [[ -n "$VERBOSE" ]]; then
> + echo "Matched $exact functions exactly."
> + echo "Ok"
> + echo "Validation of skipped function logic..."
> +fi
> +
> +skipped_cnt=$(wc -l ${outdir}/test_bin_skipped_fns | awk '{ print $1}')
> +if [[ "$skipped_cnt" == "0" ]]; then
> + echo "No skipped functions. Done."
> + exit 0
> +fi
> +
> +skipped_fns=$(awk '{print $1}' $outdir/test_bin_skipped_fns)
> +for s in $skipped_fns ; do
> + # Ensure the skipped function are not in BTF
> + inbtf=$(grep " $s(" $outdir/test_bin_btf.funcs)
> + if [[ -n "$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/test_bin_skipped_fns)
> +legitimate_skip=0
> +
> +for f in $uncertain_loc ; do
> + # Extract parameters types
> + raw_params=$(grep ${f} $outdir/test_bin_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 -F dwarf -C "${struct_type}" ${tests_dir}/bin/test_bin|tail -n 2|grep -q __packed__
> + then
> + legitimate_skip=$((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

--
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering