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Message-ID: <CACT4Y+YSfOE6Y9y-8mUwUOyyE-L3PUHUr6PuNX=iu-zyMyv3=A@mail.gmail.com>
Date: Wed, 9 Jul 2025 17:15:39 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Alexander Potapenko <glider@...gle.com>
Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org,
kasan-dev@...glegroups.com, Aleksandr Nogikh <nogikh@...gle.com>,
Andrey Konovalov <andreyknvl@...il.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
Josh Poimboeuf <jpoimboe@...nel.org>, Marco Elver <elver@...gle.com>,
Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 10/11] kcov: selftests: add kcov_test
On Thu, 26 Jun 2025 at 15:42, Alexander Potapenko <glider@...gle.com> wrote:
>
> Implement test fixtures for testing different combinations of coverage
> collection modes:
> - unique and non-unique coverage;
> - collecting PCs and comparison arguments;
> - mapping the buffer as RO and RW.
>
> To build:
> $ make -C tools/testing/selftests/kcov kcov_test
>
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
> MAINTAINERS | 1 +
> tools/testing/selftests/kcov/Makefile | 6 +
> tools/testing/selftests/kcov/kcov_test.c | 364 +++++++++++++++++++++++
Let's also add 'config' fragment (see e.g. ./dma/config)
Otherwise it's impossible to run these tests in automated fashion.
> 3 files changed, 371 insertions(+)
> create mode 100644 tools/testing/selftests/kcov/Makefile
> create mode 100644 tools/testing/selftests/kcov/kcov_test.c
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 5bbc78b0fa6ed..0ec909e085077 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -12833,6 +12833,7 @@ F: include/linux/kcov_types.h
> F: include/uapi/linux/kcov.h
> F: kernel/kcov.c
> F: scripts/Makefile.kcov
> +F: tools/testing/selftests/kcov/
>
> KCSAN
> M: Marco Elver <elver@...gle.com>
> diff --git a/tools/testing/selftests/kcov/Makefile b/tools/testing/selftests/kcov/Makefile
> new file mode 100644
> index 0000000000000..08abf8b60bcf9
> --- /dev/null
> +++ b/tools/testing/selftests/kcov/Makefile
> @@ -0,0 +1,6 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +LDFLAGS += -static
> +
> +TEST_GEN_PROGS := kcov_test
> +
> +include ../lib.mk
> diff --git a/tools/testing/selftests/kcov/kcov_test.c b/tools/testing/selftests/kcov/kcov_test.c
> new file mode 100644
> index 0000000000000..4d3ca41f28af4
> --- /dev/null
> +++ b/tools/testing/selftests/kcov/kcov_test.c
> @@ -0,0 +1,364 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Test the kernel coverage (/sys/kernel/debug/kcov).
> + *
> + * Copyright 2025 Google LLC.
> + */
> +#include <fcntl.h>
> +#include <linux/kcov.h>
> +#include <stdint.h>
> +#include <stddef.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <sys/ioctl.h>
> +#include <sys/mman.h>
> +#include <sys/types.h>
> +#include <unistd.h>
> +
> +#include "../kselftest_harness.h"
> +
> +/* Normally these defines should be provided by linux/kcov.h, but they aren't there yet. */
Then I think we need to do:
#ifndef KCOV_UNIQUE_ENABLE
> +#define KCOV_UNIQUE_ENABLE _IOW('c', 103, unsigned long)
> +#define KCOV_RESET_TRACE _IO('c', 104)
> +
> +#define COVER_SIZE (64 << 10)
> +#define BITMAP_SIZE (4 << 10)
> +
> +#define DEBUG_COVER_PCS 0
> +
> +FIXTURE(kcov)
> +{
> + int fd;
> + unsigned long *mapping;
> + size_t mapping_size;
> +};
> +
> +FIXTURE_VARIANT(kcov)
> +{
> + int mode;
> + bool fast_reset;
> + bool map_readonly;
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(kcov, mode_trace_pc)
> +{
> + /* clang-format on */
> + .mode = KCOV_TRACE_PC,
> + .fast_reset = true,
> + .map_readonly = false,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(kcov, mode_trace_cmp)
> +{
> + /* clang-format on */
> + .mode = KCOV_TRACE_CMP,
> + .fast_reset = true,
> + .map_readonly = false,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(kcov, reset_ioctl_rw)
> +{
> + /* clang-format on */
> + .mode = KCOV_TRACE_PC,
> + .fast_reset = false,
> + .map_readonly = false,
> +};
> +
> +FIXTURE_VARIANT_ADD(kcov, reset_ioctl_ro)
> +/* clang-format off */
> +{
> + /* clang-format on */
> + .mode = KCOV_TRACE_PC,
> + .fast_reset = false,
> + .map_readonly = true,
> +};
> +
> +int kcov_open_init(struct __test_metadata *_metadata, unsigned long size,
> + int prot, unsigned long **out_mapping)
> +{
> + unsigned long *mapping;
> +
> + /* A single fd descriptor allows coverage collection on a single thread. */
> + int fd = open("/sys/kernel/debug/kcov", O_RDWR);
> +
> + ASSERT_NE(fd, -1)
> + {
> + perror("open");
> + }
> +
> + EXPECT_EQ(ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE), 0)
> + {
> + perror("ioctl KCOV_INIT_TRACE");
> + close(fd);
> + }
> +
> + /* Mmap buffer shared between kernel- and user-space. */
> + mapping = (unsigned long *)mmap(NULL, size * sizeof(unsigned long),
> + prot, MAP_SHARED, fd, 0);
> + ASSERT_NE((void *)mapping, MAP_FAILED)
> + {
> + perror("mmap");
> + close(fd);
> + }
> + *out_mapping = mapping;
> +
> + return fd;
> +}
> +
> +FIXTURE_SETUP(kcov)
> +{
> + int prot = variant->map_readonly ? PROT_READ : (PROT_READ | PROT_WRITE);
> +
> + /* Read-only mapping is incompatible with fast reset. */
> + ASSERT_FALSE(variant->map_readonly && variant->fast_reset);
> +
> + self->mapping_size = COVER_SIZE;
> + self->fd = kcov_open_init(_metadata, self->mapping_size, prot,
> + &(self->mapping));
> +
> + /* Enable coverage collection on the current thread. */
> + EXPECT_EQ(ioctl(self->fd, KCOV_ENABLE, variant->mode), 0)
> + {
> + perror("ioctl KCOV_ENABLE");
> + munmap(self->mapping, COVER_SIZE * sizeof(unsigned long));
> + close(self->fd);
> + }
> +}
> +
> +void kcov_uninit_close(struct __test_metadata *_metadata, int fd,
> + unsigned long *mapping, size_t size)
> +{
> + /* Disable coverage collection for the current thread. */
> + EXPECT_EQ(ioctl(fd, KCOV_DISABLE, 0), 0)
> + {
> + perror("ioctl KCOV_DISABLE");
> + }
> +
> + /* Free resources. */
> + EXPECT_EQ(munmap(mapping, size * sizeof(unsigned long)), 0)
> + {
> + perror("munmap");
> + }
> +
> + EXPECT_EQ(close(fd), 0)
> + {
> + perror("close");
> + }
> +}
> +
> +FIXTURE_TEARDOWN(kcov)
> +{
> + kcov_uninit_close(_metadata, self->fd, self->mapping,
> + self->mapping_size);
> +}
> +
> +void dump_collected_pcs(struct __test_metadata *_metadata, unsigned long *cover,
> + size_t start, size_t end)
> +{
> + int i = 0;
> +
> + TH_LOG("Collected %lu PCs", end - start);
> +#if DEBUG_COVER_PCS
> + for (i = start; i < end; i++)
> + TH_LOG("0x%lx", cover[i + 1]);
> +#endif
> +}
> +
> +/* Coverage collection helper without assertions. */
> +unsigned long collect_coverage_unchecked(struct __test_metadata *_metadata,
> + unsigned long *cover, bool dump)
> +{
> + unsigned long before, after;
> +
> + before = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> + /*
> + * Call the target syscall call. Here we use read(-1, NULL, 0) as an example.
> + * This will likely return an error (-EFAULT or -EBADF), but the goal is to
> + * collect coverage for the syscall's entry/exit paths.
> + */
> + read(-1, NULL, 0);
> +
> + after = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> +
> + if (dump)
> + dump_collected_pcs(_metadata, cover, before, after);
> + return after - before;
> +}
> +
> +unsigned long collect_coverage_once(struct __test_metadata *_metadata,
> + unsigned long *cover)
> +{
> + unsigned long collected =
> + collect_coverage_unchecked(_metadata, cover, /*dump*/ true);
> +
> + /* Coverage must be non-zero. */
> + EXPECT_GT(collected, 0);
> + return collected;
> +}
> +
> +void reset_coverage(struct __test_metadata *_metadata, bool fast, int fd,
> + unsigned long *mapping)
> +{
> + unsigned long count;
> +
> + if (fast) {
> + __atomic_store_n(&mapping[0], 0, __ATOMIC_RELAXED);
> + } else {
> + EXPECT_EQ(ioctl(fd, KCOV_RESET_TRACE, 0), 0)
> + {
> + perror("ioctl KCOV_RESET_TRACE");
> + }
> + count = __atomic_load_n(&mapping[0], __ATOMIC_RELAXED);
> + EXPECT_NE(count, 0);
> + }
> +}
> +
> +TEST_F(kcov, kcov_basic_syscall_coverage)
> +{
> + unsigned long first, second, before, after, i;
> +
> + /* Reset coverage that may be left over from the fixture setup. */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> +
> + /* Collect the coverage for a single syscall two times in a row. */
> + first = collect_coverage_once(_metadata, self->mapping);
> + second = collect_coverage_once(_metadata, self->mapping);
> + /* Collected coverage should not differ too much. */
> + EXPECT_GT(first * 10, second);
> + EXPECT_GT(second * 10, first);
> +
> + /* Now reset the buffer and collect the coverage again. */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> + collect_coverage_once(_metadata, self->mapping);
> +
> + /* Now try many times to fill up the buffer. */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> + while (collect_coverage_unchecked(_metadata, self->mapping,
> + /*dump*/ false)) {
> + /* Do nothing. */
> + }
> + before = __atomic_load_n(&(self->mapping[0]), __ATOMIC_RELAXED);
> + /*
> + * Resetting with ioctl may still generate some coverage, but much less
> + * than there was before.
> + */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> + after = __atomic_load_n(&(self->mapping[0]), __ATOMIC_RELAXED);
> + EXPECT_GT(before, after);
> + /* Collecting coverage after reset will now succeed. */
> + collect_coverage_once(_metadata, self->mapping);
> +}
> +
> +FIXTURE(kcov_uniq)
> +{
> + int fd;
> + unsigned long *mapping;
> + size_t mapping_size;
> + unsigned long *bitmap;
> + size_t bitmap_size;
> + unsigned long *cover;
> + size_t cover_size;
> +};
> +
> +FIXTURE_VARIANT(kcov_uniq)
> +{
> + bool fast_reset;
> + bool map_readonly;
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(kcov_uniq, fast_rw)
> +{
> + /* clang-format on */
> + .fast_reset = true,
> + .map_readonly = false,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(kcov_uniq, slow_rw)
> +{
> + /* clang-format on */
> + .fast_reset = false,
> + .map_readonly = false,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(kcov_uniq, slow_ro)
> +{
> + /* clang-format on */
> + .fast_reset = false,
> + .map_readonly = true,
> +};
> +
> +FIXTURE_SETUP(kcov_uniq)
> +{
> + int prot = variant->map_readonly ? PROT_READ : (PROT_READ | PROT_WRITE);
> +
> + /* Read-only mapping is incompatible with fast reset. */
> + ASSERT_FALSE(variant->map_readonly && variant->fast_reset);
> +
> + self->mapping_size = COVER_SIZE;
> + self->fd = kcov_open_init(_metadata, self->mapping_size, prot,
> + &(self->mapping));
> +
> + /* Enable coverage collection on the current thread. */
> + EXPECT_EQ(ioctl(self->fd, KCOV_UNIQUE_ENABLE, BITMAP_SIZE), 0)
> + {
> + perror("ioctl KCOV_ENABLE");
> + munmap(self->mapping, COVER_SIZE * sizeof(unsigned long));
> + close(self->fd);
> + }
> +}
> +
> +FIXTURE_TEARDOWN(kcov_uniq)
> +{
> + kcov_uninit_close(_metadata, self->fd, self->mapping,
> + self->mapping_size);
> +}
> +
> +TEST_F(kcov_uniq, kcov_uniq_coverage)
> +{
> + unsigned long first, second, before, after, i;
> +
> + /* Reset coverage that may be left over from the fixture setup. */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> +
> + /*
> + * Collect the coverage for a single syscall two times in a row.
> + * Use collect_coverage_unchecked(), because it may return zero coverage.
> + */
> + first = collect_coverage_unchecked(_metadata, self->mapping,
> + /*dump*/ true);
> + second = collect_coverage_unchecked(_metadata, self->mapping,
> + /*dump*/ true);
> +
> + /* Now reset the buffer and collect the coverage again. */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> + collect_coverage_once(_metadata, self->mapping);
> +
> + /* Now try many times to saturate the unique coverage bitmap. */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> + for (i = 0; i < 1000; i++)
> + collect_coverage_unchecked(_metadata, self->mapping,
> + /*dump*/ false);
> + /* Another invocation of collect_coverage_unchecked() should not produce new coverage. */
> + EXPECT_EQ(collect_coverage_unchecked(_metadata, self->mapping,
> + /*dump*/ false),
> + 0);
> +
> + before = __atomic_load_n(&(self->mapping[0]), __ATOMIC_RELAXED);
> + /*
> + * Resetting with ioctl may still generate some coverage, but much less
> + * than there was before.
> + */
> + reset_coverage(_metadata, variant->fast_reset, self->fd, self->mapping);
> + after = __atomic_load_n(&(self->mapping[0]), __ATOMIC_RELAXED);
> + EXPECT_GT(before, after);
> + /* Collecting coverage after reset will now succeed. */
> + collect_coverage_once(_metadata, self->mapping);
> +}
> +
> +TEST_HARNESS_MAIN
> --
> 2.50.0.727.gbf7dc18ff4-goog
>
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