Re: [PATCHv3 bpf-next 7/9] selftests/bpf: Allow to use kfunc from testmod.ko in test_verifier

David Vernet
2023-02-07T15:34:37Z

On Fri, Feb 03, 2023 at 05:23:34PM +0100, Jiri Olsa wrote:
> Currently the test_verifier allows test to specify kfunc symbol
> and search for it in the kernel BTF.
>
> Adding the possibility to search for kfunc also in bpf_testmod
> module when it's not found in kernel BTF.
>
> To find bpf_testmod btf we need to get back SYS_ADMIN cap.

This observation and any subsequent discussion is certainly outside the
scope of your patch set, but it feels like a bit of a weird /
inconsistent UX to force users to have SYS_ADMIN cap for loading kfuncs
from modules, but not from vmlinux BTF.

I realize that you need to have SYS_ADMIN cap for BPF_PROG_GET_FD_BY_ID,
BPF_MAP_GET_FD_BY_ID, etc, so the consistency makes sense there, but it
would be nice if we could eventually make the UX consistent for programs
linking against module kfuncs, because I don't really see the difference
in terms of permissions from the user's perspective.

>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

LGTM in general -- just left one comment below.

Acked-by: David Vernet <void@manifault.com>

> ---
> tools/testing/selftests/bpf/test_verifier.c | 161 +++++++++++++++++---
> 1 file changed, 139 insertions(+), 22 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index 14f11f2dfbce..0a570195be37 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -879,8 +879,140 @@ static int create_map_kptr(void)
> return fd;
> }
>
> +static void set_root(bool set)
> +{
> + __u64 caps;
> +
> + if (set) {
> + if (cap_enable_effective(1ULL << CAP_SYS_ADMIN, &caps))
> + perror("cap_disable_effective(CAP_SYS_ADMIN)");
> + } else {
> + if (cap_disable_effective(1ULL << CAP_SYS_ADMIN, &caps))
> + perror("cap_disable_effective(CAP_SYS_ADMIN)");
> + }
> +}
> +
> +static inline __u64 ptr_to_u64(const void *ptr)
> +{
> + return (__u64) (unsigned long) ptr;

Small nit / suggestion -- IMO this is slightly preferable just to keep
it a bit more in-line with the C-standard:

return (uintptr_t)ptr;

The standard of course doesn't dictate that you can do
ptr -> uintptr_t -> __u64 -> uintptr_t -> ptr, but it at least does dictate that you can do
ptr -> uintptr_t -> ptr, whereas it does not say the same for
ptr -> unsigned long -> ptr

Also, I don't think the 'inline' keyword is necessary. The compiler will
probably figure this out on its own.

> +}
> +
> +static struct btf *btf__load_testmod_btf(struct btf *vmlinux)

Would be nice if some of this code could be shared from libbpf at some
point, but ok, a cleanup for another time.

> +{
> + struct bpf_btf_info info;
> + __u32 len = sizeof(info);
> + struct btf *btf = NULL;
> + char name[64];
> + __u32 id = 0;
> + int err, fd;
> +
> + /* Iterate all loaded BTF objects and find bpf_testmod,
> + * we need SYS_ADMIN cap for that.
> + */
> + set_root(true);
> +
> + while (true) {
> + err = bpf_btf_get_next_id(id, &id);
> + if (err) {
> + if (errno == ENOENT)
> + break;
> + perror("bpf_btf_get_next_id failed");
> + break;
> + }
> +
> + fd = bpf_btf_get_fd_by_id(id);
> + if (fd < 0) {
> + if (errno == ENOENT)
> + continue;
> + perror("bpf_btf_get_fd_by_id failed");
> + break;
> + }
> +
> + memset(&info, 0, sizeof(info));
> + info.name_len = sizeof(name);
> + info.name = ptr_to_u64(name);
> + len = sizeof(info);
> +
> + err = bpf_obj_get_info_by_fd(fd, &info, &len);
> + if (err) {
> + close(fd);
> + perror("bpf_obj_get_info_by_fd failed");
> + break;
> + }
> +
> + if (strcmp("bpf_testmod", name)) {
> + close(fd);
> + continue;
> + }
> +
> + btf = btf__load_from_kernel_by_id_split(id, vmlinux);
> + if (!btf) {
> + close(fd);
> + break;
> + }
> +
> + /* We need the fd to stay open so it can be used in fd_array.
> + * The final cleanup call to btf__free will free btf object
> + * and close the file descriptor.
> + */
> + btf__set_fd(btf, fd);
> + break;
> + }
> +
> + set_root(false);
> + return btf;
> +}
> +
> +static struct btf *testmod_btf;
> +static struct btf *vmlinux_btf;
> +
> +static void kfuncs_cleanup(void)
> +{
> + btf__free(testmod_btf);
> + btf__free(vmlinux_btf);
> +}
> +
> +static void fixup_prog_kfuncs(struct bpf_insn *prog, int *fd_array,
> + struct kfunc_btf_id_pair *fixup_kfunc_btf_id)
> +{
> + /* Patch in kfunc BTF IDs */
> + while (fixup_kfunc_btf_id->kfunc) {
> + int btf_id = 0;
> +
> + /* try to find kfunc in kernel BTF */
> + vmlinux_btf = vmlinux_btf ?: btf__load_vmlinux_btf();
> + if (vmlinux_btf) {
> + btf_id = btf__find_by_name_kind(vmlinux_btf,
> + fixup_kfunc_btf_id->kfunc,
> + BTF_KIND_FUNC);
> + btf_id = btf_id < 0 ? 0 : btf_id;
> + }
> +
> + /* kfunc not found in kernel BTF, try bpf_testmod BTF */
> + if (!btf_id) {
> + testmod_btf = testmod_btf ?: btf__load_testmod_btf(vmlinux_btf);
> + if (testmod_btf) {
> + btf_id = btf__find_by_name_kind(testmod_btf,
> + fixup_kfunc_btf_id->kfunc,
> + BTF_KIND_FUNC);
> + btf_id = btf_id < 0 ? 0 : btf_id;
> + if (btf_id) {
> + /* We put bpf_testmod module fd into fd_array
> + * and its index 1 into instruction 'off'.
> + */
> + *fd_array = btf__fd(testmod_btf);
> + prog[fixup_kfunc_btf_id->insn_idx].off = 1;
> + }
> + }
> + }
> +
> + prog[fixup_kfunc_btf_id->insn_idx].imm = btf_id;
> + fixup_kfunc_btf_id++;
> + }
> +}
> +
> static void do_test_fixup(struct bpf_test *test, enum bpf_prog_type prog_type,
> - struct bpf_insn *prog, int *map_fds)
> + struct bpf_insn *prog, int *map_fds, int *fd_array)
> {
> int *fixup_map_hash_8b = test->fixup_map_hash_8b;
> int *fixup_map_hash_48b = test->fixup_map_hash_48b;
> @@ -905,7 +1037,6 @@ static void do_test_fixup(struct bpf_test *test, enum bpf_prog_type prog_type,
> int *fixup_map_ringbuf = test->fixup_map_ringbuf;
> int *fixup_map_timer = test->fixup_map_timer;
> int *fixup_map_kptr = test->fixup_map_kptr;
> - struct kfunc_btf_id_pair *fixup_kfunc_btf_id = test->fixup_kfunc_btf_id;
>
> if (test->fill_helper) {
> test->fill_insns = calloc(MAX_TEST_INSNS, sizeof(struct bpf_insn));
> @@ -1106,25 +1237,7 @@ static void do_test_fixup(struct bpf_test *test, enum bpf_prog_type prog_type,
> } while (*fixup_map_kptr);
> }
>
> - /* Patch in kfunc BTF IDs */
> - if (fixup_kfunc_btf_id->kfunc) {
> - struct btf *btf;
> - int btf_id;
> -
> - do {
> - btf_id = 0;
> - btf = btf__load_vmlinux_btf();
> - if (btf) {
> - btf_id = btf__find_by_name_kind(btf,
> - fixup_kfunc_btf_id->kfunc,
> - BTF_KIND_FUNC);
> - btf_id = btf_id < 0 ? 0 : btf_id;
> - }
> - btf__free(btf);
> - prog[fixup_kfunc_btf_id->insn_idx].imm = btf_id;
> - fixup_kfunc_btf_id++;
> - } while (fixup_kfunc_btf_id->kfunc);
> - }
> + fixup_prog_kfuncs(prog, fd_array, test->fixup_kfunc_btf_id);
> }
>
> struct libcap {
> @@ -1451,6 +1564,7 @@ static void do_test_single(struct bpf_test *test, bool unpriv,
> int run_errs, run_successes;
> int map_fds[MAX_NR_MAPS];
> const char *expected_err;
> + int fd_array[2] = { -1, -1 };
> int saved_errno;
> int fixup_skips;
> __u32 pflags;
> @@ -1464,7 +1578,7 @@ static void do_test_single(struct bpf_test *test, bool unpriv,
> if (!prog_type)
> prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
> fixup_skips = skips;
> - do_test_fixup(test, prog_type, prog, map_fds);
> + do_test_fixup(test, prog_type, prog, map_fds, &fd_array[1]);
> if (test->fill_insns) {
> prog = test->fill_insns;
> prog_len = test->prog_len;
> @@ -1498,6 +1612,8 @@ static void do_test_single(struct bpf_test *test, bool unpriv,
> else
> opts.log_level = DEFAULT_LIBBPF_LOG_LEVEL;
> opts.prog_flags = pflags;
> + if (fd_array[1] != -1)
> + opts.fd_array = &fd_array[0];
>
> if ((prog_type == BPF_PROG_TYPE_TRACING ||
> prog_type == BPF_PROG_TYPE_LSM) && test->kfunc) {
> @@ -1740,6 +1856,7 @@ static int do_test(bool unpriv, unsigned int from, unsigned int to)
> }
>
> unload_bpf_testmod(verbose);
> + kfuncs_cleanup();
>
> printf("Summary: %d PASSED, %d SKIPPED, %d FAILED\n", passes,
> skips, errors);
> --
> 2.39.1
>