Re: [PATCH bpf v2 2/2] selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL

[Song Liu]

> On Nov 4, 2020, at 6:25 PM, Daniel Xu <dxu@xxxxxxxxx> wrote:
> 
> Previously, bpf_probe_read_user_str() could potentially overcopy the
> trailing bytes after the NUL due to how do_strncpy_from_user() does the
> copy in long-sized strides. The issue has been fixed in the previous
> commit.
> 
> This commit adds a selftest that ensures we don't regress
> bpf_probe_read_user_str() again.
> 
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
> .../bpf/prog_tests/probe_read_user_str.c      | 60 +++++++++++++++++++
> .../bpf/progs/test_probe_read_user_str.c      | 34 +++++++++++
> 2 files changed, 94 insertions(+)
> create mode 100644 tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> create mode 100644 tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> new file mode 100644
> index 000000000000..597a166e6c8d
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> @@ -0,0 +1,60 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <test_progs.h>
> +#include "test_probe_read_user_str.skel.h"
> +
> +static const char str[] = "mestring";
> +
> +void test_probe_read_user_str(void)
> +{
> +	struct test_probe_read_user_str *skel;
> +	int fd, err, duration = 0;
> +	char buf[256];
> +	ssize_t n;
> +
> +	skel = test_probe_read_user_str__open_and_load();
> +	if (CHECK(!skel, "test_probe_read_user_str__open_and_load",
> +		  "skeleton open and load failed\n"))
> +		goto out;

nit: we can just return here. 

> +
> +	err = test_probe_read_user_str__attach(skel);
> +	if (CHECK(err, "test_probe_read_user_str__attach",
> +		  "skeleton attach failed: %d\n", err))
> +		goto out;
> +
> +	fd = open("/dev/null", O_WRONLY);
> +	if (CHECK(fd < 0, "open", "open /dev/null failed: %d\n", fd))
> +		goto out;
> +
> +	/* Give pid to bpf prog so it doesn't read from anyone else */
> +	skel->bss->pid = getpid();

It is better to set pid before attaching skel. 

> +
> +	/* Ensure bytes after string are ones */
> +	memset(buf, 1, sizeof(buf));
> +	memcpy(buf, str, sizeof(str));
> +
> +	/* Trigger tracepoint */
> +	n = write(fd, buf, sizeof(buf));
> +	if (CHECK(n != sizeof(buf), "write", "write failed: %ld\n", n))
> +		goto fd_out;
> +
> +	/* Did helper fail? */
> +	if (CHECK(skel->bss->ret < 0, "prog ret", "prog returned: %d\n",

In most cases, we use underscore instead of spaces in the second argument 
of CHECK(). IOW, please use "prog_ret" instead of "prog ret". 

> +		  skel->bss->ret))
> +		goto fd_out;
> +
> +	/* Check that string was copied correctly */
> +	err = memcmp(skel->bss->buf, str, sizeof(str));
> +	if (CHECK(err, "memcmp", "prog copied wrong string"))
> +		goto fd_out;
> +
> +	/* Now check that no extra trailing bytes were copied */
> +	memset(buf, 0, sizeof(buf));
> +	err = memcmp(skel->bss->buf + sizeof(str), buf, sizeof(buf) - sizeof(str));
> +	if (CHECK(err, "memcmp", "trailing bytes were not stripped"))
> +		goto fd_out;
> +
> +fd_out:
> +	close(fd);
> +out:
> +	test_probe_read_user_str__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> new file mode 100644
> index 000000000000..41c3e296566e
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> @@ -0,0 +1,34 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +#include <sys/types.h>
> +
> +struct sys_enter_write_args {
> +	unsigned long long pad;
> +	int syscall_nr;
> +	int pad1; /* 4 byte hole */
> +	unsigned int fd;
> +	int pad2; /* 4 byte hole */
> +	const char *buf;
> +	size_t count;
> +};
> +
> +pid_t pid = 0;
> +int ret = 0;
> +char buf[256] = {};
> +
> +SEC("tracepoint/syscalls/sys_enter_write")
> +int on_write(struct sys_enter_write_args *ctx)
> +{
> +	if (pid != (bpf_get_current_pid_tgid() >> 32))
> +		return 0;
> +
> +	ret = bpf_probe_read_user_str(buf, sizeof(buf), ctx->buf);

bpf_probe_read_user_str() returns "long". Let's use "long ret;"

> +
> +	return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> -- 
> 2.28.0
>