netdev - Re: [PATCH bpf-next 2/2] tools/bpf: add log_level to bpf_load_program

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Date:   Tue, 5 Feb 2019 18:26:42 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, kernel-team@...com
Subject: Re: [PATCH bpf-next 2/2] tools/bpf: add log_level to
 bpf_load_program_attr

On Tue, Feb 05, 2019 at 11:48:23AM -0800, Yonghong Song wrote:
> The kernel verifier has three levels of logs:
>     0: no logs
>     1: logs mostly useful
>   > 1: verbose
> 
> Current libbpf API functions bpf_load_program_xattr() and
> bpf_load_program() cannot specify log_level.
> The bcc, however, provides an interface for user to
> specify log_level 2 for verbose output.
> 
> This patch added log_level into structure
> bpf_load_program_attr, so users, including bcc, can use
> bpf_load_program_xattr() to change log_level.
> 
> The bpf selftest test_sock.c is modified to enable log_level = 2.
> If the "verbose" in test_sock.c is changed to true,
> the test will output logs like below:
>   $ ./test_sock
>   func#0 @0
>   0: R1=ctx(id=0,off=0,imm=0) R10=fp0,call_-1
>   0: (bf) r6 = r1
>   1: R1=ctx(id=0,off=0,imm=0) R6_w=ctx(id=0,off=0,imm=0) R10=fp0,call_-1
>   1: (61) r7 = *(u32 *)(r6 +28)
>   invalid bpf_context access off=28 size=4
> 
>   Test case: bind4 load with invalid access: src_ip6 .. [PASS]
>   ...
>   Test case: bind6 allow all .. [PASS]
>   Summary: 16 PASSED, 0 FAILED
> 
> Some test_sock tests are negative tests and verbose verifier
> log will be printed out as shown in the above.
> 
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  tools/lib/bpf/bpf.c                     | 4 +++-
>  tools/lib/bpf/bpf.h                     | 1 +
>  tools/testing/selftests/bpf/test_sock.c | 9 ++++++++-
>  3 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index 3defad77dc7a..78aa8c2b1a5c 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -214,6 +214,7 @@ int bpf_load_program_xattr(const struct bpf_load_program_attr *load_attr,
>  {
>  	void *finfo = NULL, *linfo = NULL;
>  	union bpf_attr attr;
> +	__u32 log_level;
>  	__u32 name_len;
>  	int fd;
>  
> @@ -292,7 +293,8 @@ int bpf_load_program_xattr(const struct bpf_load_program_attr *load_attr,
>  	/* Try again with log */
>  	attr.log_buf = ptr_to_u64(log_buf);
>  	attr.log_size = log_buf_sz;
> -	attr.log_level = 1;
> +	log_level = load_attr->log_level;
> +	attr.log_level = (log_level >= 2) ? log_level : 1;
>  	log_buf[0] = 0;
>  	fd = sys_bpf_prog_load(&attr, sizeof(attr));
>  done:
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index ed09eed2dc3b..15a8e22e8eae 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -76,6 +76,7 @@ struct bpf_load_program_attr {
>  	const struct bpf_insn *insns;
>  	size_t insns_cnt;
>  	const char *license;
> +	__u32 log_level;
>  	__u32 kern_version;
>  	__u32 prog_ifindex;
>  	__u32 prog_btf_fd;

this will break binary compatibility in libbpf api.
Please add new fields always to the end of *_attr structs.

Also why not to silence bcc instead?
Let it treat log_level > 1 as log_level=1
I don't think anyone but the most extreme verifier hacker used level=2.
Personally I don't remember when was the last time I used it.
It seem like a niche feature that we can safely remove in bcc.