Re: [PATCH v8 tip 5/9] tracing: allow BPF programs to call bpf_trace_printk()

[Steven Rostedt]

On Thu, 19 Mar 2015 18:59:43 -0700
Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:

> Debugging of BPF programs needs some form of printk from the program,
> so let programs call limited trace_printk() with %d %u %x %p modifiers only.
> 
> Similar to kernel modules, during program load verifier checks whether program
> is calling bpf_trace_printk() and if so, kernel allocates trace_printk buffers
> and emits big 'this is debug only' banner.
> 
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> ---
>  include/uapi/linux/bpf.h |    1 +
>  kernel/trace/bpf_trace.c |   75 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 76 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 238c6883877b..cc47ef41076a 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -166,6 +166,7 @@ enum bpf_func_id {
>  	BPF_FUNC_map_delete_elem, /* int map_delete_elem(&map, &key) */
>  	BPF_FUNC_probe_read,      /* int bpf_probe_read(void *dst, int size, void *src) */
>  	BPF_FUNC_ktime_get_ns,    /* u64 bpf_ktime_get_ns(void) */
> +	BPF_FUNC_trace_printk,    /* int bpf_trace_printk(const char *fmt, int fmt_size, ...) */
>  	__BPF_FUNC_MAX_ID,
>  };
>  
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 769c577a1fc8..d5b6284edd75 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -10,6 +10,7 @@
>  #include <linux/bpf.h>
>  #include <linux/filter.h>
>  #include <linux/uaccess.h>
> +#include <linux/ctype.h>
>  #include "trace.h"
>  
>  static DEFINE_PER_CPU(int, bpf_prog_active);
> @@ -83,6 +84,72 @@ static const struct bpf_func_proto bpf_ktime_get_ns_proto = {
>  	.ret_type = RET_INTEGER,
>  };
>  
> +/* limited trace_printk()
> + * only %d %u %x %ld %lu %lx %lld %llu %llx %p conversion specifiers allowed
> + */

Ah! Again, don't contaminate the rest of the kernel with net comment
styles! :-)

I change my comment style when I submit net patches. So patches that go
into tracing must conform to the tracing style.

> +static u64 bpf_trace_printk(u64 r1, u64 fmt_size, u64 r3, u64 r4, u64 r5)
> +{
> +	char *fmt = (char *) (long) r1;
> +	int mod[3] = {};
> +	int fmt_cnt = 0;
> +	int i;
> +
> +	/* bpf_check()->check_func_arg()->check_stack_boundary()

It spreads like a disease!

> +	 * guarantees that fmt points to bpf program stack,
> +	 * fmt_size bytes of it were initialized and fmt_size > 0
> +	 */
> +	if (fmt[--fmt_size] != 0)
> +		return -EINVAL;
> +
> +	/* check format string for allowed specifiers */
> +	for (i = 0; i < fmt_size; i++) {
> +		if ((!isprint(fmt[i]) && !isspace(fmt[i])) || !isascii(fmt[i]))
> +			return -EINVAL;
> +
> +		if (fmt[i] != '%')
> +			continue;
> +
> +		if (fmt_cnt >= 3)
> +			return -EINVAL;
> +
> +		/* fmt[i] != 0 && fmt[last] == 0, so we can access fmt[i + 1] */
> +		i++;
> +		if (fmt[i] == 'l') {
> +			mod[fmt_cnt]++;
> +			i++;
> +		} else if (fmt[i] == 'p') {
> +			mod[fmt_cnt]++;
> +			i++;
> +			if (!isspace(fmt[i]) && fmt[i] != 0)

I wonder if we should allow punctuation here too? None alpha-numeric
characters?

> +				return -EINVAL;
> +			fmt_cnt++;
> +			continue;
> +		}
> +
> +		if (fmt[i] == 'l') {
> +			mod[fmt_cnt]++;
> +			i++;
> +		}
> +
> +		if (fmt[i] != 'd' && fmt[i] != 'u' && fmt[i] != 'x')
> +			return -EINVAL;
> +		fmt_cnt++;
> +	}
> +
> +	return __trace_printk(1/* fake ip will not be printed */, fmt,
> +			      mod[0] == 2 ? r3 : mod[0] == 1 ? (long) r3 : (u32) r3,
> +			      mod[1] == 2 ? r4 : mod[1] == 1 ? (long) r4 : (u32) r4,
> +			      mod[2] == 2 ? r5 : mod[2] == 1 ? (long) r5 : (u32) r5);

Nice trick.

Except for the net-contaminated-comments, the rest looks good.

Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

-- Steve

> +}
> +
> +static const struct bpf_func_proto bpf_trace_printk_proto = {
> +	.func = bpf_trace_printk,
> +	.gpl_only = true,
> +	.ret_type = RET_INTEGER,
> +	.arg1_type = ARG_PTR_TO_STACK,
> +	.arg2_type = ARG_CONST_STACK_SIZE,
> +};
> +
>  static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func_id)
>  {
>  	switch (func_id) {
> @@ -96,6 +163,14 @@ static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func
>  		return &bpf_probe_read_proto;
>  	case BPF_FUNC_ktime_get_ns:
>  		return &bpf_ktime_get_ns_proto;
> +
> +	case BPF_FUNC_trace_printk:
> +		/* this program might be calling bpf_trace_printk,
> +		 * so allocate per-cpu printk buffers
> +		 */
> +		trace_printk_init_buffers();
> +
> +		return &bpf_trace_printk_proto;
>  	default:
>  		return NULL;
>  	}

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