linux-kernel - Re: [PATCH] libbpf: Avoid allocating reg_name with sscanf in parse_usdt

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Date:   Tue, 18 Oct 2022 11:32:46 -0700
From:   sdf@...gle.com
To:     Xu Kuohai <xukuohai@...weicloud.com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH] libbpf: Avoid allocating reg_name with sscanf in parse_usdt_arg()

On 10/18, Xu Kuohai wrote:
> From: Xu Kuohai <xukuohai@...wei.com>

> The reg_name in parse_usdt_arg() is used to hold register name, which
> is short enough to be held in a 16-byte array, so we could define
> reg_name as char reg_name[16] to avoid dynamically allocating reg_name
> with sscanf.

> Suggested-by: Andrii Nakryiko <andrii.nakryiko@...il.com>
> Signed-off-by: Xu Kuohai <xukuohai@...wei.com>

Addresses Andrii's suggestion from the following:

https://lore.kernel.org/bpf/86c88c01-22eb-b7f8-9c65-0faf97b4096b@huawei.com/

Acked-by: Stanislav Fomichev <sdf@...gle.com>

> ---
>   tools/lib/bpf/usdt.c | 16 ++++++----------
>   1 file changed, 6 insertions(+), 10 deletions(-)

> diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> index 49f3c3b7f609..28fa1b2283de 100644
> --- a/tools/lib/bpf/usdt.c
> +++ b/tools/lib/bpf/usdt.c
> @@ -1225,26 +1225,24 @@ static int calc_pt_regs_off(const char *reg_name)

>   static int parse_usdt_arg(const char *arg_str, int arg_num, struct  
> usdt_arg_spec *arg)
>   {
> -	char *reg_name = NULL;
> +	char reg_name[16];
>   	int arg_sz, len, reg_off;
>   	long off;

> -	if (sscanf(arg_str, " %d @ %ld ( %%%m[^)] ) %n", &arg_sz, &off,  
> &reg_name, &len) == 3) {
> +	if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", &arg_sz, &off,  
> reg_name, &len) == 3) {
>   		/* Memory dereference case, e.g., -4@-20(%rbp) */
>   		arg->arg_type = USDT_ARG_REG_DEREF;
>   		arg->val_off = off;
>   		reg_off = calc_pt_regs_off(reg_name);
> -		free(reg_name);
>   		if (reg_off < 0)
>   			return reg_off;
>   		arg->reg_off = reg_off;
> -	} else if (sscanf(arg_str, " %d @ %%%ms %n", &arg_sz, &reg_name, &len)  
> == 2) {
> +	} else if (sscanf(arg_str, " %d @ %%%15s %n", &arg_sz, reg_name, &len)  
> == 2) {
>   		/* Register read case, e.g., -4@...x */
>   		arg->arg_type = USDT_ARG_REG;
>   		arg->val_off = 0;

>   		reg_off = calc_pt_regs_off(reg_name);
> -		free(reg_name);
>   		if (reg_off < 0)
>   			return reg_off;
>   		arg->reg_off = reg_off;
> @@ -1456,16 +1454,15 @@ static int calc_pt_regs_off(const char *reg_name)

>   static int parse_usdt_arg(const char *arg_str, int arg_num, struct  
> usdt_arg_spec *arg)
>   {
> -	char *reg_name = NULL;
> +	char reg_name[16];
>   	int arg_sz, len, reg_off;
>   	long off;

> -	if (sscanf(arg_str, " %d @ %ld ( %m[a-z0-9] ) %n", &arg_sz, &off,  
> &reg_name, &len) == 3) {
> +	if (sscanf(arg_str, " %d @ %ld ( %15[a-z0-9] ) %n", &arg_sz, &off,  
> reg_name, &len) == 3) {
>   		/* Memory dereference case, e.g., -8@-88(s0) */
>   		arg->arg_type = USDT_ARG_REG_DEREF;
>   		arg->val_off = off;
>   		reg_off = calc_pt_regs_off(reg_name);
> -		free(reg_name);
>   		if (reg_off < 0)
>   			return reg_off;
>   		arg->reg_off = reg_off;
> @@ -1474,12 +1471,11 @@ static int parse_usdt_arg(const char *arg_str,  
> int arg_num, struct usdt_arg_spec
>   		arg->arg_type = USDT_ARG_CONST;
>   		arg->val_off = off;
>   		arg->reg_off = 0;
> -	} else if (sscanf(arg_str, " %d @ %m[a-z0-9] %n", &arg_sz, &reg_name,  
> &len) == 2) {
> +	} else if (sscanf(arg_str, " %d @ %15[a-z0-9] %n", &arg_sz, reg_name,  
> &len) == 2) {
>   		/* Register read case, e.g., -8@a1 */
>   		arg->arg_type = USDT_ARG_REG;
>   		arg->val_off = 0;
>   		reg_off = calc_pt_regs_off(reg_name);
> -		free(reg_name);
>   		if (reg_off < 0)
>   			return reg_off;
>   		arg->reg_off = reg_off;
> --
> 2.30.2