Re: [PATCH bpf v3 2/6] libbpf: Fix memory leak in parse_usdt_arg()

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Xu Kuohai <xukuohai@huawei.com>
Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, netdev@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko <mykolal@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Kumar Kartikeya Dwivedi <memxor@gmail.com>,
	Alan Maguire <alan.maguire@oracle.com>,
	Delyan Kratunov <delyank@fb.com>,
	Lorenzo Bianconi <lorenzo@kernel.org>
Subject: Re: [PATCH bpf v3 2/6] libbpf: Fix memory leak in parse_usdt_arg()
Date: Mon, 10 Oct 2022 18:34:33 -0700	[thread overview]
Message-ID: <CAEf4BzbJ8LW1Q_hBc-eB25f=F+jdQ5aPucEv_oDNrbjB=GGR+g@mail.gmail.com> (raw)
In-Reply-To: <20221010142553.776550-3-xukuohai@huawei.com>

On Mon, Oct 10, 2022 at 7:08 AM Xu Kuohai <xukuohai@huawei.com> wrote:
>
> In the arm64 version of parse_usdt_arg(), when sscanf returns 2, reg_name
> is allocated but not freed. Fix it.
>
> Fixes: 0f8619929c57 ("libbpf: Usdt aarch64 arg parsing support")
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
> ---
>  tools/lib/bpf/usdt.c | 59 +++++++++++++++++++++++++-------------------
>  1 file changed, 33 insertions(+), 26 deletions(-)
>
> diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> index e83b497c2245..f3b5be7415b5 100644
> --- a/tools/lib/bpf/usdt.c
> +++ b/tools/lib/bpf/usdt.c
> @@ -1351,8 +1351,10 @@ static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec
>         char *reg_name = NULL;
>         int arg_sz, len, reg_off;
>         long off;
> +       int ret;
>
> -       if (sscanf(arg_str, " %d @ \[ %m[a-z0-9], %ld ] %n", &arg_sz, &reg_name, &off, &len) == 3) {
> +       ret = sscanf(arg_str, " %d @ \[ %m[a-z0-9], %ld ] %n", &arg_sz, &reg_name, &off, &len);
> +       if (ret == 3) {
>                 /* Memory dereference case, e.g., -4@[sp, 96] */
>                 arg->arg_type = USDT_ARG_REG_DEREF;
>                 arg->val_off = off;
> @@ -1361,32 +1363,37 @@ static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec
>                 if (reg_off < 0)
>                         return reg_off;
>                 arg->reg_off = reg_off;
> -       } else if (sscanf(arg_str, " %d @ \[ %m[a-z0-9] ] %n", &arg_sz, &reg_name, &len) == 2) {
> -               /* Memory dereference case, e.g., -4@[sp] */
> -               arg->arg_type = USDT_ARG_REG_DEREF;
> -               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;
> -       } else if (sscanf(arg_str, " %d @ %ld %n", &arg_sz, &off, &len) == 2) {
> -               /* Constant value case, e.g., 4@5 */
> -               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) {
> -               /* Register read case, e.g., -8@x4 */
> -               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;
>         } else {
> -               pr_warn("usdt: unrecognized arg #%d spec '%s'\n", arg_num, arg_str);
> -               return -EINVAL;
> +               if (ret == 2)
> +                       free(reg_name);
> +
> +               if (sscanf(arg_str, " %d @ \[ %m[a-z0-9] ] %n", &arg_sz, &reg_name, &len) == 2) {
> +                       /* Memory dereference case, e.g., -4@[sp] */
> +                       arg->arg_type = USDT_ARG_REG_DEREF;
> +                       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;
> +               } else if (sscanf(arg_str, " %d @ %ld %n", &arg_sz, &off, &len) == 2) {
> +                       /* Constant value case, e.g., 4@5 */
> +                       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) {
> +                       /* Register read case, e.g., -8@x4 */
> +                       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;
> +               } else {
> +                       pr_warn("usdt: unrecognized arg #%d spec '%s'\n", arg_num, arg_str);
> +                       return -EINVAL;
> +               }
>         }
>

I think all this is more complicated than it has to be. How big  can
register names be? Few characters? Let's get rid of %m[a-z0-9] and
instead use fixed-max-length strings, e.g., %5s. And read register
names into such local char buffers. It will simplify everything
tremendously. Let's use 16-byte buffers and use %15s to match it?
Would that be enough?

>         arg->arg_signed = arg_sz < 0;
> --
> 2.30.2
>