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

[Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>]

On Mon, Oct 10, 2022 at 7:08 AM Xu Kuohai <xukuohai@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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
>