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

[Xu Kuohai]

On 10/11/2022 9:34 AM, Andrii Nakryiko wrote:
> 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?

The valid register names accepted by calc_pt_regs_off() are x0~x31 and sp, so
16-byte buffer is enough. Since %15s matches all non-space characters, will use
%15[a-z0-9] to match it.

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