Subject: Re: Help with the BPF verifier

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Thu, 1 Nov 2018 16:13:10 -0300

Em Thu, Nov 01, 2018 at 12:10:39PM -0700, David Miller escreveu:
> From: Arnaldo Carvalho de Melo <[email-masked]>
> Date: Thu, 1 Nov 2018 15:52:17 -0300
> 
> >     50		unsigned int filename_arg = 6;
>  ...
> > --- /wb/augmented_raw_syscalls.c.old	2018-11-01 15:43:55.000394234 -0300
> > +++ /wb/augmented_raw_syscalls.c	2018-11-01 15:44:15.102367838 -0300
> > @@ -67,7 +67,7 @@
> >  		augmented_args.filename.reserved = 0;
> >  		augmented_args.filename.size = probe_read_str(&augmented_args.filename.value,
> >  							      sizeof(augmented_args.filename.value),
> > -							      (const void *)args->args[0]);
> > +							      (const void *)args->args[filename_arg]);
> 
> args[] is sized to '6', therefore the last valid index is '5', yet you're using '6' here which
> is one entry past the end of the declared array.

Nope... this is inside an if:

        if (filename_arg <= 5) {
                augmented_args.filename.reserved = 0;
                augmented_args.filename.size = probe_read_str(&augmented_args.filename.value,
                                                              sizeof(augmented_args.filename.value),
                                                              (const void *)args->args[filename_arg]);
                if (augmented_args.filename.size < sizeof(augmented_args.filename.value)) {
                        len -= sizeof(augmented_args.filename.value) - augmented_args.filename.size;
                        len &= sizeof(augmented_args.filename.value) - 1;
                }
        } else {

I use 6 to mean "hey, this syscall doesn't have any string argument, don't
bother with it".

- Arnaldo