Message-ID: <CAEf4BzZVEGM4esi-Rz67_xX_RTDrgxViy0gHfpeauECR5bmRNA@mail.gmail.com>
Date:   Wed, 7 Apr 2021 14:53:50 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Florent Revest <revest@...omium.org>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 1/6] bpf: Factorize bpf_trace_printk and bpf_seq_printf

On Tue, Apr 6, 2021 at 8:35 AM Florent Revest <revest@...omium.org> wrote:
>
> [Sorry for the late replies, I'm just back from a long easter break :)]
>
> On Fri, Mar 26, 2021 at 11:51 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> > On Fri, Mar 26, 2021 at 2:53 PM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > > On Tue, Mar 23, 2021 at 7:23 PM Florent Revest <revest@...omium.org> wrote:
> > > > Unfortunately, the implementation of the two existing helpers already
> > > > drifted quite a bit and unifying them entailed a lot of changes:
> > >
> > > "Unfortunately" as in a lot of extra work for you? I think overall
> > > though it was very fortunate that you ended up doing it, all
> > > implementations are more feature-complete and saner now, no? Thanks a
> > > lot for your hard work!
>
> Ahah, "unfortunately" a bit of extra work for me, indeed. But I find
> this kind of refactoring patches even harder to review than to write
> so thank you too!
>
> > > > - bpf_trace_printk always expected fmt[fmt_size] to be the terminating
> > > >   NULL character, this is no longer true, the first 0 is terminating.
> > >
> > > You mean if you had bpf_trace_printk("bla bla\0some more bla\0", 24)
> > > it would emit that zero character? If yes, I don't think it was a sane
> > > behavior anyways.
>
> The call to snprintf in bpf_do_trace_printk would eventually ignore
> "some more bla" but the parsing done in bpf_trace_printk would indeed
> read the whole string.
>
> > > This is great, you already saved some lines of code! I suspect I'll
> > > have some complaints about mods (it feels like this preample should
> > > provide extra information about which arguments have to be read from
> > > kernel/user memory, but I'll see next patches first.
> >
> > Disregard the last part (at least for now). I had a mental model that
> > it should be possible to parse a format string once and then remember
> > "instructions" (i.e., arg1 is long, arg2 is string, and so on). But
> > that's too complicated, so I think re-parsing the format string is
> > much simpler.
>
> I also wanted to do that originally but realized it would keep a lot
> of the complexity in the helpers themselves and not really move the
> needle.
>
> > > > +/* Horrid workaround for getting va_list handling working with different
> > > > + * argument type combinations generically for 32 and 64 bit archs.
> > > > + */
> > > > +#define BPF_CAST_FMT_ARG(arg_nb, args, mod)                            \
> > > > +       ((mod[arg_nb] == BPF_PRINTF_LONG_LONG ||                        \
> > > > +        (mod[arg_nb] == BPF_PRINTF_LONG && __BITS_PER_LONG == 64))     \
> > > > +         ? args[arg_nb]                                                \
> > > > +         : ((mod[arg_nb] == BPF_PRINTF_LONG ||                         \
> > > > +            (mod[arg_nb] == BPF_PRINTF_INT && __BITS_PER_LONG == 32))  \
> > >
> > > is this right? INT is always 32-bit, it's only LONG that differs.
> > > Shouldn't the rule be
> > >
> > > (LONG_LONG || LONG && __BITS_PER_LONG) -> (__u64)args[args_nb]
> > > (INT || LONG && __BITS_PER_LONG == 32) -> (__u32)args[args_nb]
> > >
> > > Does (long) cast do anything fancy when casting from u64? Sorry, maybe
> > > I'm confused.
>
> To be honest, I am also confused by that logic... :p My patch tries to
> conserve exactly the same logic as "88a5c690b6 bpf: fix
> bpf_trace_printk on 32 bit archs" because I was also afraid of missing
> something and could not test it on 32 bit arches. From that commit
> description, it is unclear to me what "u32 and long are passed
> differently to u64, since the result of C conditional operators
> follows the "usual arithmetic conversions" rules" means. Maybe Daniel
> can comment on this ?

Yeah, no idea. Seems like the code above should work fine for 32 and
64 bitness and both little- and big-endianness.

>
> > > > +int bpf_printf_preamble(char *fmt, u32 fmt_size, const u64 *raw_args,
> > > > +                       u64 *final_args, enum bpf_printf_mod_type *mod,
> > > > +                       u32 num_args)
> > > > +{
> > > > +       struct bpf_printf_buf *bufs = this_cpu_ptr(&bpf_printf_buf);
> > > > +       int err, i, fmt_cnt = 0, copy_size, used;
> > > > +       char *unsafe_ptr = NULL, *tmp_buf = NULL;
> > > > +       bool prepare_args = final_args && mod;
> > >
> > > probably better to enforce that both or none are specified, otherwise
> > > return error
>
> Fair :)
>
> > it's actually three of them: raw_args, mod, and num_args, right? All
> > three are either NULL or non-NULL.
>
> It is a bit tricky to see from that patch but in "3/6 bpf: Add a
> bpf_snprintf helper" the verifier code calls this function with
> num_args != 0 to check whether the number of arguments is correct
> without actually converting anything.
>
> Also when the helper gets called, raw_args can come from the BPF
> program and be NULL but in that case we will also have num_args = 0
> guaranteed by the helper so the loop will bail out if it encounters a
> format specifier.

ok, but at least final_args and mod are locked together, so should be
enforced to be either null or not, right?

>
> > > > +       enum bpf_printf_mod_type current_mod;
> > > > +       size_t tmp_buf_len;
> > > > +       u64 current_arg;
> > > > +       char fmt_ptype;
> > > > +
> > > > +       for (i = 0; i < fmt_size && fmt[i] != '\0'; i++) {
> > >
> > > Can we say that if the last character is not '\0' then it's a bad
> > > format string and return -EINVAL? And if \0 is inside the format
> > > string, then it's also a bad format string? I wonder what others think
> > > about this?... I think sanity should prevail.
>
> Overall, there are two situations:
> - bpf_seq_printf, bpf_trace_printk: we have a pointer and size but we
> are not guaranteed zero-termination
> - bpf_snprintf: we have a pointer, no size but it's guaranteed to be
> zero-terminated (by ARG_PTR_TO_CONST_STR)
>
> Currently, in the bpf_snprintf helper, I set fmt_size to UINT_MAX and
> the terminating condition will be fmt[i] == '\0'.
> As you pointed out a bit further, I got a bit carried away with the
> refactoring and dropped the zero-termination checks for the existing
> helpers !
>
> So I see two possibilities:
> - either we check fmt[last] == '\0', add a bail out condition in the
> loop if we encounter another `\0` and set fmt_size to sprintf(fmt) in
> the bpf_snprintf verifier and helper code.
> - or we unconditionally call strnlen(fmt, fmt_size) in
> bpf_printf_preamble. If no 0 is found, we return an error, if there is
> one we treat it as the NULL terminating character.

I was thinking about the second one. It is clearly acceptable on BPF
verifier side, though one might argue that we are doing extra work on
the BPF helper side. I don't think it matters in practice, so I'll be
fine with that, if that makes code cleaner and simpler.

>
> > > > +               if ((!isprint(fmt[i]) && !isspace(fmt[i])) ||
> > > > +                   !isascii(fmt[i])) {
> > >
> > > && always binds tighter than ||, so you can omit extra (). I'd put
> > > this on a single line as well, but that's a total nit.
>
> Neat! :)

I just got a compilation warning in a similar situation yesterday when
I dropped unnecessary parentheses, so some versions of compilers might
think it is not a good practice. Just keep that in mind. I don't think
I care enough.

>
> > > > +                       err = -EINVAL;
> > > > +                       goto out;
> > > > +               }
> > > >
> > > >                 if (fmt[i] != '%')
> > > >                         continue;
> > > >
> > > > -               if (fmt_cnt >= 3)
> > > > -                       return -EINVAL;
> > > > +               if (fmt[i + 1] == '%') {
> > > > +                       i++;
> > > > +                       continue;
> > > > +               }
> > > > +
> > > > +               if (fmt_cnt >= num_args) {
> > > > +                       err = -EINVAL;
> > > > +                       goto out;
> > > > +               }
> > > >
> > > >                 /* fmt[i] != 0 && fmt[last] == 0, so we can access fmt[i + 1] */
> > > >                 i++;
> > > > -               if (fmt[i] == 'l') {
> > > > -                       mod[fmt_cnt]++;
> > > > +
> > > > +               /* skip optional "[0 +-][num]" width formating field */
> > >
> > > typo: formatting
>
> Fixed
>
> > > > +               while (fmt[i] == '0' || fmt[i] == '+'  || fmt[i] == '-' ||
> > > > +                      fmt[i] == ' ')
> > > > +                       i++;
> > > > +               if (fmt[i] >= '1' && fmt[i] <= '9') {
> > > >                         i++;
> > >
> > > Are we worried about integer overflow here? %123123123123123d
> > > hopefully won't crash anything, right?
>
> I expect that this should be handled gracefully by the subsequent call
> to snprintf(). Our parsing logic does not guarantee that the format
> string is 100% legit but it guarantees that it's safe to call
> vsnprintf with arguments coming from BPF. If the output buffer is too
> small to hold the output, the output will be truncated.
>
> Note that this is already how bpf_seq_printf already works.

Ok, but let's not hope and add the test for this.

>
> > > > -               } else if (fmt[i] == 'p') {
> > > > -                       mod[fmt_cnt]++;
> > > > -                       if ((fmt[i + 1] == 'k' ||
> > > > -                            fmt[i + 1] == 'u') &&
> > > > +                       while (fmt[i] >= '0' && fmt[i] <= '9')
> > > > +                               i++;
> > >
> > > whoa, fmt_size shouldn't be ignored
>
> Oh no, I'll attach the stone of shame! It all made sense with
> bpf_snprintf() in mind because, there, we are guaranteed to have a
> NULL terminated string already but in an excess of refactoring
> enthusiasm I dropped the zero-termination check for the other helpers.
>
> But if we implement either of the options discussed above, then we do
> not need to constantly check fmt_size.

let's see when we get to the next version ;) I don't remember code
enough by now, but I'll keep that in mind for the next revision
anyways

>
> > > > +               }
> > > > +
> > >
> > > and here if we exhausted all format string but haven't gotten to
> > > format specified, we should -EINVAL
> > >
> > > if (i >= fmt_size) return -EINVAL?
>
> Same comment as above, if we are already guaranteed zero-termination
> by a prior check, we don't need that.
>
> > > > +               if (fmt[i] == 'p') {
> > > > +                       current_mod = BPF_PRINTF_LONG;
> > > > +
> > > > +                       if ((fmt[i + 1] == 'k' || fmt[i + 1] == 'u') &&
> > > >                             fmt[i + 2] == 's') {
> > >
> > > right, if i + 2 is ok to access? always be remembering about fmt_size
>
> Same.
>
> > > >                                 fmt_ptype = fmt[i + 1];
> > > >                                 i += 2;
> > > >                                 goto fmt_str;
> > > >                         }
> > > >
> > > > -                       if (fmt[i + 1] == 'B') {
> > > > -                               i++;
> > > > +                       if (fmt[i + 1] == 0 || isspace(fmt[i + 1]) ||
> > > > +                           ispunct(fmt[i + 1]) || fmt[i + 1] == 'K' ||
> > > > +                           fmt[i + 1] == 'x' || fmt[i + 1] == 'B' ||
> > > > +                           fmt[i + 1] == 's' || fmt[i + 1] == 'S') {
> > > > +                               /* just kernel pointers */
> > > > +                               if (prepare_args)
> > > > +                                       current_arg = raw_args[fmt_cnt];
> > >
> > > fmt_cnt is not the best name, imo. arg_cnt makes more sense
>
> Mh, we already have "num_args" that can make it confusing. The way I see it:
> - the number of format specifiers is the number of %d %s... in the format string
> - the number of arguments is the number of values given in the raw_args array.
>

Well, if you read "fmt_cnt" as "number of formatters" then yeah, I
suppose it's fine. Never mind. Just fmt_cnt and fmt_size refers to
slightly different "fmt"s, which confused me for a bit, but that's ok.
You use different naming conventions, which is inconsistent, so maybe
adjust that for purists (i.e., if you have num_args, then you should
have num_fmts; or, alternatively, arg_cnt and fmt_cnt). But I'm just
nitpicking, obviously.

> Potentially, the number of arguments can be higher than the number of
> format specifiers, for example printf("%d\n", i, j); so calling them
> differently sorta makes sense.
> But to be honest I don't have a strong opinion about this and this is
> mainly just a remaining from the current bpf_seq_printf
> implementation.

Yep. I think it's ok to allow num_args > fmt_cnt.

>
> > > > +                       if (!tmp_buf) {
> > > > +                               used = this_cpu_inc_return(bpf_printf_buf_used);
> > > > +                               if (WARN_ON_ONCE(used > 1)) {
> > > > +                                       this_cpu_dec(bpf_printf_buf_used);
> > > > +                                       return -EBUSY;
> > > > +                               }
> > > > +                               preempt_disable();
> > >
> > > shouldn't we preempt_disable before we got bpf_printf_buf_used? if we
> > > get preempted after incrementing counter, buffer will be unusable for
> > > a while, potentially, right?
>
> Good catch :)
>
> > > > +                       if (!tmp_buf) {
> > > > +                               used = this_cpu_inc_return(bpf_printf_buf_used);
> > > > +                               if (WARN_ON_ONCE(used > 1)) {
> > > > +                                       this_cpu_dec(bpf_printf_buf_used);
> > > > +                                       return -EBUSY;
> > > > +                               }
> > > > +                               preempt_disable();
> > > > +                               tmp_buf = bufs->tmp_buf;
> > > > +                               tmp_buf_len = MAX_PRINTF_BUF_LEN;
> > > > +                       }
> > >
> > > how about helper used like this:
> > >
> > > if (try_get_fmt_tmp_buf(&tmp_buf, &tmp_buf_len))
> > >    return -EBUSY;
> > >
> > > which will do nothing if tmp_buf != NULL?
>
> Yep, I quite like that. :)
>
> > > >  fmt_next:
> > > > +               if (prepare_args) {
> > >
> > > I'd ditch prepare_args variable and just check final_args (and that
> > > check to ensure both mods and final_args are specified I suggested
> > > above)
>
> Agreed.
>
> > > > +                       mod[fmt_cnt] = current_mod;
> > > > +                       final_args[fmt_cnt] = current_arg;
> > > > +               }
> > > >                 fmt_cnt++;
> > > >         }
> > >
> > > [...]
> > >
> > > > -
> > > > -       return __BPF_TP_EMIT();
> > > > +       err = 0;
> > > > +out:
> > > > +       bpf_printf_postamble();
> > >
> > > naming is hard, but preamble and postamble reads way too fancy :)
> > > bpf_printf_prepare() and bpf_printf_cleanup() or something like that
> > > is a bit more to the point, no?
>
> Haha, you're totally right.
>
> > > > +       if (data_len & 7 || data_len > MAX_SEQ_PRINTF_VARARGS * 8 ||
> > > > +           (data_len && !data))
> > >
> > > data && !data_len is also an error, no?
>
> Isn't that checked by the verifier ?

data_len is ARG_CONST_SIZE_OR_ZERO, so data_len == 0 is allowed by
verifier. But it's probably no harm either to allow data != NULL and
data_len = 0. Might simplify some more dynamic use of snprintf(),
actually.

>
> I don't mind adding an explicit check for it (data_len ^ data or two
> clearer conditions ?) but I think that even if this were to happen,
> this would not be a problem: if we encounter a format specifier,
> num_args will be zero so bpf_printf_preamble will bail out before it
> tries to access data.

agree

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