Re: [PATCH] bpf: remove pointless code from bpf_do_trace_printk()

[Andrii Nakryiko]

On Thu, Apr 22, 2021 at 2:23 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> On Thu, Apr 22, 2021 at 9:13 AM Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > On 22/04/2021 05.32, Andrii Nakryiko wrote:
> > > On Wed, Apr 21, 2021 at 6:19 PM Rasmus Villemoes
> > > <linux@xxxxxxxxxxxxxxxxxx> wrote:
> > >>
> > >> The comment is wrong. snprintf(buf, 16, "") and snprintf(buf, 16,
> > >> "%s", "") etc. will certainly put '\0' in buf[0]. The only case where
> > >> snprintf() does not guarantee a nul-terminated string is when it is
> > >> given a buffer size of 0 (which of course prevents it from writing
> > >> anything at all to the buffer).
> > >>
> > >> Remove it before it gets cargo-culted elsewhere.
> > >>
> > >> Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> > >> ---
> > >>  kernel/trace/bpf_trace.c | 3 ---
> > >>  1 file changed, 3 deletions(-)
> > >>
> > >
> > > The change looks good to me, but please rebase it on top of the
> > > bpf-next tree. This is not a bug, so it doesn't have to go into the
> > > bpf tree. As it is right now, it doesn't apply cleanly onto bpf-next.
>
> FWIW the idea of the patch also looks good to me :)
>
> > Thanks for the pointer. Looking in next-20210420, it seems to me that
> >
> > commit d9c9e4db186ab4d81f84e6f22b225d333b9424e3
> > Author: Florent Revest <revest@xxxxxxxxxxxx>
> > Date:   Mon Apr 19 17:52:38 2021 +0200
> >
> >     bpf: Factorize bpf_trace_printk and bpf_seq_printf
> >
> > is buggy. In particular, these two snippets:
> >
> > +#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)      \
> > +         ? (u64)args[arg_nb]                                           \
> > +         : (u32)args[arg_nb])
> >
> >
> > +       ret = snprintf(buf, sizeof(buf), fmt, BPF_CAST_FMT_ARG(0, args,
> > mod),
> > +               BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2,
> > args, mod));
> >
> > Regardless of the casts done in that macro, the type of the resulting
> > expression is that resulting from C promotion rules. And (foo ? (u64)bla
> > : (u32)blib) has type u64, which is thus the type the compiler uses when
> > building the vararg list being passed into snprintf(). C simply doesn't
> > allow you to change types at run-time in this way.
> >
> > It probably works fine on x86-64, which passes the first six or so
> > argument in registers, va_start() puts those registers into the va_list
> > opaque structure, and when it comes time to do a va_arg(int), just the
> > lower 32 bits are used. It is broken on i386 and other architectures
> > where arguments are passed on the stack (and for x86-64 as well had
> > there been a few more arguments) and va_arg(ap, int) is essentially ({
> > int res = *(int *)ap; ap += 4; res; }) [or maybe it's -= 4 because stack
> > direction etc., that's not really relevant here].
> >
> > Rasmus
>
> Thank you Rasmus :)
>
> It seems that we went offtrack in
> https://lore.kernel.org/bpf/CAEf4BzZVEGM4esi-Rz67_xX_RTDrgxViy0gHfpeauECR5bmRNA@xxxxxxxxxxxxxx/
> and we do need something like "88a5c690b6 bpf: fix bpf_trace_printk on
> 32 bit archs". Thinking about it again, it's clearer now why the
> __BPF_TP_EMIT macro emits 2^3=8 different __trace_printk() indeed.

Yeah, we wondering but no one could guess why it was done the way it
was done :) Next time we should invest in a better comment ;-P

>
> In the case of bpf_trace_printk with a maximum of 3 args, it's
> relatively cheap; but for bpf_seq_printf and bpf_snprintf which accept
> up to 12 arguments, that would be 2^12=4096 calls. Until now
> bpf_seq_printf has just ignored this problem and just considered
> everything as u64, I wonder if that'd be the best approach for these
> two helpers anyway.