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

[Florent Revest]

On Thu, Apr 22, 2021 at 2:36 PM Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> On Thu, Apr 22, 2021 at 12:09 PM Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > On 22/04/2021 11.23, Florent Revest 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 :)
> >
> >
> > I think you were lucky (or unlucky, depending on how you look at it)
> > with your test case
> >
> > +       num_ret  = BPF_SNPRINTF(num_out, sizeof(num_out),
> > +                               "%d %u %x %li %llu %lX",
> > +                               -8, 9, 150, -424242, 1337, 0xDABBAD00);
> >
> > because it just so happens that the eventual snprintf() call uses three
> > arguments for itself, so the first three 32-bit arguments end up being
> > passed via registers, while the 64 bit arguments are passed via the
> > stack. Can I get you to test what would happen if you interchanged
> > these, i.e. changed the test case to do
> >
> > +       num_ret  = BPF_SNPRINTF(num_out, sizeof(num_out),
> > +                               "%li %llu %lX %d %u %x",
> > +                               -424242, 1337, 0xDABBAD00, -8, 9, 150);
> >
> > (or just add a few more expects-a-32-bit argument format specifiers and
> > corresponding arguments). My guess is that up until formatting -8 it
> > goes well, but when vsnprintf() is to grab the argument corresponding to
> > %u, it will get the 0xffffffff from the upper half of (u64)-8.
>
> I will need to come up with a repro and let you know yes :)
>
> > > 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.
> >
> > Isn't it 3^3 = 27, or has that been reduced in -next compared to Linus'
> > master? Doesn't matter much, just curious.
> >
> > > 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.
> >
> > Yeah, that doesn't scale at all.
> >
> >  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.
> > >
> >
> > [wild handwaving ahead]
> >
> > One possibility, if one is willing to get hands dirty and dig into ABI
> > details on various arches, is to create a
> >
> >   struct fake_va_list {
> >     union {
> >       va_list      ap; /* opaque, compiler-provided */
> >       arch_va_list _ap; /* arch-provided, must match layout of ap */
> >     };
> >     void *stack;
> >   };
> >
> > Then do
> >
> >   struct fake_va_list fva;
> >   u64 buf[24]; /* or whatever you want to support, can be different in
> > different functions */
> >
> >   fake_va_init(&fva, buf);
> >   /* various C code, parsing format string etc. */
> >   if (arg[i] is really 32 bits)
> >     fake_va_push(&fva, (u32)arg[i]);
> >   else
> >     fake_va_push(&fva, (u64)arg[i]);
> >   /* etc. */
> >   ...
> >   vsnprintf(out, size, fmt, fva.va);
> >
> > On arches like x86-64, where va_list is really a typedef for a
> > one-element array of
> >
> > struct __va_list_tag {
> >         unsigned int               gp_offset;
> >         unsigned int               fp_offset;
> >         void *                     overflow_arg_area;
> >         void *                     reg_save_area;
> > };
> >
> >
> > fake_va_init() would make the va_list look like the reg_save_area is
> > already used (i.e., set gp_offset to 48), and initialize both
> > ->_ap.overflow_arg_area and ->stack to point at the given buffer.
> > fake_va_push() would use and update stack appropriately. For 32 bit x86,
> > va_list is really just a pointer, so fake_va_init would essentially just
> > do "fva->_ap = fva->stack = buf", and fake_va_push() would again just
> > need to manipulate ->stack.
> >
> > It's not pretty, but I don't think it necessarily requires too much
> > arch-specific work (fake_va_push() could be common, perhaps just with a
> > arch define to say whether 64 bit arguments need ->stack to first be
> > up-aligned to an 8 byte boundary).
> >
> > Rasmus
>
> Creative! :D I think these arch-specific structures would be a hard
> sell though ahah.
>
> I was having a stroll through lib/vsprintf.c and noticed bstr_printf:
>
>  * This function like C99 vsnprintf, but the difference is that vsnprintf gets
>  * arguments from stack, and bstr_printf gets arguments from @bin_buf which is
>  * a binary buffer that generated by vbin_printf.
>
> Maybe it would be easier to just build our argument buffer similarly
> to what vbin_printf does.

I've been experimenting with this idea and it is quite promising :) it
also makes the code much cleaner, I find. I'll send a series asap.

BPF maintainers: should we fix forward or do you prefer reverting the
snprintf series and then re-applying another snprintf series without
the regression in bpf_trace_printk that mangles some argument types ?
(bpf_seq_printf has always been like that so no regression there)