From: Ilya Leoshkevich <iii@linux.ibm.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
bpf@vger.kernel.org, Heiko Carstens <hca@linux.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>
Subject: Re: [PATCH bpf-next 17/24] libbpf: Read usdt arg spec with bpf_probe_read_kernel()
Date: Fri, 27 Jan 2023 12:01:17 +0100 [thread overview]
Message-ID: <37b2889c5284a02ecf3de84b07efa3219584252b.camel@linux.ibm.com> (raw)
In-Reply-To: <CAEf4BzYcGSnmXVr52KcqtJrid6moyFqSL0R86S6LTiuvnQK9_g@mail.gmail.com>
On Thu, 2023-01-26 at 11:03 -0800, Andrii Nakryiko wrote:
> On Thu, Jan 26, 2023 at 3:41 AM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> >
> > On Wed, 2023-01-25 at 16:26 -0800, Andrii Nakryiko wrote:
> > > On Wed, Jan 25, 2023 at 1:39 PM Ilya Leoshkevich
> > > <iii@linux.ibm.com>
> > > wrote:
> > > >
> > > > Loading programs that use bpf_usdt_arg() on s390x fails with:
> > > >
> > > > ; switch (arg_spec->arg_type) {
> > > > 139: (61) r1 = *(u32 *)(r2 +8)
> > > > R2 unbounded memory access, make sure to bounds check any
> > > > such
> > > > access
> > >
> > > can you show a bit longer log? we shouldn't just use
> > > bpf_probe_read_kernel for this. I suspect strategically placed
> > > barrier_var() calls will solve this. This is usually an issue
> > > with
> > > compiler reordering operations and doing actual check after it
> > > already
> > > speculatively adjusted pointer (which is technically safe and ok
> > > if
> > > we
> > > never deref that pointer, but verifier doesn't recognize such
> > > pattern)
> >
> > The full log is here:
> >
> > https://gist.github.com/iii-i/b6149ee99b37078ec920ab1d3bb45134
[...]
> > --- a/tools/lib/bpf/usdt.bpf.h
> > +++ b/tools/lib/bpf/usdt.bpf.h
> > @@ -130,7 +130,10 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64
> > arg_num, long *res)
> > if (!spec)
> > return -ESRCH;
> >
> > - if (arg_num >= BPF_USDT_MAX_ARG_CNT || arg_num >= spec-
> > > arg_cnt)
> > + if (arg_num >= BPF_USDT_MAX_ARG_CNT)
> > + return -ENOENT;
> > + barrier_var(arg_num);
> > + if (arg_num >= spec->arg_cnt)
> > return -ENOENT;
> >
> > arg_spec = &spec->args[arg_num];
> >
> > I can use this in v2 if it looks good.
>
> arg_num -> spec->arg_cnt is "real" check, arg_num >=
> BPF_USDT_MAX_ARG_CNT is more to satisfy verifier (we know that
> spec->arg_cnt won't be >= BPF_USDT_MAX_ARG_CNT). Let's swap two
> checks
> in order and keep BPF_USDT_MAX_ARG_CNT close to spec->args[arg_num]
> use? And if barrier_var() is necessary, then so be it.
Unfortunately just swapping did not help, so let's use the barrier.
> > Btw, I looked at the barrier_var() definition:
> >
> > #define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
> >
> > and I'm curious why it's not defined like this:
> >
> > #define barrier_var(var) asm volatile("" : "+r"(var))
> >
> > which is a bit simpler?
> > >
>
> no reason, just unfamiliarity with embedded asm back then, we can
> update it we they are equivalent
Thanks! I will add a simplification in v2 then.
next prev parent reply other threads:[~2023-01-27 11:01 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-25 21:37 [PATCH bpf-next 00/24] Support bpf trampoline for s390x Ilya Leoshkevich
2023-01-25 21:37 ` [PATCH bpf-next 01/24] selftests/bpf: Fix liburandom_read.so linker error Ilya Leoshkevich
2023-01-26 1:07 ` Andrii Nakryiko
2023-01-26 13:30 ` Ilya Leoshkevich
2023-01-25 21:37 ` [PATCH bpf-next 02/24] selftests/bpf: Fix symlink creation error Ilya Leoshkevich
2023-01-25 21:37 ` [PATCH bpf-next 03/24] selftests/bpf: Fix fexit_stress on s390x Ilya Leoshkevich
2023-01-25 21:37 ` [PATCH bpf-next 04/24] selftests/bpf: Fix trampoline_count " Ilya Leoshkevich
2023-01-25 21:37 ` [PATCH bpf-next 05/24] selftests/bpf: Fix kfree_skb " Ilya Leoshkevich
2023-01-25 21:37 ` [PATCH bpf-next 06/24] selftests/bpf: Set errno when urand_spawn() fails Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 07/24] selftests/bpf: Fix decap_sanity_ns cleanup Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 08/24] selftests/bpf: Fix verify_pkcs7_sig on s390x Ilya Leoshkevich
2023-01-26 1:06 ` Andrii Nakryiko
2023-01-27 12:36 ` Ilya Leoshkevich
2023-01-27 17:26 ` Andrii Nakryiko
2023-01-25 21:38 ` [PATCH bpf-next 09/24] selftests/bpf: Fix xdp_do_redirect " Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 10/24] selftests/bpf: Fix cgrp_local_storage " Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 11/24] selftests/bpf: Check stack_mprotect() return value Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 12/24] selftests/bpf: Increase SIZEOF_BPF_LOCAL_STORAGE_ELEM on s390x Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 13/24] selftests/bpf: Add a sign-extension test for kfuncs Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 14/24] selftests/bpf: Fix test_lsm on s390x Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 15/24] selftests/bpf: Fix test_xdp_adjust_tail_grow2 " Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 16/24] selftests/bpf: Fix vmlinux test " Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 17/24] libbpf: Read usdt arg spec with bpf_probe_read_kernel() Ilya Leoshkevich
2023-01-26 0:26 ` Andrii Nakryiko
2023-01-26 11:41 ` Ilya Leoshkevich
2023-01-26 19:03 ` Andrii Nakryiko
2023-01-27 11:01 ` Ilya Leoshkevich [this message]
2023-01-25 21:38 ` [PATCH bpf-next 18/24] s390/bpf: Fix a typo in a comment Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 19/24] s390/bpf: Add expoline to tail calls Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 20/24] s390/bpf: Implement bpf_arch_text_poke() Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 21/24] bpf: btf: Add BTF_FMODEL_SIGNED_ARG flag Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 22/24] s390/bpf: Implement arch_prepare_bpf_trampoline() Ilya Leoshkevich
2023-01-26 1:15 ` Andrii Nakryiko
2023-01-26 14:30 ` Ilya Leoshkevich
2023-01-26 19:06 ` Andrii Nakryiko
2023-01-27 11:15 ` Ilya Leoshkevich
2023-01-27 17:30 ` Andrii Nakryiko
2023-01-25 21:38 ` [PATCH bpf-next 23/24] s390/bpf: Implement bpf_jit_supports_subprog_tailcalls() Ilya Leoshkevich
2023-01-25 21:38 ` [PATCH bpf-next 24/24] s390/bpf: Implement bpf_jit_supports_kfunc_call() Ilya Leoshkevich
2023-01-26 1:28 ` Alexei Starovoitov
2023-01-27 11:36 ` Ilya Leoshkevich
2023-01-27 16:04 ` Alexei Starovoitov
2023-01-26 0:45 ` [PATCH bpf-next 00/24] Support bpf trampoline for s390x Andrii Nakryiko
2023-01-27 16:51 ` Ilya Leoshkevich
2023-01-27 17:24 ` Andrii Nakryiko
2023-01-27 22:50 ` Ilya Leoshkevich
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