On 7/29/20 6:06 AM, Andrii Nakryiko wrote:
On Tue, Jul 28, 2020 at 2:16 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
On 7/28/20 9:11 PM, Andrii Nakryiko wrote:
On Tue, Jul 28, 2020 at 5:15 AM Ilya Leoshkevich <iii@xxxxxxxxxxxxx> wrote:
Yet another adaptation to commit 0ebeea8ca8a4 ("bpf: Restrict
bpf_probe_read{, str}() only to archs where they work") that makes more
samples compile on s390.
Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
Sorry, we can't do this yet. This will break on older kernels that
don't yet have bpf_probe_read_kernel() implemented. Met and Yonghong
are working on extending a set of CO-RE relocations, that would allow
to do bpf_probe_read_kernel() detection on BPF side, transparently for
an application, and will pick either bpf_probe_read() or
bpf_probe_read_kernel(). It should be ready soon (this or next week,
most probably), though it will have dependency on the latest Clang.
But for now, please don't change this.
Could you elaborate what this means wrt dependency on latest clang? Given clang
releases have a rather long cadence, what about existing users with current clang
releases?
So the overall idea is to use something like this to do kernel reads:
static __always_inline int bpf_probe_read_universal(void *dst, u32 sz,
const void *src)
{
if (bpf_core_type_exists(btf_bpf_probe_read_kernel))
return bpf_probe_read_kernel(dst, sz, src);
else
return bpf_probe_read(dst, sz, src);
}
And then use bpf_probe_read_universal() in BPF_CORE_READ and family.
This approach relies on few things:
1. each BPF helper has a corresponding btf_<helper-name> type defined for it
2. bpf_core_type_exists(some_type) returns 0 or 1, depending if
specified type is found in kernel BTF (so needs kernel BTF, of
course). This is the part me and Yonghong are working on at the
moment.
3. verifier's dead code elimination, which will leave only
bpf_probe_read() or bpf_probe_read_kernel() calls and will remove the
other one. So on older kernels, there will never be unsupoorted call
to bpf_probe_read_kernel().
The new type existence relocation requires the latest Clang. So the
way to deal with older Clangs would be to just fallback to
bpf_probe_read, if we detect that Clang is too old and can't emit
necessary relocation.
Okay, seems reasonable overall. One question though: couldn't libbpf transparently
fix up the selection of bpf_probe_read() vs bpf_probe_read_kernel()? E.g. it would
probe the kernel whether bpf_probe_read_kernel() is available and if it is then it
would rewrite the raw call number from the instruction from bpf_probe_read() into
the one for bpf_probe_read_kernel()? I guess the question then becomes whether the
original use for bpf_probe_read() was related to CO-RE. But I think this could also
be overcome by adding a fake helper signature in libbpf with a unreasonable high
number that is dedicated to probing mem via CO-RE and then libbpf picks the right
underlying helper call number for the insn. That avoids fiddling with macros and
need for new clang version, no (unless I'm missing something)?
If that's not an acceptable plan, then one can "parameterize"
BPF_CORE_READ macro family by re-defining bpf_core_read() macro. Right
now it's defined as:
#define bpf_core_read(dst, sz, src) \
bpf_probe_read(dst, sz, (const void *)__builtin_preserve_access_index(src))
Re-defining it in terms of bpf_probe_read_kernel is trivial, but I
can't do it for BPF_CORE_READ, because it will break all the users of
bpf_core_read.h that run on older kernels.
tools/lib/bpf/bpf_core_read.h | 51 ++++++++++++++++++-----------------
tools/lib/bpf/bpf_tracing.h | 15 +++++++----
2 files changed, 37 insertions(+), 29 deletions(-)
[...]
