On Wed, Jul 29, 2020 at 11:29:43PM +0200, Daniel Borkmann wrote:

On 7/29/20 7:28 PM, Song Liu wrote:

quoted

On Tue, Jul 28, 2020 at 8:37 AM Jean-Philippe Brucker
[off-list ref] wrote:

quoted

When a tracing BPF program attempts to read memory without using the
bpf_probe_read() helper, the verifier marks the load instruction with
the BPF_PROBE_MEM flag. Since the arm64 JIT does not currently recognize
this flag it falls back to the interpreter.

Add support for BPF_PROBE_MEM, by appending an exception table to the
BPF program. If the load instruction causes a data abort, the fixup
infrastructure finds the exception table and fixes up the fault, by
clearing the destination register and jumping over the faulting
instruction.

To keep the compact exception table entry format, inspect the pc in
fixup_exception(). A more generic solution would add a "handler" field
to the table entry, like on x86 and s390.

Signed-off-by: Jean-Philippe Brucker <redacted>

This patch looks good to me.

Acked-by: Song Liu <redacted>

+1, applied, thanks a lot!

quoted

It is possible to add a selftest for this? I thought about this a
little bit, but
didn't get a good idea.

Why not adding a test_verifier.c test case which calls into bpf_get_current_task()
to fetch pointer to current and then read out some field via BPF_PROBE_MEM which
should then succeed on x86/s390x/arm64 but be skipped on the other archs? Jean-Philippe,
could you look into following up with such test case(s)?

Sure I'll take a look. Ilya also added a selftests to trigger exceptions
in https://lore.kernel.org/bpf/20200715233301.933201-5-iii@linux.ibm.com/ (local)
It's useful but I think it relies on the verifier not mandating NULL
checks for next-level pointers (they are ptr_ instead of ptr_or_null_),
which might change in the future. So I'm wondering if we can deliberately
access an invalid pointer with the help of bpf_test_run, and check that
the result is zero. 

Thanks,
Jean

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