[PATCH net-next 2/3] bpf: change helper bpf_probe_read arg2 type to ARG_CONST_SIZE_OR_ZERO

[Yonghong Song]

The helper bpf_probe_read arg2 type is changed
from ARG_CONST_SIZE to ARG_CONST_SIZE_OR_ZERO to permit
size-0 buffer. Together with newer ARG_CONST_SIZE_OR_ZERO
semantics which allows non-NULL buffer with size 0,
this allows simpler bpf programs with verifier acceptance.
The prvious commit which changes ARG_CONST_SIZE_OR_ZERO semantics
has details on examples.

Signed-off-by: Yonghong Song <y...@fb.com>
Acked-by: Alexei Starovoitov <a...@kernel.org>
---
 kernel/trace/bpf_trace.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 506efe6..a5580c6 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -78,12 +78,16 @@ EXPORT_SYMBOL_GPL(trace_call_bpf);
 
 BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
 {
-       int ret;
+       int ret = 0;
+
+       if (unlikely(size == 0))
+               goto out;
 
        ret = probe_kernel_read(dst, unsafe_ptr, size);
        if (unlikely(ret < 0))
                memset(dst, 0, size);
 
+ out:
        return ret;
 }
 
@@ -92,7 +96,7 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
        .gpl_only       = true,
        .ret_type       = RET_INTEGER,
        .arg1_type      = ARG_PTR_TO_UNINIT_MEM,
-       .arg2_type      = ARG_CONST_SIZE,
+       .arg2_type      = ARG_CONST_SIZE_OR_ZERO,
        .arg3_type      = ARG_ANYTHING,
 };
 
-- 
2.9.5