Re: [PATCH] RISC-V/bpf: Enable bpf_probe_read{, str}()

Yonghong Song
2022-07-06T06:42:21Z

On 7/5/22 10:00 PM, Andrii Nakryiko wrote:
> On Sun, Jul 3, 2022 at 10:53 PM Christoph Hellwig <hch@infradead.org> wrote:
>>
>> On Sun, Jul 03, 2022 at 09:09:24PM +0800, Yixun Lan wrote:
>>> Enable this option to fix a bcc error in RISC-V platform
>>>
>>> And, the error shows as follows:
>>
>> These should not be enabled on new platforms. Use the proper helpers
>> to probe kernel vs user pointers instead.
>
> riscv existed as of [0], so I'd argue it is a proper bug fix, as
> corresponding select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE should
> have been added back then.
>
> But I also agree that BCC tools should be updated to use proper
> bpf_probe_read_{kernel,user}[_str()] helpers, please contribute such
> fixes to BCC tools and BCC itself as well. Cc'ed Alan as his ksnoop in
> libbpf-tools seems to be using bpf_probe_read() as well and needs to
> be fixed.

Yixun, the bcc change looks like below:

--- a/src/cc/frontends/clang/b_frontend_action.cc
+++ b/src/cc/frontends/clang/b_frontend_action.cc
@@ -132,7 +132,7 @@ static std::string
check_bpf_probe_read_user(llvm::StringRef probe,

/* For arch with overlapping address space, dont use
bpf_probe_read for
* user read. Just error out */
-#if defined(__s390x__)
+#if defined(__s390x__) || defined(__riscv_)
overlap_addr = true;
return "";
#endif

Basically, prevent using bpf_probe_read() helper, so it will force user
to use bpf_probe_read_user() or bpf_probe_read_kernel(). and this should
make it work for old kernels.

>
> [0] 0ebeea8ca8a4 ("bpf: Restrict bpf_probe_read{, str}() only to
> archs where they work")