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

[Yonghong Song]

On 7/5/22 10:00 PM, Andrii Nakryiko wrote:
> On Sun, Jul 3, 2022 at 10:53 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> 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")