RISC-V: cachestat tool fails to run: "no member named 'pc' in 'struct pt_regs'"

[euspectre]

root@riscv64-test:/# ./cachestat
/virtual/main.c:13:14: error: no member named 'pc' in 'struct pt_regs'
    key.ip = PT_REGS_IP(ctx);
             ^~~~~~~~~~~~~~~
/virtual/include/bcc/helpers.h:1376:29: note: expanded from macro 'PT_REGS_IP'
#define PT_REGS_IP(x) ((x)->pc)
                       ~~~  ^
1 error generated.
Traceback (most recent call last):
  File "/usr/share/bcc/tools/./cachestat", line 96, in <module>
    b = BPF(text=bpf_text)
  File "/usr/lib/python3/dist-packages/bcc/__init__.py", line 476, in __init__
    raise Exception("Failed to compile BPF module %s" % (src_file or "<text>"))
Exception: Failed to compile BPF module <text>

System: QEMU VM with riscv64 CPU, OS: Ubuntu 22.04 with kernel 5.15.0-1014-generic.

If I understand it correctly, 'epc' should be used instead of 'pc', same as in arch/riscv/include/asm/ptrace.h.

diff --git a/src/cc/export/helpers.h b/src/cc/export/helpers.h
index 71ed8c8a..0f6e59ef 100644
--- a/src/cc/export/helpers.h
+++ b/src/cc/export/helpers.h
@@ -1373,7 +1373,7 @@ int bpf_usdt_readarg_p(int argc, struct pt_regs *ctx, void *buf, u64 len) asm("l
 #define PT_REGS_FP(x) ((x)->s0) /* Works only with CONFIG_FRAME_POINTER */
 #define PT_REGS_RC(x) ((x)->a0)
 #define PT_REGS_SP(x) ((x)->sp)
-#define PT_REGS_IP(x) ((x)->pc)
+#define PT_REGS_IP(x) ((x)->epc)
 #else
 #error "bcc does not support this platform yet"
 #endif

Right?
@xmzzz, what do you think?

[chenhengqi]

This is because ctx should be of type struct user_regs_struct * for RISC-V.

[xmzzz]

I think @chenhengqi is right, you can refer to:
#4085 (comment)
Thanks ^0^

[euspectre]

So, libbpf already uses struct user_regs_struct there on RISC-V and the correct field is in fact 'pc'. OK.
I checked that src/cc/libbpf/cc/libbpf/src/bpf_tracing.h does have that part.

But cachestat built from the latest sources of bcc as described at https://github.com/iovisor/bcc/blob/master/INSTALL.md#source still tries to use struct pt_regs rather than struct user_regs_struct and fails. How so? I don't understand.

Some issue in the build scripts? Or?

[chenhengqi]

This should be fixed in BCC, you can just copy the macros from libbpf.

[xmzzz]

This should be fixed in BCC, you can just copy the macros from libbpf.

Hi, @euspectre . I fixed it according to @chenhengqi ‘s suggestion and it worked.

Please let me know if I'm missing anything, thanks a lot.

[euspectre]

This should be fixed in BCC, you can just copy the macros from libbpf.

I see. Thanks for the explanation.

[euspectre]

The fix from #4116 helps in my system too. Great!