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cilium
introduce builtin memcmp etc., like bpf in cilium
When writing bpf programs, you may need to use some basic memory operations, such as memcmp, etc.
https://github.com/cilium/cilium/blob/master/bpf/include/bpf/builtins.h
In Cilium those builtins are useful to (1) reduce the BPF complexity and (2) reduce runtime overhead. I doubt (1) is an issue for pwru today and it's not clear if (2) would be noticeable. In Cilium's case, (1) is regularly an issue, but we didn't measure the performance difference for (2).
Yes, I introduced this earlier, but I had a problem compiling it, so I removed it. I can see how to fix it later.
Rewrite addr_equal using __builtin_memcmp as follows
static __always_inline bool
addr_equal(union addr a, u8 b[16]) {
if (__builtin_memcmp(&a, b, sizeof(a)) != 0) {
return false;
}
return true;
}
After that, go generate works fine, and then the following error is reported when executing pwru
2021/10/26 15:36:24 Loading objects: field KprobeSkb1: program kprobe_skb_1: call at 367: reference to missing symbol "memcmp"
Looking at the symbol table by executing readelf, I found that __builtin_memcmp is not inlined
[duanjiong@fedora pwru]$ readelf -s kprobepwru_bpfeb.o
158: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND memcmp
My version of clang is as follows
[duanjiong@fedora pwru]$ clang --version
clang version 12.0.1 (Fedora 12.0.1-1.fc34)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Clang only supports those builtins in specific cases: https://clang.llvm.org/docs/LanguageExtensions.html#string-builtins.
I encountered the same problem. __builtin_memcmp does not work any more, it was replaced by memcmp.
[root@host file_read]# bpftool prog load bpf_test.o /sys/fs/bpf/bpf_test
libbpf: elf: skipping unrecognized data section(5) .rodata.str1.1
libbpf: failed to find BTF for extern 'memcmp': -3
Error: failed to open object file
[root@host file_read]# readelf -s bpf_test.o
Symbol table '.symtab' contains 6 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS bpf_test.c
2: 00000000000000e0 0 NOTYPE LOCAL DEFAULT 3 LBB0_3
3: 0000000000000000 0 SECTION LOCAL DEFAULT 5
4: 0000000000000000 240 FUNC GLOBAL DEFAULT 3 kretprobe_sys_read
5: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND memcmp
[root@host file_read]#
How do I use memcmp in a BPF program? What header would I include? I get the same error message as above @chenk008
@thealexcons same issue for me. __builtin_memcmp is replaced by memcmp by clang and bpf loading fail. any update here?
