On 8/26/25 10:02 PM, Eduard Zingerman wrote:
On Tue, 2025-08-26 at 13:17 -0700, Yonghong Song wrote: [...]I tried with gcc14 and can reproduced the issue described in the above. I build the kernel like below with gcc14 make KCFLAGS='-O3' -j and get the following build error WARN: resolve_btfids: unresolved symbol bpf_strnchr make[2]: *** [/home/yhs/work/bpf-next/scripts/Makefile.vmlinux:91: vmlinux] Error 255 make[2]: *** Deleting file 'vmlinux' Checking the symbol table: 22276: ffffffff81b15260 249 FUNC LOCAL DEFAULT 1 bpf_strnchr.cons[...] 235128: ffffffff81b1f540 296 FUNC GLOBAL DEFAULT 1 bpf_strnchr and the disasm code: bpf_strnchr: ... bpf_strchr: ... bpf_strnchr.constprop.0 ... So in symbol table, we have both bpf_strnchr.constprop.0 and bpf_strnchr. For such case, pahole will skip func bpf_strnchr hence the above resolve_btfids failure. The solution in this patch can indeed resolve this issue.It looks like instead of adding __noclone there is an option to improve pahole's filtering of ambiguous functions. Abstractly, there is nothing wrong with having a clone of a global function that has undergone additional optimizations. As long as the original symbol exists, everything should be fine.
Right. The generated code itself is totally fine. The problem is currently pahole will filter out bpf_strnchr since in the symbol table having both bpf_strnchr and bpf_strnchr.constprop.0. It there is no explicit dwarf-level signature in dwarf for bpf_strnchr.constprop.0. (For this particular .constprop.0 case, it is possible to derive the signature. but it will be hard for other suffixes like .isra). The current pahole will have strip out suffixes so the function name is 'bpf_strnchr' which covers bpf_strnchr and bpf_strnchr.constprop.0. Since two underlying signature is different, the 'bpf_strnchr' will be filtered out. I am actually working to improve such cases in llvm to address like foo() and foo.<...>() functions and they will have their own respective functions. We will discuss with gcc folks about how to implement similar approaches in gcc.
Since kfuncs are global, this should guarantee that the compiler does not change their signature, correct? Does this also hold for LTO builds?
Yes, the original signature will not changed. This holds for LTO build and global variables/functions will not be renamed.
If so, when pahole sees a set of symbols like [foo, foo.1, foo.2, ...],
The compiler needs to emit the signature in dwarf for foo.1, foo.2, etc. and this is something I am working on.
with 'foo' being global and the rest local, then there is no real need to filter out 'foo'.
I think the current __noclone approach is okay as the full implementation for signature changes (foo, foo.1, ...) might takes a while for both llvm and gcc.
Wdyt? [...]
