On Fri, Mar 6, 2026 at 9:03 PM Yonghong Song <[email protected]> wrote: > > The current clang thin-lto build often produces lots of symbols with > suffix. The following is a partial list of such function call symbols: > ... > ethnl_module_fw_flash_ntf.llvm.7631589765585346066 > __nf_conntrack_alloc.llvm.6438426151906658917 > tcp_can_early_drop.llvm.11937612064648250727 > tcp_print_conntrack.llvm.11937612064648250727 > ... > > In my particular build with current bpf-next, the number of '*.llvm.<hash>' > function calls is 1212. As the side effect of cross-file inlining, > some static variables may be promoted with '*.llvm.<hash>' as well. > In my same setup, the number of variables with such suffixes is 9. > > Such symbols make kernel live patching difficult since > - a minor code change will change the hash and then the '*.llvm.<hash>' > symbol becomes another one with a different hash. Sometimes, maybe > the suffix is gone. > - a previous source-level symbol may become a one with suffix after live > patching code. > > In [1], Song Liu suggested to reduce the number of '*.llvm.<hash>' functions > to make live patch easier. In respond of this, I implemented this > in llvm ([2]). The same thin-lto build with [2] only has two symbols with > suffix: > m_stop.llvm.14460341347352036579 > m_next.llvm.14460341347352036579 > This should make live patch much easier. > > To support suffix symbol reduction, two lld flags are necessary to enable > this feature in kernel: > - Flag '--lto-whole-program-visibility' is needed as it ensures that all > non-assembly files are available in the same thin-lto lld, which is true > for kernel. > - Flag '-mllvm -always-rename-promoted-locals=false' is needed to enable > suffix reduction. Currently in llvm [1], only process mode is supported. > There is another distributed mode (across different processes or even > different machines) which is not supported yet ([2]). The kernel uses > process mode so it should work. > > The assembly files may have some global functions/data which may potentially > conflict with thin-lto global symbols after the above two flags. But such > assembly > global symbols are limited and tend to be uniquely named for its context. > Hence the conflict with globals in non-assembly codes is rare. If indeed the > conflict happens, we can rename either of them to avoid conflicts. > > Nathan Chancellor suggested the following under thin-lto: > KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm > -always-rename-promoted-locals=false) > The '-mllvm -always-rename-promoted-locals=false' flag is only available for > llvm23. > So for llvm22 or earlier, the above KBUILD_LDFLAGS will ignore those two > flags. > For llvm23 and later, two flags will be added to KBUILD_LDFLAGS. > > [1] https://lpc.events/event/19/contributions/2212 > [2] https://github.com/llvm/llvm-project/pull/178587 > > Signed-off-by: Yonghong Song <[email protected]>
Acked-by: Song Liu <[email protected]> Thanks for making this improvement happen!
