On Thu, Mar 21, 2024 at 1:01 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote: > > With CONFIG_LTO_CLANG_THIN enabled, with some of previous > version of kernel code base ([1]), I hit the following > error: > test_ksyms:PASS:kallsyms_fopen 0 nsec > test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found > #118 ksyms:FAIL > > The reason is that 'bpf_link_fops' is renamed to > bpf_link_fops.llvm.8325593422554671469 > Due to cross-file inlining, the static variable 'bpf_link_fops' > in syscall.c is used by a function in another file. To avoid > potential duplicated names, the llvm added suffix > '.llvm.<hash>' ([2]) to 'bpf_link_fops' variable. > Such renaming caused a problem in libbpf if 'bpf_link_fops' > is used in bpf prog as a ksym as 'bpf_link_fops' does not > match any symbol in /proc/kallsyms. > > To fix this issue, libbpf needs to understand that suffix '.llvm.<hash>' > is caused by clang lto kernel and to process such symbols properly. > > With latest bpf-next code base built with CONFIG_LTO_CLANG_THIN, > I cannot reproduce the above failure any more. But such an issue > could happen with other symbols. > > For example, with my current kernel, I got the following from > /proc/kallsyms: > ffffffff84782154 d __func__.net_ratelimit.llvm.6135436931166841955 > ffffffff85f0a500 d tk_core.llvm.726630847145216431 > ffffffff85fdb960 d __fs_reclaim_map.llvm.10487989720912350772 > ffffffff864c7300 d fake_dst_ops.llvm.54750082607048300 > > I could not easily create a selftest to test newly-added > libbpf functionality with a static C test since I do not know > which symbol is cross-file inlined. But based on my particular kernel, > the following test change can run successfully. > > diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c b/tools/testing/selftests/bpf/prog_tests/ksyms.c > index 6a86d1f07800..904a103f7b1d 100644 > --- a/tools/testing/selftests/bpf/prog_tests/ksyms.c > +++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c > @@ -42,6 +42,7 @@ void test_ksyms(void) > ASSERT_EQ(data->out__bpf_link_fops, link_fops_addr, "bpf_link_fops"); > ASSERT_EQ(data->out__bpf_link_fops1, 0, "bpf_link_fops1"); > ASSERT_EQ(data->out__btf_size, btf_size, "btf_size"); > + ASSERT_NEQ(data->out__fake_dst_ops, 0, "fake_dst_ops"); > ASSERT_EQ(data->out__per_cpu_start, per_cpu_start_addr, "__per_cpu_start"); > > cleanup: > diff --git a/tools/testing/selftests/bpf/progs/test_ksyms.c b/tools/testing/selftests/bpf/progs/test_ksyms.c > index 6c9cbb5a3bdf..fe91eef54b66 100644 > --- a/tools/testing/selftests/bpf/progs/test_ksyms.c > +++ b/tools/testing/selftests/bpf/progs/test_ksyms.c > @@ -9,11 +9,13 @@ __u64 out__bpf_link_fops = -1; > __u64 out__bpf_link_fops1 = -1; > __u64 out__btf_size = -1; > __u64 out__per_cpu_start = -1; > +__u64 out__fake_dst_ops = -1; > > extern const void bpf_link_fops __ksym; > extern const void __start_BTF __ksym; > extern const void __stop_BTF __ksym; > extern const void __per_cpu_start __ksym; > +extern const void fake_dst_ops __ksym; > /* non-existing symbol, weak, default to zero */ > extern const void bpf_link_fops1 __ksym __weak; > > @@ -23,6 +25,7 @@ int handler(const void *ctx) > out__bpf_link_fops = (__u64)&bpf_link_fops; > out__btf_size = (__u64)(&__stop_BTF - &__start_BTF); > out__per_cpu_start = (__u64)&__per_cpu_start; > + out__fake_dst_ops = (__u64)&fake_dst_ops; > > out__bpf_link_fops1 = (__u64)&bpf_link_fops1; > > This patch fixed the issue in libbpf such that if clang lto kernel > is enabled and the symbol resolution is for ksym's, > the suffix '.llvm.<hash>' will be ignored during comparison of > bpf prog ksym vs. symbols in /proc/kallsyms, this resolved the issue. > > Note that currently kernel does not support gcc build with lto. > > [1] https://lore.kernel.org/bpf/20240302165017.1627295-1-yonghong.song@xxxxxxxxx/ > [2] https://github.com/llvm/llvm-project/blob/release/18.x/llvm/include/llvm/IR/ModuleSummaryIndex.h#L1714-L1719 > > Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx> > --- > tools/lib/bpf/libbpf.c | 64 +++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 63 insertions(+), 1 deletion(-) > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > index a7a89269148c..8c3861192bc8 100644 > --- a/tools/lib/bpf/libbpf.c > +++ b/tools/lib/bpf/libbpf.c > @@ -664,6 +664,7 @@ struct bpf_object { > bool loaded; > bool has_subcalls; > bool has_rodata; > + bool need_kallsyms; > > struct bpf_gen *gen_loader; > > @@ -8016,14 +8017,73 @@ static int libbpf_kallsyms_parse(kallsyms_cb_t cb, void *ctx) > return err; > } > > +static int check_lto_kernel(void) > +{ > + static int check_lto = 2; > + char buf[PATH_MAX]; > + struct utsname uts; > + gzFile file; > + int len; > + > + if (check_lto != 2) > + return check_lto; > + > + uname(&uts); > + len = snprintf(buf, PATH_MAX, "/boot/config-%s", uts.release); > + if (len < 0) { > + check_lto = -EINVAL; > + goto out; > + } else if (len >= PATH_MAX) { > + check_lto = -ENAMETOOLONG; > + goto out; > + } > + > + /* gzopen also accepts uncompressed files. */ > + file = gzopen(buf, "re"); > + if (!file) > + file = gzopen("/proc/config.gz", "re"); > + > + if (!file) { > + check_lto = -ENOENT; > + goto out; > + } > + > + check_lto = 0; > + while (gzgets(file, buf, sizeof(buf))) { > + /* buf also contains '\n', skip it during comparison. */ > + if (!strncmp(buf, "CONFIG_LTO_CLANG=y", 18)) { > + check_lto = 1; > + break; > + } > + } > + > + gzclose(file); > +out: > + return check_lto; > +} > + > static int kallsyms_cb(unsigned long long sym_addr, char sym_type, > const char *sym_name, void *ctx) > { > + int lto_enabled = check_lto_kernel(); > + char orig_name[PATH_MAX], *res; > struct bpf_object *obj = ctx; > const struct btf_type *t; > struct extern_desc *ext; > > - ext = find_extern_by_name(obj, sym_name); > + /* Only check static variables in data sections */ > + if (sym_type == 'd' && obj->need_kallsyms && lto_enabled == 1) { > + strcpy(orig_name, sym_name); > + res = strstr(orig_name, ".llvm."); > + if (res) { > + *res = '\0'; > + pr_debug("extern (ksym) '%s': use original name '%s' for comparison\n", > + sym_name, orig_name); > + } > + ext = find_extern_by_name(obj, orig_name); let's teach find_extern_by_name() to take string length so we don't have to do those strcpy, we can just pass "effective length" I'll take a look at the patch set tomorrow, ran out of time today, sorry. > + } else { > + ext = find_extern_by_name(obj, sym_name); > + } > if (!ext || ext->type != EXT_KSYM) > return 0; > > @@ -8322,7 +8382,9 @@ static int bpf_object__resolve_externs(struct bpf_object *obj, > return -EINVAL; > } > if (need_kallsyms) { > + obj->need_kallsyms = true; > err = bpf_object__read_kallsyms_file(obj); > + obj->need_kallsyms = false; > if (err) > return -EINVAL; > } > -- > 2.43.0 >
