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);
+ } 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;
