On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
Currently libbpf is very strict about parsing BPF program isnstruction
isnstruction => instruction
sections. No gaps are allowed between sequential BPF programs within a given
ELF section. Libbpf enforced that by keeping track of the next section offset
that should start a new BPF (sub)program and cross-checks that by searching for
a corresponding STT_FUNC ELF symbol.
But this is too restrictive once we allow to have weak BPF programs and link
together two or more BPF object files. In such case, some weak BPF programs
might be "overriden" by either non-weak BPF program with the same name and
overriden => overridden
signature, or even by another weak BPF program that just happened to be linked
first. That, in turn, leaves BPF instructions of the "lost" BPF (sub)program
intact, but there is no corresponding ELF symbol, because no one is going to
be referencing it.
Libbpf already correctly handles such cases in the sense that it won't append
such dead code to actual BPF programs loaded into kernel. So the only change
that needs to be done is to relax the logic of parsing BPF instruction
sections. Instead of assuming next BPF (sub)program section offset, iterate
available STT_FUNC ELF symbols to discover all available BPF subprograms and
programs.
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Ack with a minor suggestion below.
Acked-by: Yonghong Song <yhs@xxxxxx>
---
tools/lib/bpf/libbpf.c | 56 ++++++++++++++++--------------------------
1 file changed, 21 insertions(+), 35 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ce5558d0a61b..a0e6d6bc47f3 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -502,8 +502,6 @@ static Elf_Scn *elf_sec_by_name(const struct bpf_object *obj, const char *name);
static int elf_sec_hdr(const struct bpf_object *obj, Elf_Scn *scn, GElf_Shdr *hdr);
static const char *elf_sec_name(const struct bpf_object *obj, Elf_Scn *scn);
static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn);
-static int elf_sym_by_sec_off(const struct bpf_object *obj, size_t sec_idx,
- size_t off, __u32 sym_type, GElf_Sym *sym);
void bpf_program__unload(struct bpf_program *prog)
{
@@ -644,10 +642,12 @@ static int
bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
const char *sec_name, int sec_idx)
{
+ Elf_Data *symbols = obj->efile.symbols;
struct bpf_program *prog, *progs;
void *data = sec_data->d_buf;
size_t sec_sz = sec_data->d_size, sec_off, prog_sz;
- int nr_progs, err;
+ size_t n = symbols->d_size / sizeof(GElf_Sym);
Maybe use "nr_syms" instead of "n" to be more descriptive?
+ int nr_progs, err, i;
const char *name;
GElf_Sym sym;
@@ -655,14 +655,16 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
nr_progs = obj->nr_programs;
sec_off = 0;
- while (sec_off < sec_sz) {
- if (elf_sym_by_sec_off(obj, sec_idx, sec_off, STT_FUNC, &sym)) {
- pr_warn("sec '%s': failed to find program symbol at offset %zu\n",
- sec_name, sec_off);
- return -LIBBPF_ERRNO__FORMAT;
- }
+ for (i = 0; i < n; i++) {
+ if (!gelf_getsym(symbols, i, &sym))
+ continue;
+ if (sym.st_shndx != sec_idx)
+ continue;
+ if (GELF_ST_TYPE(sym.st_info) != STT_FUNC)
+ continue;
prog_sz = sym.st_size;
+ sec_off = sym.st_value;
name = elf_sym_str(obj, sym.st_name);
if (!name) {
@@ -711,8 +713,6 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
nr_progs++;
obj->nr_programs = nr_progs;
-
- sec_off += prog_sz;
}
return 0;
@@ -2825,26 +2825,6 @@ static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn)
return data;
}
[...]
