On Mon, Jul 7, 2025 at 9:09 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 7/7/25 1:35 PM, Menglong Dong wrote:
> > For now, we lookup the address of the attach target in
> > bpf_check_attach_target() with find_kallsyms_symbol_value or
> > kallsyms_lookup_name, which is not accurate in some cases.
> >
> > For example, we want to attach to the target "t_next", but there are
> > multiple symbols with the name "t_next" exist in the kallsyms. The one
> > that kallsyms_lookup_name() returned may have no ftrace record, which
> > makes the attach target not available. So we want the one that has ftrace
> > record to be returned.
> >
> > Meanwhile, there may be multiple symbols with the name "t_next" in ftrace
> > record. In this case, the attach target is ambiguous, so the attach should
> > fail.
> >
> > Introduce the function bpf_lookup_attach_addr() to do the address lookup,
> > which is able to solve this problem.
> >
> > Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
>
> Breaks CI, see also:
Yeah, I should run the whole selftests :/
>
> First test_progs failure (test_progs-aarch64-gcc-14):
> #467/1 tracing_failure/bpf_spin_lock
> test_bpf_spin_lock:PASS:tracing_failure__open 0 nsec
> libbpf: prog 'test_spin_lock': BPF program load failed: -ENOENT
> libbpf: prog 'test_spin_lock': -- BEGIN PROG LOAD LOG --
> The address of function bpf_spin_lock cannot be found
> processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> -- END PROG LOAD LOG --
> libbpf: prog 'test_spin_lock': failed to load: -ENOENT
> libbpf: failed to load object 'tracing_failure'
> libbpf: failed to load BPF skeleton 'tracing_failure': -ENOENT
> test_bpf_spin_lock:FAIL:tracing_failure__load unexpected error: -2 (errno 2)
> #467/2 tracing_failure/bpf_spin_unlock
> test_bpf_spin_lock:PASS:tracing_failure__open 0 nsec
> libbpf: prog 'test_spin_unlock': BPF program load failed: -ENOENT
> libbpf: prog 'test_spin_unlock': -- BEGIN PROG LOAD LOG --
> The address of function bpf_spin_unlock cannot be found
> processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> -- END PROG LOAD LOG --
> libbpf: prog 'test_spin_unlock': failed to load: -ENOENT
> libbpf: failed to load object 'tracing_failure'
> libbpf: failed to load BPF skeleton 'tracing_failure': -ENOENT
> test_bpf_spin_lock:FAIL:tracing_failure__load unexpected error: -2 (errno 2)
>
> > kernel/bpf/verifier.c | 76 ++++++++++++++++++++++++++++++++++++++++---
> > 1 file changed, 71 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 0f6cc2275695..9a7128da6d13 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -23436,6 +23436,72 @@ static int check_non_sleepable_error_inject(u32 btf_id)
> > return btf_id_set_contains(&btf_non_sleepable_error_inject, btf_id);
> > }
> >
> > +struct symbol_lookup_ctx {
> > + const char *name;
> > + unsigned long addr;
> > +};
> > +
> > +static int symbol_callback(void *data, unsigned long addr)
> > +{
> > + struct symbol_lookup_ctx *ctx = data;
> > +
> > + if (!ftrace_location(addr))
> > + return 0;
> > +
> > + if (ctx->addr)
> > + return -EADDRNOTAVAIL;
> > +
> > + ctx->addr = addr;
> > +
> > + return 0;
> > +}
> > +
> > +static int symbol_mod_callback(void *data, const char *name, unsigned long addr)
> > +{
> > + if (strcmp(((struct symbol_lookup_ctx *)data)->name, name) != 0)
> > + return 0;
> > +
> > + return symbol_callback(data, addr);
> > +}
> > +
> > +/**
> > + * bpf_lookup_attach_addr: Lookup address for a symbol
> > + *
> > + * @mod: kernel module to lookup the symbol, NULL means to lookup the kernel
> > + * symbols
> > + * @sym: the symbol to resolve
> > + * @addr: pointer to store the result
> > + *
> > + * Lookup the address of the symbol @sym, and the address should has
> > + * corresponding ftrace location. If multiple symbols with the name @sym
> > + * exist, the one that has ftrace location will be returned. If more than
> > + * 1 has ftrace location, -EADDRNOTAVAIL will be returned.
> > + *
> > + * Returns: 0 on success, -errno otherwise.
> > + */
> > +static int bpf_lookup_attach_addr(const struct module *mod, const char *sym,
> > + unsigned long *addr)
> > +{
> > + struct symbol_lookup_ctx ctx = { .addr = 0, .name = sym };
> > + int err;
> > +
> > + if (!mod)
> > + err = kallsyms_on_each_match_symbol(symbol_callback, sym, &ctx);
>
> This is also not really equivalent to kallsyms_lookup_name(). kallsyms_on_each_match_symbol()
> only iterates over all symbols in vmlinux whereas kallsyms_lookup_name() looks up both vmlinux
> and modules.
Yeah, my mistake. I'll fixup this logic in the next version.
Thanks!
Menglong Dong
>
> > + else
> > + err = module_kallsyms_on_each_symbol(mod->name, symbol_mod_callback,
> > + &ctx);
> > +
> > + if (!ctx.addr)
> > + return -ENOENT;
> > +
> > + if (err)
> > + return err;
> > +
> > + *addr = ctx.addr;
> > +
> > + return 0;
> > +}
> > +
> > int bpf_check_attach_target(struct bpf_verifier_log *log,
> > const struct bpf_prog *prog,
> > const struct bpf_prog *tgt_prog,
> > @@ -23689,18 +23755,18 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
> > if (btf_is_module(btf)) {
> > mod = btf_try_get_module(btf);
> > if (mod)
> > - addr = find_kallsyms_symbol_value(mod, tname);
> > + ret = bpf_lookup_attach_addr(mod, tname, &addr);
> > else
> > - addr = 0;
> > + ret = -ENOENT;
> > } else {
> > - addr = kallsyms_lookup_name(tname);
> > + ret = bpf_lookup_attach_addr(NULL, tname, &addr);
> > }
> > - if (!addr) {
> > + if (ret) {
> > module_put(mod);
> > bpf_log(log,
> > "The address of function %s cannot be found\n",
> > tname);
> > - return -ENOENT;
> > + return ret;
> > }
> > }
> >
>
