On Tue, Jul 08, 2025 at 03:21:40PM +0800, 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.
could you reproduce this somehow (bpftrace/selftest) for some symbol?
I'd think pahole now filters all such symbols out of BTF and you need
BTF func record to load the program in the first place
jirka
>
> 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>
> ---
> v2:
> - Lookup both vmlinux and modules symbols when mod is NULL, just like
> kallsyms_lookup_name().
>
> If the btf is not a modules, shouldn't we lookup on the vmlinux only?
> I'm not sure if we should keep the same logic with
> kallsyms_lookup_name().
>
> - Return the kernel symbol that don't have ftrace location if the symbols
> with ftrace location are not available
> ---
> kernel/bpf/verifier.c | 77 ++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 72 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 53007182b46b..4bacd0abf207 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -23476,6 +23476,73 @@ 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;
> + bool ftrace_addr;
> +};
> +
> +static int symbol_callback(void *data, unsigned long addr)
> +{
> + struct symbol_lookup_ctx *ctx = data;
> +
> + ctx->addr = addr;
> + if (!ftrace_location(addr))
> + return 0;
> +
> + if (ctx->ftrace_addr)
> + return -EADDRNOTAVAIL;
> + ctx->ftrace_addr = true;
> +
> + 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 both vmlinux
> + * and modules symbols
> + * @sym: the symbol to resolve
> + * @addr: pointer to store the result
> + *
> + * Lookup the address of the symbol @sym. If multiple symbols with the name
> + * @sym exist, the one that has ftrace location is preferred. 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 };
> + const char *mod_name = NULL;
> + int err = 0;
> +
> +#ifdef CONFIG_MODULES
> + mod_name = mod ? mod->name : NULL;
> +#endif
> + if (!mod_name)
> + err = kallsyms_on_each_match_symbol(symbol_callback, sym, &ctx);
> +
> + if (!err && !ctx.addr)
> + err = module_kallsyms_on_each_symbol(mod_name, symbol_mod_callback,
> + &ctx);
> +
> + if (!ctx.addr)
> + err = -ENOENT;
> + *addr = err ? 0 : ctx.addr;
> +
> + return err;
> +}
> +
> int bpf_check_attach_target(struct bpf_verifier_log *log,
> const struct bpf_prog *prog,
> const struct bpf_prog *tgt_prog,
> @@ -23729,18 +23796,18 @@ 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;
> }
> }
>
> --
> 2.39.5
>
