Re: [PATCH bpf-next v2] bpf: make the attach target more accurate

[Jiri Olsa <olsajiri@gmail.com>]

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@chinatelecom.cn>
> ---
> 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
>