[PATCH bpf v2] bpf: remove unnecessary BTF lookups in bpf_sk_storage_tracing_allowed

[Jared Kangas <jkangas@redhat.com>]

When loading BPF programs, bpf_sk_storage_tracing_allowed() does a
series of lookups to get a type name from the program's attach_btf_id,
making the assumption that the type is present in the vmlinux BTF along
the way. However, this results in btf_type_by_id() returning a null
pointer if a non-vmlinux kernel BTF is attached to. Proof-of-concept on
a kernel with CONFIG_IPV6=m:

    $ cat bpfcrash.c
    #include <unistd.h>
    #include <linux/bpf.h>
    #include <sys/syscall.h>

    static int bpf(enum bpf_cmd cmd, union bpf_attr *attr)
    {
        return syscall(__NR_bpf, cmd, attr, sizeof(*attr));
    }

    int main(void)
    {
        const int btf_fd = bpf(BPF_BTF_GET_FD_BY_ID, &(union bpf_attr) {
            .btf_id = BTF_ID,
        });
        if (btf_fd < 0)
            return 1;

        const int bpf_sk_storage_get = 107;
        const struct bpf_insn insns[] = {
            { .code = BPF_JMP | BPF_CALL, .imm = bpf_sk_storage_get},
            { .code = BPF_JMP | BPF_EXIT },
        };
        return bpf(BPF_PROG_LOAD, &(union bpf_attr) {
            .prog_type            = BPF_PROG_TYPE_TRACING,
            .expected_attach_type = BPF_TRACE_FENTRY,
            .license              = (unsigned long)"GPL",
            .insns                = (unsigned long)&insns,
            .insn_cnt             = sizeof(insns) / sizeof(insns[0]),
            .attach_btf_obj_fd    = btf_fd,
            .attach_btf_id        = TYPE_ID,
        });
    }
    $ sudo bpftool btf list | grep ipv6
    2: name [ipv6]  size 928200B
    $ sudo bpftool btf dump id 2 | awk '$3 ~ /inet6_sock_destruct/'
    [130689] FUNC 'inet6_sock_destruct' type_id=130677 linkage=static
    $ gcc -D_DEFAULT_SOURCE -DBTF_ID=2 -DTYPE_ID=130689 \
        bpfcrash.c -o bpfcrash
    $ sudo ./bpfcrash

This causes a null pointer dereference:

    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
    Call trace:
     bpf_sk_storage_tracing_allowed+0x8c/0xb0 P
     check_helper_call.isra.0+0xa8/0x1730
     do_check+0xa18/0xb40
     do_check_common+0x140/0x640
     bpf_check+0xb74/0xcb8
     bpf_prog_load+0x598/0x9a8
     __sys_bpf+0x580/0x980
     __arm64_sys_bpf+0x28/0x40
     invoke_syscall.constprop.0+0x54/0xe8
     do_el0_svc+0xb4/0xd0
     el0_svc+0x44/0x1f8
     el0t_64_sync_handler+0x13c/0x160
     el0t_64_sync+0x184/0x188

Resolve this by using prog->aux->attach_func_name and removing the
lookups.

Fixes: 8e4597c627fb ("bpf: Allow using bpf_sk_storage in FENTRY/FEXIT/RAW_TP")
Link: https://lore.kernel.org/bpf/20250116162356.1054047-1-jkangas@redhat.com
Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
Signed-off-by: Jared Kangas <jkangas@redhat.com>
---
V1 -> V2: Used Martin KaFai Lau's suggestion (see lore.kernel.org link),
          edited patch summary + description accordingly.

 net/core/bpf_sk_storage.c | 13 +------------
 1 file changed, 1 insertion(+), 12 deletions(-)

diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
index 2f4ed83a75ae..72a0dc8b1646 100644
--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c
@@ -352,11 +352,6 @@ const struct bpf_func_proto bpf_sk_storage_delete_proto = {
 
 static bool bpf_sk_storage_tracing_allowed(const struct bpf_prog *prog)
 {
-	const struct btf *btf_vmlinux;
-	const struct btf_type *t;
-	const char *tname;
-	u32 btf_id;
-
 	if (prog->aux->dst_prog)
 		return false;
 
@@ -371,13 +366,7 @@ static bool bpf_sk_storage_tracing_allowed(const struct bpf_prog *prog)
 		return true;
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
-		btf_vmlinux = bpf_get_btf_vmlinux();
-		if (IS_ERR_OR_NULL(btf_vmlinux))
-			return false;
-		btf_id = prog->aux->attach_btf_id;
-		t = btf_type_by_id(btf_vmlinux, btf_id);
-		tname = btf_name_by_offset(btf_vmlinux, t->name_off);
-		return !!strncmp(tname, "bpf_sk_storage",
+		return !!strncmp(prog->aux->attach_func_name, "bpf_sk_storage",
 				 strlen("bpf_sk_storage"));
 	default:
 		return false;
-- 
2.47.1