bpf/btf: Accept function names that contain dots

When building a kernel with LLVM=1, LLVM_IAS=0 and CONFIG_KASAN=y, LLVM
leaves DWARF tags for the "asan.module_ctor" & co symbols. In turn,
pahole creates BTF_KIND_FUNC entries for these and this makes the BTF
metadata validation fail because they contain a dot.

In a dramatic turn of event, this BTF verification failure can cause
the netfilter_bpf initialization to fail, causing netfilter_core to
free the netfilter_helper hashmap and netfilter_ftp to trigger a
use-after-free. The risk of u-a-f in netfilter will be addressed
separately but the existence of "asan.module_ctor" debug info under some
build conditions sounds like a good enough reason to accept functions
that contain dots in BTF.

Although using only LLVM=1 is the recommended way to compile clang-based
kernels, users can certainly do LLVM=1, LLVM_IAS=0 as well and we still
try to support that combination according to Nick. To clarify:

  - > v5.10 kernel, LLVM=1 (LLVM_IAS=0 is not the default) is recommended,
    but user can still have LLVM=1, LLVM_IAS=0 to trigger the issue

  - <= 5.10 kernel, LLVM=1 (LLVM_IAS=0 is the default) is recommended in
    which case GNU as will be used

Fixes: 1dc92851849c ("bpf: kernel side support for BTF Var and DataSec")
Signed-off-by: Florent Revest <revest@chromium.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Cc: Yonghong Song <yhs@meta.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/bpf/20230615145607.3469985-1-revest@chromium.org

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 6b682b8..72b32b7 100644 (file)
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -744,13 +744,12 @@ static bool btf_name_offset_valid(const struct btf *btf, u32 offset)
        return offset < btf->hdr.str_len;
 }
 
-static bool __btf_name_char_ok(char c, bool first, bool dot_ok)
+static bool __btf_name_char_ok(char c, bool first)
 {
        if ((first ? !isalpha(c) :
                     !isalnum(c)) &&
            c != '_' &&
-           ((c == '.' && !dot_ok) ||
-             c != '.'))
+           c != '.')
                return false;
        return true;
 }
@@ -767,20 +766,20 @@ static const char *btf_str_by_offset(const struct btf *btf, u32 offset)
        return NULL;
 }
 
-static bool __btf_name_valid(const struct btf *btf, u32 offset, bool dot_ok)
+static bool __btf_name_valid(const struct btf *btf, u32 offset)
 {
        /* offset must be valid */
        const char *src = btf_str_by_offset(btf, offset);
        const char *src_limit;
 
-       if (!__btf_name_char_ok(*src, true, dot_ok))
+       if (!__btf_name_char_ok(*src, true))
                return false;
 
        /* set a limit on identifier length */
        src_limit = src + KSYM_NAME_LEN;
        src++;
        while (*src && src < src_limit) {
-               if (!__btf_name_char_ok(*src, false, dot_ok))
+               if (!__btf_name_char_ok(*src, false))
                        return false;
                src++;
        }
@@ -788,17 +787,14 @@ static bool __btf_name_valid(const struct btf *btf, u32 offset, bool dot_ok)
        return !*src;
 }
 
-/* Only C-style identifier is permitted. This can be relaxed if
- * necessary.
- */
 static bool btf_name_valid_identifier(const struct btf *btf, u32 offset)
 {
-       return __btf_name_valid(btf, offset, false);
+       return __btf_name_valid(btf, offset);
 }
 
 static bool btf_name_valid_section(const struct btf *btf, u32 offset)
 {
-       return __btf_name_valid(btf, offset, true);
+       return __btf_name_valid(btf, offset);
 }
 
 static const char *__btf_name_by_offset(const struct btf *btf, u32 offset)
@@ -4422,7 +4418,7 @@ static s32 btf_var_check_meta(struct btf_verifier_env *env,
        }
 
        if (!t->name_off ||
-           !__btf_name_valid(env->btf, t->name_off, true)) {
+           !__btf_name_valid(env->btf, t->name_off)) {
                btf_verifier_log_type(env, t, "Invalid name");
                return -EINVAL;
        }