[PATCH bpf-next v3 14/15] bpf: allow '?' at the beginning of DATASEC names

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

Currently kernel does not allow question marks in BTF names.
This commit makes an exception, allowing first character of the
DATASEC name to be a question mark.

The intent is to allow libbpf to use SEC("?.struct_ops") to identify
struct_ops maps that are optional, e.g. like in the following BPF code:

    SEC("?.struct_ops")
    struct test_ops optional_map = { ... };

Which yields the following BTF:

    ...
    [13] DATASEC '?.struct_ops' size=0 vlen=...
    ...

To load such BTF libbpf rewrites DATASEC name before load.
After this patch the rewrite won't be necessary.

Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
 kernel/bpf/btf.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 6ff0bd1a91d5..a25fb6bce808 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -761,12 +761,13 @@ 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)
+static bool __btf_name_char_ok(char c, bool first, bool allow_qmark)
 {
 	if ((first ? !isalpha(c) :
 		     !isalnum(c)) &&
 	    c != '_' &&
-	    c != '.')
+	    c != '.' &&
+	    (allow_qmark && first ? c != '?' : true))
 		return false;
 	return true;
 }
@@ -783,20 +784,20 @@ 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)
+static bool __btf_name_valid(const struct btf *btf, u32 offset, bool allow_qmark)
 {
 	/* offset must be valid */
 	const char *src = btf_str_by_offset(btf, offset);
 	const char *src_limit;
 
-	if (!__btf_name_char_ok(*src, true))
+	if (!__btf_name_char_ok(*src, true, allow_qmark))
 		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))
+		if (!__btf_name_char_ok(*src, false, false))
 			return false;
 		src++;
 	}
@@ -806,12 +807,12 @@ static bool __btf_name_valid(const struct btf *btf, u32 offset)
 
 static bool btf_name_valid_identifier(const struct btf *btf, u32 offset)
 {
-	return __btf_name_valid(btf, offset);
+	return __btf_name_valid(btf, offset, false);
 }
 
 static bool btf_name_valid_section(const struct btf *btf, u32 offset)
 {
-	return __btf_name_valid(btf, offset);
+	return __btf_name_valid(btf, offset, true);
 }
 
 static const char *__btf_name_by_offset(const struct btf *btf, u32 offset)
@@ -4481,7 +4482,7 @@ static s32 btf_var_check_meta(struct btf_verifier_env *env,
 	}
 
 	if (!t->name_off ||
-	    !__btf_name_valid(env->btf, t->name_off)) {
+	    !btf_name_valid_identifier(env->btf, t->name_off)) {
 		btf_verifier_log_type(env, t, "Invalid name");
 		return -EINVAL;
 	}
-- 
2.43.0