Commit 6ebaa3fb authored by Eduard Zingerman's avatar Eduard Zingerman Committed by Andrii Nakryiko
Browse files

libbpf: Rewrite btf datasec names starting from '?' 



Optional struct_ops maps are defined using question mark at the start
of the section name, e.g.:

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

This commit teaches libbpf to detect if kernel allows '?' prefix
in datasec names, and if it doesn't then to rewrite such names
by replacing '?' with '_', e.g.:

    DATASEC ?.struct_ops -> DATASEC _.struct_ops

Signed-off-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240306104529.6453-13-eddyz87@gmail.com
parent 5ad0ecbe

tools/lib/bpf/features.c

+22 −0
Original line number Diff line number Diff line
@@ -147,6 +147,25 @@ static int probe_kern_btf_datasec(int token_fd) 
					     strs, sizeof(strs), token_fd));

}



static int probe_kern_btf_qmark_datasec(int token_fd)

{

	static const char strs[] = "\0x\0?.data";

	/* static int a; */

	__u32 types[] = {

		/* int */

		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),  /* [1] */

		/* VAR x */                                     /* [2] */

		BTF_TYPE_ENC(1, BTF_INFO_ENC(BTF_KIND_VAR, 0, 0), 1),

		BTF_VAR_STATIC,

		/* DATASEC ?.data */                            /* [3] */

		BTF_TYPE_ENC(3, BTF_INFO_ENC(BTF_KIND_DATASEC, 0, 1), 4),

		BTF_VAR_SECINFO_ENC(2, 0, 4),

	};



	return probe_fd(libbpf__load_raw_btf((char *)types, sizeof(types),

					     strs, sizeof(strs), token_fd));

}



static int probe_kern_btf_float(int token_fd)

{

	static const char strs[] = "\0float";
@@ -534,6 +553,9 @@ static struct kern_feature_desc { 
	[FEAT_ARG_CTX_TAG] = {

		"kernel-side __arg_ctx tag", probe_kern_arg_ctx_tag,

	},

	[FEAT_BTF_QMARK_DATASEC] = {

		"BTF DATASEC names starting from '?'", probe_kern_btf_qmark_datasec,

	},

};



bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)

tools/lib/bpf/libbpf.c

+17 −2
Original line number Diff line number Diff line
@@ -2869,6 +2869,11 @@ static bool section_have_execinstr(struct bpf_object *obj, int idx) 
	return sh->sh_flags & SHF_EXECINSTR;

}



static bool starts_with_qmark(const char *s)

{

	return s && s[0] == '?';

}



static bool btf_needs_sanitization(struct bpf_object *obj)

{

	bool has_func_global = kernel_supports(obj, FEAT_BTF_GLOBAL_FUNC);
@@ -2878,9 +2883,10 @@ static bool btf_needs_sanitization(struct bpf_object *obj) 
	bool has_decl_tag = kernel_supports(obj, FEAT_BTF_DECL_TAG);

	bool has_type_tag = kernel_supports(obj, FEAT_BTF_TYPE_TAG);

	bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);

	bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);



	return !has_func || !has_datasec || !has_func_global || !has_float ||

	       !has_decl_tag || !has_type_tag || !has_enum64;

	       !has_decl_tag || !has_type_tag || !has_enum64 || !has_qmark_datasec;

}



static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
@@ -2892,6 +2898,7 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf) 
	bool has_decl_tag = kernel_supports(obj, FEAT_BTF_DECL_TAG);

	bool has_type_tag = kernel_supports(obj, FEAT_BTF_TYPE_TAG);

	bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);

	bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);

	int enum64_placeholder_id = 0;

	struct btf_type *t;

	int i, j, vlen;
@@ -2918,7 +2925,7 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf) 


			name = (char *)btf__name_by_offset(btf, t->name_off);

			while (*name) {

				if (*name == '.')

				if (*name == '.' || *name == '?')

					*name = '_';

				name++;

			}
@@ -2933,6 +2940,14 @@ static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf) 
				vt = (void *)btf__type_by_id(btf, v->type);

				m->name_off = vt->name_off;

			}

		} else if (!has_qmark_datasec && btf_is_datasec(t) &&

			   starts_with_qmark(btf__name_by_offset(btf, t->name_off))) {

			/* replace '?' prefix with '_' for DATASEC names */

			char *name;



			name = (char *)btf__name_by_offset(btf, t->name_off);

			if (name[0] == '?')

				name[0] = '_';

		} else if (!has_func && btf_is_func_proto(t)) {

			/* replace FUNC_PROTO with ENUM */

			vlen = btf_vlen(t);

tools/lib/bpf/libbpf_internal.h

+2 −0
Original line number Diff line number Diff line
@@ -374,6 +374,8 @@ enum kern_feature_id { 
	FEAT_UPROBE_MULTI_LINK,

	/* Kernel supports arg:ctx tag (__arg_ctx) for global subprogs natively */

	FEAT_ARG_CTX_TAG,

	/* Kernel supports '?' at the front of datasec names */

	FEAT_BTF_QMARK_DATASEC,

	__FEAT_CNT,

};