libbpf: Use mmap to parse vmlinux BTF from sysfs

Teach libbpf to use mmap when parsing vmlinux BTF from /sys. We don't
apply this to fall-back paths on the regular file system because there
is no way to ensure that modifications underlying the MAP_PRIVATE
mapping are not visible to the process.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250520-vmlinux-mmap-v5-3-e8c941acc414@isovalent.com

tools/lib/bpf/btf.c

@@ -12,6 +12,7 @@
 #include <sys/utsname.h>
 #include <sys/param.h>
 #include <sys/stat.h>
+#include <sys/mman.h>
 #include <linux/kernel.h>
 #include <linux/err.h>
 #include <linux/btf.h>
@@ -120,6 +121,9 @@ struct btf {
 	/* whether base_btf should be freed in btf_free for this instance */
 	bool owns_base;
 
+	/* whether raw_data is a (read-only) mmap */
+	bool raw_data_is_mmap;
+
 	/* BTF object FD, if loaded into kernel */
 	int fd;
 
@@ -951,6 +955,17 @@ static bool btf_is_modifiable(const struct btf *btf)
 	return (void *)btf->hdr != btf->raw_data;
 }
 
+static void btf_free_raw_data(struct btf *btf)
+{
+	if (btf->raw_data_is_mmap) {
+		munmap(btf->raw_data, btf->raw_size);
+		btf->raw_data_is_mmap = false;
+	} else {
+		free(btf->raw_data);
+	}
+	btf->raw_data = NULL;
+}
+
 void btf__free(struct btf *btf)
 {
 	if (IS_ERR_OR_NULL(btf))
@@ -970,7 +985,7 @@ void btf__free(struct btf *btf)
 		free(btf->types_data);
 		strset__free(btf->strs_set);
 	}
-	free(btf->raw_data);
+	btf_free_raw_data(btf);
 	free(btf->raw_data_swapped);
 	free(btf->type_offs);
 	if (btf->owns_base)
@@ -1030,7 +1045,7 @@ struct btf *btf__new_empty_split(struct btf *base_btf)
 	return libbpf_ptr(btf_new_empty(base_btf));
 }
 
-static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
+static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf, bool is_mmap)
 {
 	struct btf *btf;
 	int err;
@@ -1050,12 +1065,18 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 		btf->start_str_off = base_btf->hdr->str_len;
 	}
 
+	if (is_mmap) {
+		btf->raw_data = (void *)data;
+		btf->raw_data_is_mmap = true;
+	} else {
 		btf->raw_data = malloc(size);
 		if (!btf->raw_data) {
 			err = -ENOMEM;
 			goto done;
 		}
 		memcpy(btf->raw_data, data, size);
+	}
+
 	btf->raw_size = size;
 
 	btf->hdr = btf->raw_data;
@@ -1083,12 +1104,12 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 
 struct btf *btf__new(const void *data, __u32 size)
 {
-	return libbpf_ptr(btf_new(data, size, NULL));
+	return libbpf_ptr(btf_new(data, size, NULL, false));
 }
 
 struct btf *btf__new_split(const void *data, __u32 size, struct btf *base_btf)
 {
-	return libbpf_ptr(btf_new(data, size, base_btf));
+	return libbpf_ptr(btf_new(data, size, base_btf, false));
 }
 
 struct btf_elf_secs {
@@ -1209,7 +1230,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 
 	if (secs.btf_base_data) {
 		dist_base_btf = btf_new(secs.btf_base_data->d_buf, secs.btf_base_data->d_size,
-					NULL);
+					NULL, false);
 		if (IS_ERR(dist_base_btf)) {
 			err = PTR_ERR(dist_base_btf);
 			dist_base_btf = NULL;
@@ -1218,7 +1239,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 	}
 
 	btf = btf_new(secs.btf_data->d_buf, secs.btf_data->d_size,
-		      dist_base_btf ?: base_btf);
+		      dist_base_btf ?: base_btf, false);
 	if (IS_ERR(btf)) {
 		err = PTR_ERR(btf);
 		goto done;
@@ -1335,7 +1356,7 @@ static struct btf *btf_parse_raw(const char *path, struct btf *base_btf)
 	}
 
 	/* finally parse BTF data */
-	btf = btf_new(data, sz, base_btf);
+	btf = btf_new(data, sz, base_btf, false);
 
 err_out:
 	free(data);
@@ -1354,6 +1375,37 @@ struct btf *btf__parse_raw_split(const char *path, struct btf *base_btf)
 	return libbpf_ptr(btf_parse_raw(path, base_btf));
 }
 
+static struct btf *btf_parse_raw_mmap(const char *path, struct btf *base_btf)
+{
+	struct stat st;
+	void *data;
+	struct btf *btf;
+	int fd, err;
+
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return libbpf_err_ptr(-errno);
+
+	if (fstat(fd, &st) < 0) {
+		err = -errno;
+		close(fd);
+		return libbpf_err_ptr(err);
+	}
+
+	data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	err = -errno;
+	close(fd);
+
+	if (data == MAP_FAILED)
+		return libbpf_err_ptr(err);
+
+	btf = btf_new(data, st.st_size, base_btf, true);
+	if (IS_ERR(btf))
+		munmap(data, st.st_size);
+
+	return btf;
+}
+
 static struct btf *btf_parse(const char *path, struct btf *base_btf, struct btf_ext **btf_ext)
 {
 	struct btf *btf;
@@ -1618,7 +1670,7 @@ struct btf *btf_get_from_fd(int btf_fd, struct btf *base_btf)
 		goto exit_free;
 	}
 
-	btf = btf_new(ptr, btf_info.btf_size, base_btf);
+	btf = btf_new(ptr, btf_info.btf_size, base_btf, false);
 
 exit_free:
 	free(ptr);
@@ -1658,10 +1710,8 @@ struct btf *btf__load_from_kernel_by_id(__u32 id)
 
 static void btf_invalidate_raw_data(struct btf *btf)
 {
-	if (btf->raw_data) {
+	if (btf->raw_data)
-		free(btf->raw_data);
+		btf_free_raw_data(btf);
-		btf->raw_data = NULL;
-	}
 	if (btf->raw_data_swapped) {
 		free(btf->raw_data_swapped);
 		btf->raw_data_swapped = NULL;
@@ -5331,7 +5381,10 @@ struct btf *btf__load_vmlinux_btf(void)
 		pr_warn("kernel BTF is missing at '%s', was CONFIG_DEBUG_INFO_BTF enabled?\n",
 			sysfs_btf_path);
 	} else {
+		btf = btf_parse_raw_mmap(sysfs_btf_path, NULL);
+		if (IS_ERR(btf))
 			btf = btf__parse(sysfs_btf_path, NULL);
+
 		if (!btf) {
 			err = -errno;
 			pr_warn("failed to read kernel BTF from '%s': %s\n",