[PATCH bpf-next v3 3/3] libbpf: Use mmap to parse vmlinux BTF from sysfs

[Lorenz Bauer <lmb@isovalent.com>]

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>
---
 tools/lib/bpf/btf.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 72 insertions(+), 11 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index b7513d4cce55b263310c341bc254df6364e829d9..3006c1ebb97ed899eb519b10927491d87ccdaca5 100644
--- a/tools/lib/bpf/btf.c
+++ b/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_no_copy(void *data, __u32 size, struct btf *base_btf)
 {
 	struct btf *btf;
 	int err;
@@ -1050,12 +1065,7 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 		btf->start_str_off = base_btf->hdr->str_len;
 	}
 
-	btf->raw_data = malloc(size);
-	if (!btf->raw_data) {
-		err = -ENOMEM;
-		goto done;
-	}
-	memcpy(btf->raw_data, data, size);
+	btf->raw_data = data;
 	btf->raw_size = size;
 
 	btf->hdr = btf->raw_data;
@@ -1081,6 +1091,24 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 	return btf;
 }
 
+static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
+{
+	struct btf *btf;
+	void *raw_data;
+
+	raw_data = malloc(size);
+	if (!raw_data)
+		return ERR_PTR(-ENOMEM);
+
+	memcpy(raw_data, data, size);
+
+	btf = btf_new_no_copy(raw_data, size, base_btf);
+	if (IS_ERR(btf))
+		free(raw_data);
+
+	return btf;
+}
+
 struct btf *btf__new(const void *data, __u32 size)
 {
 	return libbpf_ptr(btf_new(data, size, NULL));
@@ -1354,6 +1382,37 @@ 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;
+
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return libbpf_err_ptr(-errno);
+
+	if (fstat(fd, &st) < 0) {
+		close(fd);
+		return libbpf_err_ptr(-errno);
+	}
+
+	data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	close(fd);
+
+	if (data == MAP_FAILED)
+		return NULL;
+
+	btf = btf_new_no_copy(data, st.st_size, base_btf);
+	if (!btf)
+		munmap(data, st.st_size);
+	else
+		btf->raw_data_is_mmap = true;
+
+	return btf;
+}
+
 static struct btf *btf_parse(const char *path, struct btf *base_btf, struct btf_ext **btf_ext)
 {
 	struct btf *btf;
@@ -1659,8 +1718,7 @@ struct btf *btf__load_from_kernel_by_id(__u32 id)
 static void btf_invalidate_raw_data(struct btf *btf)
 {
 	if (btf->raw_data) {
-		free(btf->raw_data);
-		btf->raw_data = NULL;
+		btf_free_raw_data(btf);
 	}
 	if (btf->raw_data_swapped) {
 		free(btf->raw_data_swapped);
@@ -5290,7 +5348,10 @@ 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(sysfs_btf_path, NULL);
+		btf = btf_parse_raw_mmap(sysfs_btf_path, NULL);
+		if (IS_ERR_OR_NULL(btf))
+			btf = btf__parse(sysfs_btf_path, NULL);
+
 		if (!btf) {
 			err = -errno;
 			pr_warn("failed to read kernel BTF from '%s': %s\n",

-- 
2.49.0