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 | 85 +++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 69 insertions(+), 16 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index f18d7e6a453cd9e5c384487659df04f7efafdf5a..42815a29c0a52a1a7eed2c6b22b9b1754ae01c9a 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(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;
 	}
 
-	btf->raw_data = malloc(size);
-	if (!btf->raw_data) {
-		err = -ENOMEM;
-		goto done;
+	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);
 	}
-	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 +1375,36 @@ 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);
+	close(fd);
+
+	if (data == MAP_FAILED)
+		return NULL;
+
+	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 +1669,7 @@ 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);
@@ -1659,8 +1710,7 @@ 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);
@@ -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(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

On Sat, May 10, 2025 at 5:35 AM Lorenz Bauer <lmb@isovalent.com> wrote:
>
> 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 | 85 +++++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 69 insertions(+), 16 deletions(-)
>

Almost there, see below. With those changes feel free to add my ack

Acked-by: Andrii Nakryiko <andrii@kernel.org>

> +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 NULL;

s/return NULL;/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 +1669,7 @@ 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);
> @@ -1659,8 +1710,7 @@ 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);
>         }

drop now unnecessary {} ?

>         if (btf->raw_data_swapped) {
>                 free(btf->raw_data_swapped);
> @@ -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(sysfs_btf_path, NULL);
> +               btf = btf_parse_raw_mmap(sysfs_btf_path, NULL);
> +               if (IS_ERR_OR_NULL(btf))

just IS_ERR() with the fixes I pointed out above

> +                       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
>