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",
--
2.49.0On Mon, May 5, 2025 at 11:39 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 | 83 ++++++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 72 insertions(+), 11 deletions(-)
>
[...]
> @@ -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)
btf_new() is internal, so I'd extend existing btf_new() with `bool
is_mmap` and not add btf_new_no_copy(), I think it's simpler.
Eventually we can turn is_mmap into some sort of flags, if we need
more tuning of data ownership behavior
> +{
> + 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);
close() can clobber errno, so save `err = -errno` before it
> + return libbpf_err_ptr(-errno);
> + }
> +
> + data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
> + close(fd);
same, errno clobbering danger
> +
> + if (data == MAP_FAILED)
> + return NULL;
> +
> + btf = btf_new_no_copy(data, st.st_size, base_btf);
> + if (!btf)
btf_new_no_copy() is returning ERR_PTR() on error, no?
pw-bot: cr
> + 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
>
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