Merge branch 'allow-mmap-of-sys-kernel-btf-vmlinux'

Lorenz Bauer says:

====================
Allow mmap of /sys/kernel/btf/vmlinux

I'd like to cut down the memory usage of parsing vmlinux BTF in ebpf-go.
With some upcoming changes the library is sitting at 5MiB for a parse.
Most of that memory is simply copying the BTF blob into user space.
By allowing vmlinux BTF to be mmapped read-only into user space I can
cut memory usage by about 75%.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
---
Changes in v5:
- Fix error return of btf_parse_raw_mmap (Andrii)
- Link to v4: https://lore.kernel.org/r/20250510-vmlinux-mmap-v4-0-69e424b2a672@isovalent.com

Changes in v4:
- Go back to remap_pfn_range for aarch64 compat
- Dropped btf_new_no_copy (Andrii)
- Fixed nits in selftests (Andrii)
- Clearer error handling in the mmap handler (Andrii)
- Fixed build on s390
- Link to v3: https://lore.kernel.org/r/20250505-vmlinux-mmap-v3-0-5d53afa060e8@isovalent.com

Changes in v3:
- Remove slightly confusing calculation of trailing (Alexei)
- Use vm_insert_page (Alexei)
- Simplified libbpf code
- Link to v2: https://lore.kernel.org/r/20250502-vmlinux-mmap-v2-0-95c271434519@isovalent.com

Changes in v2:
- Use btf__new in selftest
- Avoid vm_iomap_memory in btf_vmlinux_mmap
- Add VM_DONTDUMP
- Add support to libbpf
- Link to v1: https://lore.kernel.org/r/20250501-vmlinux-mmap-v1-0-aa2724572598@isovalent.com

---
====================

Link: https://patch.msgid.link/20250520-vmlinux-mmap-v5-0-e8c941acc414@isovalent.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

include/asm-generic/vmlinux.lds.h

@@ -667,10 +667,11 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
  */
 #ifdef CONFIG_DEBUG_INFO_BTF
 #define BTF								\
+	. = ALIGN(PAGE_SIZE);						\
 	.BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {				\
 		BOUNDED_SECTION_BY(.BTF, _BTF)				\
 	}								\
-	. = ALIGN(4);							\
+	. = ALIGN(PAGE_SIZE);						\
 	.BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) {			\
 		*(.BTF_ids)						\
 	}

kernel/bpf/sysfs_btf.c

@@ -7,14 +7,46 @@
 #include <linux/kobject.h>
 #include <linux/init.h>
 #include <linux/sysfs.h>
+#include <linux/mm.h>
+#include <linux/io.h>
+#include <linux/btf.h>
 
 /* See scripts/link-vmlinux.sh, gen_btf() func for details */
 extern char __start_BTF[];
 extern char __stop_BTF[];
 
+static int btf_sysfs_vmlinux_mmap(struct file *filp, struct kobject *kobj,
+				  const struct bin_attribute *attr,
+				  struct vm_area_struct *vma)
+{
+	unsigned long pages = PAGE_ALIGN(attr->size) >> PAGE_SHIFT;
+	size_t vm_size = vma->vm_end - vma->vm_start;
+	phys_addr_t addr = virt_to_phys(__start_BTF);
+	unsigned long pfn = addr >> PAGE_SHIFT;
+
+	if (attr->private != __start_BTF || !PAGE_ALIGNED(addr))
+		return -EINVAL;
+
+	if (vma->vm_pgoff)
+		return -EINVAL;
+
+	if (vma->vm_flags & (VM_WRITE | VM_EXEC | VM_MAYSHARE))
+		return -EACCES;
+
+	if (pfn + pages < pfn)
+		return -EINVAL;
+
+	if ((vm_size >> PAGE_SHIFT) > pages)
+		return -EINVAL;
+
+	vm_flags_mod(vma, VM_DONTDUMP, VM_MAYEXEC | VM_MAYWRITE);
+	return remap_pfn_range(vma, vma->vm_start, pfn, vm_size, vma->vm_page_prot);
+}
+
 static struct bin_attribute bin_attr_btf_vmlinux __ro_after_init = {
 	.attr = { .name = "vmlinux", .mode = 0444, },
 	.read_new = sysfs_bin_attr_simple_read,
+	.mmap = btf_sysfs_vmlinux_mmap,
 };
 
 struct kobject *btf_kobj;

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 +1375,37 @@ 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) {
-		free(btf->raw_data);
-		btf->raw_data = NULL;
-	}
+	if (btf->raw_data)
+		btf_free_raw_data(btf);
 	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(sysfs_btf_path, NULL);
+		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",

tools/testing/selftests/bpf/prog_tests/btf_sysfs.c

@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+/* Copyright (c) 2025 Isovalent */
+
+#include <test_progs.h>
+#include <bpf/btf.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+static void test_btf_mmap_sysfs(const char *path, struct btf *base)
+{
+	struct stat st;
+	__u64 btf_size, end;
+	void *raw_data = NULL;
+	int fd = -1;
+	long page_size;
+	struct btf *btf = NULL;
+
+	page_size = sysconf(_SC_PAGESIZE);
+	if (!ASSERT_GE(page_size, 0, "get_page_size"))
+		goto cleanup;
+
+	if (!ASSERT_OK(stat(path, &st), "stat_btf"))
+		goto cleanup;
+
+	btf_size = st.st_size;
+	end = (btf_size + page_size - 1) / page_size * page_size;
+
+	fd = open(path, O_RDONLY);
+	if (!ASSERT_GE(fd, 0, "open_btf"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, btf_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_writable"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, btf_size, PROT_READ, MAP_SHARED, fd, 0);
+	if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_shared"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, end + 1, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_invalid_size"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, end, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_OK_PTR(raw_data, "mmap_btf"))
+		goto cleanup;
+
+	if (!ASSERT_EQ(mprotect(raw_data, btf_size, PROT_READ | PROT_WRITE), -1,
+	    "mprotect_writable"))
+		goto cleanup;
+
+	if (!ASSERT_EQ(mprotect(raw_data, btf_size, PROT_READ | PROT_EXEC), -1,
+	    "mprotect_executable"))
+		goto cleanup;
+
+	/* Check padding is zeroed */
+	for (int i = btf_size; i < end; i++) {
+		if (((__u8 *)raw_data)[i] != 0) {
+			PRINT_FAIL("tail of BTF is not zero at page offset %d\n", i);
+			goto cleanup;
+		}
+	}
+
+	btf = btf__new_split(raw_data, btf_size, base);
+	if (!ASSERT_OK_PTR(btf, "parse_btf"))
+		goto cleanup;
+
+cleanup:
+	btf__free(btf);
+	if (raw_data && raw_data != MAP_FAILED)
+		munmap(raw_data, btf_size);
+	if (fd >= 0)
+		close(fd);
+}
+
+void test_btf_sysfs(void)
+{
+	test_btf_mmap_sysfs("/sys/kernel/btf/vmlinux", NULL);
+}