linux-kernel - [PATCH bpf-next v4 1/3] btf: allow mmap of vmlinux btf

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Message-Id: <20250510-vmlinux-mmap-v4-1-69e424b2a672@isovalent.com>
Date: Sat, 10 May 2025 08:34:54 -0400
From: Lorenz Bauer <lmb@...valent.com>
To: Arnd Bergmann <arnd@...db.de>, Alexei Starovoitov <ast@...nel.org>, 
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
 Martin KaFai Lau <martin.lau@...ux.dev>, 
 Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
 Yonghong Song <yonghong.song@...ux.dev>, 
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, 
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
 Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, 
 Shuah Khan <shuah@...nel.org>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, 
 bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Lorenz Bauer <lmb@...valent.com>
Subject: [PATCH bpf-next v4 1/3] btf: allow mmap of vmlinux btf

User space needs access to kernel BTF for many modern features of BPF.
Right now each process needs to read the BTF blob either in pieces or
as a whole. Allow mmaping the sysfs file so that processes can directly
access the memory allocated for it in the kernel.

remap_pfn_range is used instead of vm_insert_page due to aarch64
compatibility issues.

Signed-off-by: Lorenz Bauer <lmb@...valent.com>
---
 include/asm-generic/vmlinux.lds.h |  3 ++-
 kernel/bpf/sysfs_btf.c            | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 58a635a6d5bdf0c53c267c2a3d21a5ed8678ce73..1750390735fac7637cc4d2fa05f96cb2a36aa448 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/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)						\
 	}
diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
index 81d6cf90584a7157929c50f62a5c6862e7a3d081..941d0d2427e3a2d27e8f1cff7b6424d0d41817c1 100644
--- a/kernel/bpf/sysfs_btf.c
+++ b/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;

-- 
2.49.0