linux-kernel - Re: [PATCH bpf-next v5 1/3] btf: allow mmap of vmlinux btf

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Message-ID: <y42h33hluaspiexeck5gkq7aow45stvf72is2k4hy46ydmlyhp@nr3lb27lrrqo>
Date: Thu, 22 May 2025 16:01:21 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Lorenz Bauer <lmb@...valent.com>
Cc: 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>, linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCH bpf-next v5 1/3] btf: allow mmap of vmlinux btf

On Tue, May 20, 2025 at 02:01:17PM +0100, Lorenz Bauer wrote:
> 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.
> 
> Tested-by: Alan Maguire <alan.maguire@...cle.com>
> 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))

With vmlinux.lds.h change above, is the page aligned check still needed?

Oh also can the size of btf region be non-page aligned?

> +		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);

Is it ok for fork() to keep the mapping in the child? (i.e. do you need
VM_DONTCOPY). BTW VM_DONTDUMP is added by remap_pfn_range(), so if you
want you can remove it here.

> +	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;
> 

Overall this looks good to me, so you can add:

Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>