linux-kernel - Re: [PATCH bpf-next v3 1/3] btf: allow mmap of vmlinux btf

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Message-ID: <CAEf4BzbsLJgbnuLj6sYFgH7sUZPfn3SqRf_5edTSGuo2oYXN4A@mail.gmail.com>
Date: Tue, 6 May 2025 14:39:15 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
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
Subject: Re: [PATCH bpf-next v3 1/3] btf: allow mmap of vmlinux btf

On Mon, May 5, 2025 at 11:39 AM Lorenz Bauer <lmb@...valent.com> 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.
>
> Signed-off-by: Lorenz Bauer <lmb@...valent.com>
> ---
>  include/asm-generic/vmlinux.lds.h |  3 ++-
>  kernel/bpf/sysfs_btf.c            | 37 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 39 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..37278d7f38ae72f2d7efcfa859e86aaf12e39a25 100644
> --- a/kernel/bpf/sysfs_btf.c
> +++ b/kernel/bpf/sysfs_btf.c
> @@ -7,14 +7,51 @@
>  #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;
> +       unsigned long addr = (unsigned long)attr->private;
> +       int i, err = 0;
> +
> +       if (addr != (unsigned long)__start_BTF || !PAGE_ALIGNED(addr))
> +               return -EINVAL;
> +
> +       if (vma->vm_pgoff)
> +               return -EINVAL;

any particular reason to not allow vm_pgoff?

> +
> +       if (vma->vm_flags & (VM_WRITE | VM_EXEC | VM_MAYSHARE))
> +               return -EACCES;
> +
> +       if (vm_size >> PAGE_SHIFT > pages)

() around shift operation, please, for those of us who haven't
memorized the entire C operator precedence table ;)

> +               return -EINVAL;
> +
> +       vm_flags_mod(vma, VM_DONTDUMP, VM_MAYEXEC | VM_MAYWRITE);
> +
> +       for (i = 0; i < pages && !err; i++, addr += PAGE_SIZE)
> +               err = vm_insert_page(vma, vma->vm_start + i * PAGE_SIZE,
> +                                    virt_to_page(addr));
> +
> +       if (err)
> +               zap_vma_pages(vma);

it's certainly subjective, but I find this error handling with !err in
for loop condition hard to follow. What's wrong with arguably more
straightforward (and as you can see I'm not a big fan of mutated addr
but calculated vma->vm_start + i * PAGE_SIZE: pick one style one
follow it for both entities?):


for (i = 0; i < pages; i++) {
    err = vm_insert_page(vma, vma->vm_start + i * PAGE_SIZE,
                         virt_to_page(addr + i * PAGE_SIZE));
    if (err) {
        zap_vma_pages(vma);
        return err;
    }
}

return 0;

?


> +
> +       return err;
> +}
> +
>  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
>