Re: [PATCH bpf-next 1/2] btf: allow mmap of vmlinux btf

[Alexei Starovoitov]

On Thu, May 1, 2025 at 7:28 AM Lorenz Bauer <lmb@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
>  include/asm-generic/vmlinux.lds.h |  3 ++-
>  kernel/bpf/sysfs_btf.c            | 25 +++++++++++++++++++++++--
>  2 files changed, 25 insertions(+), 3 deletions(-)
>
> 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..7651f37b82c78b8afd96078567a5b6612f5f4d97 100644
> --- a/kernel/bpf/sysfs_btf.c
> +++ b/kernel/bpf/sysfs_btf.c
> @@ -7,18 +7,39 @@
>  #include <linux/kobject.h>
>  #include <linux/init.h>
>  #include <linux/sysfs.h>
> +#include <linux/mm.h>
> +#include <linux/io.h>
>
>  /* See scripts/link-vmlinux.sh, gen_btf() func for details */
>  extern char __start_BTF[];
>  extern char __stop_BTF[];
>
> +struct kobject *btf_kobj;
> +
> +static int btf_vmlinux_mmap(struct file *filp, struct kobject *kobj,
> +                           const struct bin_attribute *attr,
> +                           struct vm_area_struct *vma)
> +{
> +       size_t btf_size = __stop_BTF - __start_BTF;
> +
> +       if (kobj != btf_kobj)
> +               return -EINVAL;
> +
> +       if (vma->vm_flags & (VM_WRITE|VM_EXEC|VM_MAYSHARE))
> +               return -EACCES;
> +
> +       vm_flags_clear(vma, VM_MAYEXEC);
> +       vm_flags_clear(vma, VM_MAYWRITE);

Probably should set VM_DONTDUMP to avoid being in the core dump.
vm_flags_mod() can set and clear in one operation.

> +
> +       return vm_iomap_memory(vma, virt_to_phys(__start_BTF), btf_size);

and this one should probably be vm_insert_pages().
Since it's not an IO area.

Overall I think it's a good idea.
As Daniel suggested pls make use of it in libbpf too.
That exercise will make sure that feature probing works
with fallback.

pw-bot: cr