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.magu...@oracle.com>
> Signed-off-by: Lorenz Bauer <l...@isovalent.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.b...@linux.dev>
