On Thu, May 22, 2025 at 4:01 PM Shakeel Butt shakeel.butt@linux.dev wrote:
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@oracle.com Signed-off-by: Lorenz Bauer lmb@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?
I'd probably leave this as a sanity/safety check, just in case someone modifies linker script and we miss this.
BTF region size isn't page-aligned but in the linker script we page-align .BTF_ids that follows it, so the padding should be zeroed out. And Lorenz added a check in the selftest to validate this, so we should be covered.
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.
I think it's good to keep it in the fork, otherwise libbpf might crash after work due to BTF data suddenly disappearing.
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@linux.dev
Thanks Shakeel, I've applied the patches to bpf-next!