On Thu, May 22, 2025 at 4:01 PM Shakeel Butt <shakeel.b...@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.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?
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.b...@linux.dev>
Thanks Shakeel, I've applied the patches to bpf-next!
