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

[Lorenz Bauer]

On Thu, May 1, 2025 at 9:26 PM Alexei Starovoitov
 wrote:
> > +
> > +   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.

FYI I went with open coding with remap_pfn_range since that allows me
to avoid struct page.

Lorenz

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  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 
> ---
>  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 
>  #include 
>  #include 
> +#include 
> +#include 
>
>  /* 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

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

[Lorenz Bauer]

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 
---
 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 
 #include 
 #include 
+#include 
+#include 
 
 /* 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);
+
+   return vm_iomap_memory(vma, virt_to_phys(__start_BTF), btf_size);
+}
+
 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_vmlinux_mmap,
 };
 
-struct kobject *btf_kobj;
-
 static int __init btf_vmlinux_init(void)
 {
bin_attr_btf_vmlinux.private = __start_BTF;

-- 
2.49.0