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Message-ID: <CAADnVQK3hSgs_hic2Yuo84vR=2GZNtryki+TDNkNGY_7URsLiw@mail.gmail.com>
Date: Thu, 1 May 2025 13:26:44 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Lorenz Bauer <lmb@...valent.com>
Cc: Arnd Bergmann <arnd@...db.de>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
linux-arch <linux-arch@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
bpf <bpf@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] btf: allow mmap of vmlinux btf
On Thu, May 1, 2025 at 7:28 AM Lorenz Bauer <lmb@...valent.com> 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@...valent.com>
> ---
> 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
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