linux-kernel - [PATCH bpf] btf: fix virt_to_phys warning on arm64 when mmapping vmlinux

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Message-Id: <20250717-vmlinux-mmap-pa-symbol-v1-1-970be6681158@isovalent.com>
Date: Thu, 17 Jul 2025 17:49:49 +0100
From: Lorenz Bauer <lmb@...valent.com>
To: Breno Leitao <leitao@...ian.org>, 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>, Shakeel Butt <shakeel.butt@...ux.dev>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Lorenz Bauer <lmb@...valent.com>
Subject: [PATCH bpf] btf: fix virt_to_phys warning on arm64 when mmapping
 vmlinux

Breno Leitao reports that arm64 emits the following warning:

    [   58.896157] virt_to_phys used for non-linear address: 000000009fea9737
      (__start_BTF+0x0/0x685530)
    [   23.988669] WARNING: CPU: 25 PID: 1442 at arch/arm64/mm/physaddr.c:15
      __virt_to_phys (arch/arm64/mm/physaddr.c:?)

        ...

    [   24.075371] Tainted: [E]=UNSIGNED_MODULE, [N]=TEST
    [   24.080276] Hardware name: Quanta S7GM 20S7GCU0010/S7G MB (CG1), BIOS 3D22
      07/03/2024
    [   24.088295] pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
    [   24.098440] pc : __virt_to_phys (arch/arm64/mm/physaddr.c:?)
    [   24.105398] lr : __virt_to_phys (arch/arm64/mm/physaddr.c:?)

	...

    [   24.197257] Call trace:
    [   24.199761] __virt_to_phys (arch/arm64/mm/physaddr.c:?) (P)
    [   24.206883] btf_sysfs_vmlinux_mmap (kernel/bpf/sysfs_btf.c:27)
    [   24.214264] sysfs_kf_bin_mmap (fs/sysfs/file.c:179)
    [   24.218536] kernfs_fop_mmap (fs/kernfs/file.c:462)
    [   24.222461] mmap_region (./include/linux/fs.h:? mm/internal.h:167
       mm/vma.c:2405 mm/vma.c:2467 mm/vma.c:2622 mm/vma.c:2692)

It seems that the memory layout on arm64 maps the kernel image in vmalloc space
which is different than x86. This makes virt_to_phys emit the warning.

Fix this by translating the address using __pa_symbol as suggested by
Breno instead.

Reported-by: Breno Leitao <leitao@...ian.org>
Closes: https://lore.kernel.org/bpf/g2gqhkunbu43awrofzqb4cs4sxkxg2i4eud6p4qziwrdh67q4g@mtw3d3aqfgmb/
Signed-off-by: Lorenz Bauer <lmb@...valent.com>
Tested-by: Breno Leitao <leitao@...ian>
Fixes: a539e2a6d51d ("btf: Allow mmap of vmlinux btf")
---
 kernel/bpf/sysfs_btf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
index 941d0d2427e3a2d27e8f1cff7b6424d0d41817c1..8e61dc555415aafd9e8f80a3408b668fd5057cb3 100644
--- a/kernel/bpf/sysfs_btf.c
+++ b/kernel/bpf/sysfs_btf.c
@@ -21,7 +21,7 @@ static int btf_sysfs_vmlinux_mmap(struct file *filp, struct kobject *kobj,
 {
 	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);
+	phys_addr_t addr = __pa_symbol(__start_BTF);
 	unsigned long pfn = addr >> PAGE_SHIFT;
 
 	if (attr->private != __start_BTF || !PAGE_ALIGNED(addr))

---
base-commit: af90e85307241ec495c2de85854cd2e35a4df16b
change-id: 20250717-vmlinux-mmap-pa-symbol-0d55cd0b32c0

Best regards,
-- 
Lorenz Bauer <lmb@...valent.com>