bpf: use __pa_symbol to retrieve physical address of __start_BTF

[lmb]

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:?)
[   24.018136] Modules linked in: nvidia_cspmu(E) mlx5_ib(E) ipmi_ssif(E) arm_smmuv3_pmu(E) arm_cspmu_module(E) coresight_trbe(E) ib_uverbs(E) ipmi_devintf(E) ipmi_msghandler(E) coresight_stm(E) coresight_etm4x(E) coresight_tmc(E) coresight_funnel(E) stm_core(E) coresight(E) cppc_cpufreq(E) sch_fq_codel(E) drm(E) backlight(E) drm_panel_orientation_quirks(E) xhci_pci(E) xhci_hcd(E) sm3_ce(E) sha3_ce(E) sha512_ce(E) spi_tegra210_quad(E) acpi_power_meter(E) loop(E) efivarfs(E) autofs4(E)
[   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 [email protected]