Re: [RFC PATCH 0/3] enable bpf_prog_pack allocator for powerpc

[Song Liu <song@kernel.org>]

On Fri, Nov 18, 2022 at 3:47 AM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 18/11/2022 à 10:39, Hari Bathini a écrit :
> >
> >
> > On 18/11/22 2:21 pm, Christophe Leroy wrote: >>>>>
> >>>>> I had the same config but hit this problem:
> >>>>>
> >>>>>        # echo 1 > /proc/sys/net/core/bpf_jit_enable; modprobe test_bpf
> >>>>>        test_bpf: #0 TAX
> >>>>>        ------------[ cut here ]------------
> >>>>>        WARNING: CPU: 0 PID: 96 at arch/powerpc/net/bpf_jit_comp.c:367
> >>>>> bpf_int_jit_compile+0x8a0/0x9f8
> >>>>
> >>>> I get no such problem, on QEMU, and I checked the .config has:
> >>>
> >>>> CONFIG_STRICT_KERNEL_RWX=y
> >>>> CONFIG_STRICT_MODULE_RWX=y
> >>>
> >>> Yeah. That did the trick.
> >>
> >> Interesting. I guess we have to find out why it fails when those config
> >> are missing.
> >>
> >> Maybe module code plays with RO and NX flags even if
> >> CONFIG_STRICT_MODULE_RWX is not selected ?
> >
> > Need to look at the code closely but fwiw, observing same failure on
> > 64-bit as well with !STRICT_RWX...
>
> The problem is in bpf_prog_pack_alloc() and in alloc_new_pack() : They
> do set_memory_ro() and set_memory_x() without taking into account
> CONFIG_STRICT_MODULE_RWX.
>
> When CONFIG_STRICT_MODULE_RWX is selected, powerpc module_alloc()
> allocates PAGE_KERNEL memory, that is RW memory, and expects the user to
> call do set_memory_ro() and set_memory_x().
>
> But when CONFIG_STRICT_MODULE_RWX is not selected, powerpc
> module_alloc() allocates PAGE_KERNEL_TEXT memory, that is RWX memory,
> and expects to be able to always write into it.

Ah, I see. x86_64 requires CONFIG_STRICT_MODULE_RWX, so this hasn't
been a problem yet.

Thanks,
Song