Re: [PATCH RESEND v8 16/16] bpf: remove CONFIG_BPF_JIT dependency on CONFIG_MODULES of

[Will Deacon]

Hi Klara,

On Fri, May 17, 2024 at 01:00:31AM +0200, Klara Modin wrote:
> On 2024-05-05 18:06, Mike Rapoport wrote:
> > From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
> > 
> > BPF just-in-time compiler depended on CONFIG_MODULES because it used
> > module_alloc() to allocate memory for the generated code.
> > 
> > Since code allocations are now implemented with execmem, drop dependency of
> > CONFIG_BPF_JIT on CONFIG_MODULES and make it select CONFIG_EXECMEM.
> > 
> > Suggested-by: Björn Töpel <bjorn@xxxxxxxxxx>
> > Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> > ---
> >   kernel/bpf/Kconfig | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
> > index bc25f5098a25..f999e4e0b344 100644
> > --- a/kernel/bpf/Kconfig
> > +++ b/kernel/bpf/Kconfig
> > @@ -43,7 +43,7 @@ config BPF_JIT
> >   	bool "Enable BPF Just In Time compiler"
> >   	depends on BPF
> >   	depends on HAVE_CBPF_JIT || HAVE_EBPF_JIT
> > -	depends on MODULES
> > +	select EXECMEM
> >   	help
> >   	  BPF programs are normally handled by a BPF interpreter. This option
> >   	  allows the kernel to generate native code when a program is loaded
> 
> This does not seem to work entirely. If build with BPF_JIT without module
> support for my Raspberry Pi 3 B I get warnings in my kernel log (easiest way
> to trigger it seems to be trying to ssh into it, which fails).

Thanks for the report. I was able to reproduce this using QEMU and it
looks like the problem is because bpf_arch_text_copy() silently fails
to write to the read-only area as a result of patch_map() faulting and
the resulting -EFAULT being chucked away.

Please can you try the diff below?

Will

--->8

diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
index 255534930368..94b9fea65aca 100644
--- a/arch/arm64/kernel/patching.c
+++ b/arch/arm64/kernel/patching.c
@@ -36,7 +36,7 @@ static void __kprobes *patch_map(void *addr, int fixmap)
 
        if (image)
                page = phys_to_page(__pa_symbol(addr));
-       else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
+       else if (IS_ENABLED(CONFIG_EXECMEM))
                page = vmalloc_to_page(addr);
        else
                return addr;