Re: [PATCH bpf-next] bpf, x86: Use global buffer for trampoline size generation

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Sep 08, 2026 at 06:19:17PM +0300, Mike Rapoport wrote:
> On Tue, Sep 08, 2026 at 02:25:24PM +0200, Jiri Olsa wrote:
> > On Mon, Sep 07, 2026 at 05:13:29PM -0700, Alexei Starovoitov wrote:
> >
> > > > > > +static void *trampoline_size_image;
> > > > > > +
> > > > > > +static int __init init_trampoline_size_image(void)
> > > > > > +{
> > > > > > +   trampoline_size_image = execmem_alloc(EXECMEM_MODULE_DATA, PAGE_SIZE);
> > > > >
> > > > > I think I asked it earlier... why does it have to be execmem ?
> > > > > Can it be normal page?
> > > >
> > > > ugh sorry I forgot.. the page/image needs to be in execmem range
> > > > for emitting call/jmp otherwise the delta won't fit in 4 bytes
> > > > and it fails on emit_patch is_simm32 check
> > > 
> > > ahh. I see. Please mention it in the comment and
> > > also use bpf_jit_alloc_exec(PAGE_SIZE).
> > > 
> > > execmem_alloc(EXECMEM_MODULE_DATA...) was working by accident.
> > > The VM ranges could have been different.
> 
> On x86 it's not an accident ;-)
> But would be a surprise if this is copied to another architecture.
>  
> > bpf_jit_alloc_exec returns read only page so I'd use bpf_jit_alloc_exec_rw,
> > but it was recently removed in:
> > 
> >   c7a2a3618290 x86/bpf: Make arch_bpf_trampoline_size allocate from EXECMEM_MODULE_DATA
> > 
> > so I ended up with:
> > 
> >   trampoline_size_image = execmem_alloc_rw(EXECMEM_BPF, PAGE_SIZE);
> 
> This will work, but it will permanently damage 2M mapping in the module
> address space.
> 
> execmem_alloc_rw() takes a page out of the ROX cache and resets that page
> permissions to RW. This splits the large mapping of the 2M cache page and
> since this is a permanent allocation, that chunk will be forever mapped
> with 4k pages.
> 
> execmem_alloc(EXECMEM_MODULE_DATA...) does not use ROX cache, so it won't
> take a page out of ROX cache and reset it's permissions.
> 
> Since this is anyway x86 code and on x86 the ranges for data and code are
> *always* the same, I'd keep execmem_alloc(EXECMEM_MODULE_DATA...) and add a
> comment.

sgtm, if there's no objection I'll add comment to init_trampoline_size_image
and send new version

thanks,
jirka