Re: [PATCH] bpf: do not inline bpf_get_smp_processor_id() with CONFIG_SMP disabled

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-12-16 at 18:24 +0100, Andrea Righi wrote:
> On Mon, Dec 16, 2024 at 05:16:33PM +0100, Daniel Borkmann wrote:
> > On 12/16/24 11:46 AM, Andrea Righi wrote:
> > > Calling bpf_get_smp_processor_id() in a kernel with CONFIG_SMP disabled
> > > can trigger the following bug, as pcpu_hot is unavailable:
> > > 
> > > [    8.471774] BUG: unable to handle page fault for address: 00000000936a290c
> > > [    8.471849] #PF: supervisor read access in kernel mode
> > > [    8.471881] #PF: error_code(0x0000) - not-present page
> > > 
> > > Fix by preventing the inlining of bpf_get_smp_processor_id() when
> > > CONFIG_SMP disabled.
> > > 
> > > Fixes: 1ae6921009e5 ("bpf: inline bpf_get_smp_processor_id() helper")
> > > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > 
> > lgtm, but can't we instead do sth like this :
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index f7f892a52a37..761c70899754 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -21281,11 +21281,15 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> >  			 * changed in some incompatible and hard to support
> >  			 * way, it's fine to back out this inlining logic
> >  			 */
> > +#ifdef CONFIG_SMP
> >  			insn_buf[0] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
> >  			insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
> >  			insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
> >  			cnt = 3;
> > -
> > +#else
> > +			BPF_ALU32_REG(BPF_XOR, BPF_REG_0, BPF_REG_0),
> > +			cnt = 1;
> > +#endif
> >  			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> >  			if (!new_prog)
> >  				return -ENOMEM;
> 
> That works as well (just tested) and it's probably better since we're
> basically inlining the return 0. Do you want me to send a v2 with this?

I think both Andrea's and Daniel's versions of the fix are good.
Note, however, that I missed one more configuration variable when
making bpf_get_smp_processor_id() inlinable: CONFIG_DEBUG_PREEMPT.

Helper body:

    BPF_CALL_0(bpf_get_smp_processor_id)
    {
    	return smp_processor_id();
    }

smp_processor_id definition:

    #ifdef CONFIG_DEBUG_PREEMPT
      extern unsigned int debug_smp_processor_id(void);
    # define smp_processor_id() debug_smp_processor_id()
    #else
    # define smp_processor_id() __smp_processor_id()
    #endif

Thanks,
Eduard.