Re: [PATCH] powerpc/32: Add support for out-of-line static calls

[Ard Biesheuvel <ardb@kernel.org>]

On Tue, 31 Aug 2021 at 10:53, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Aug 31, 2021 at 08:05:21AM +0000, Christophe Leroy wrote:
>
> > +#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)                     \
> > +     asm(".pushsection .text, \"ax\"                         \n"     \
> > +         ".align 4                                           \n"     \
> > +         ".globl " STATIC_CALL_TRAMP_STR(name) "             \n"     \
> > +         STATIC_CALL_TRAMP_STR(name) ":                      \n"     \
> > +         "   blr                                             \n"     \
> > +         "   nop                                             \n"     \
> > +         "   nop                                             \n"     \
> > +         "   nop                                             \n"     \
> > +         ".type " STATIC_CALL_TRAMP_STR(name) ", @function   \n"     \
> > +         ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
> > +         ".popsection                                        \n")
>
> > +static int patch_trampoline_32(u32 *addr, unsigned long target)
> > +{
> > +     int err;
> > +
> > +     err = patch_instruction(addr++, ppc_inst(PPC_RAW_LIS(_R12, PPC_HA(target))));
> > +     err |= patch_instruction(addr++, ppc_inst(PPC_RAW_ADDI(_R12, _R12, PPC_LO(target))));
> > +     err |= patch_instruction(addr++, ppc_inst(PPC_RAW_MTCTR(_R12)));
> > +     err |= patch_instruction(addr, ppc_inst(PPC_RAW_BCTR()));
> > +
> > +     return err;
> > +}
>
> There can be concurrent execution and modification; the above doesn't
> look safe in that regard. What happens if you've say, done the first
> two, but not the latter two and execution happens (on a different
> CPU or through IRQ context, etc..)?
>
> > +void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
> > +{
> > +     int err;
> > +     unsigned long target = (long)func;
> > +
> > +     if (!tramp)
> > +             return;
> > +
> > +     mutex_lock(&text_mutex);
> > +
> > +     if (!func)
> > +             err = patch_instruction(tramp, ppc_inst(PPC_RAW_BLR()));
> > +     else if (is_offset_in_branch_range((long)target - (long)tramp))
> > +             err = patch_branch(tramp, target, 0);
>
> These two are single instruction modifications and I'm assuming the
> hardware is sane enough that execution sees either the old or the new
> instruction. So this should work.
>
> > +     else if (IS_ENABLED(CONFIG_PPC32))
> > +             err = patch_trampoline_32(tramp, target);
> > +     else
> > +             BUILD_BUG();
> > +
> > +     mutex_unlock(&text_mutex);
> > +
> > +     if (err)
> > +             panic("%s: patching failed %pS at %pS\n", __func__, func, tramp);
> > +}
> > +EXPORT_SYMBOL_GPL(arch_static_call_transform);
>
> One possible solution that we explored on ARM64, was having the
> trampoline be in 2 slots:
>
>
>         b 1f
>
> 1:      blr
>         nop
>         nop
>         nop
>
> 2:      blr
>         nop
>         nop
>         nop
>
> Where initially the first slot is active (per "b 1f"), then you write
> the second slot, and as a final act, re-write the initial branch to
> point to slot 2.
>
> Then you execute synchronize_rcu_tasks() under your text mutex
> (careful!) to ensure all users of your slot1 are gone and the next
> modification repeats the whole thing, except for using slot1 etc..
>
> Eventually I think Ard came up with the latest ARM64 proposal which puts
> a literal in a RO section (could be the text section I suppose) and
> loads and branches to that.
>

Yes. The main reason is simply that anything else is premature
optimization: we have a clear use case (CFI) where out-of-line static
calls are faster than compiler generated indirect calls, even if the
static call sequence is based on a literal load and an indirect
branch, but CFI is not upstream [yet].

Once other use cases emerge, we will revisit this.



> Anyway, the thing is, you can really only modify a single instruction at
> the time and need to ensure concurrent execution is correct.