On Wed, Oct 10, 2018 at 11:03:43AM -0700, Andy Lutomirski wrote:
> > +#define DECLARE_STATIC_CALL(tramp, func) \
> > + extern typeof(func) tramp; \
> > + static void __used __section(.discard.static_call_tramps) \
> > + *__static_call_tramp_##tramp = tramp
> > +
>
> Confused. What's the __static_call_tramp_##tramp variable for? And
> why is a DECLARE_ macro defining a variable?
This is the magic needed for objtool to find all the call sites.
The variable itself isn't needed, but the .discard.static_call_tramps
entry is. Objtool reads that section to find out which function call
sites are targeted to a static call trampoline.
> > +#define DEFINE_STATIC_CALL(tramp, func)
> > \
> > + DECLARE_STATIC_CALL(tramp, func); \
> > + asm(".pushsection .text, \"ax\" \n" \
> > + ".align 4 \n" \
> > + ".globl " #tramp " \n" \
> > + ".type " #tramp ", @function \n" \
> > + #tramp ": \n" \
> > + "jmp " #func " \n" \
>
> I think this would be nicer as an indirect call that gets patched to a
> direct call so that the update mechanism works even before it's
> initialized. (Currently static_branch blows up horribly if you try to
> update one too early, and that's rather annoying IMO.)
Yeah, that would be better. It would also allow trampoline function
pointers to work, which I think you mentioned elsewhere. And then I
shouldn't trample this code in __static_call_update() -- that was
already kind of nasty anyway.
> Also, I think you're looking for "jmp.d32", which is available in
> newer toolchains. For older toolchains, you could use .byte 0xe9 or
> you could use a different section (I think) to force a real 32-bit
> jump.
Good idea.
> > +void __init static_call_init(void)
> > +{
> > + struct static_call_entry *entry;
> > + unsigned long insn, tramp, func;
> > + unsigned char insn_opcode, tramp_opcode;
> > + s32 call_dest;
> > +
> > + for (entry = __start_static_call_table;
> > + entry < __stop_static_call_table; entry++) {
> > +
> > + insn = (long)entry->insn + (unsigned long)&entry->insn;
> > + tramp = (long)entry->tramp + (unsigned long)&entry->tramp;
> > +
> > + insn_opcode = *(unsigned char *)insn;
> > + if (insn_opcode != 0xe8 && insn_opcode != 0xe9) {
> > + WARN_ONCE(1, "unexpected static call insn opcode %x
> > at %pS",
> > + insn_opcode, (void *)insn);
> > + continue;
> > + }
> > +
> > + tramp_opcode = *(unsigned char *)tramp;
> > + if (tramp_opcode != 0xeb && tramp_opcode != 0xe9) {
> > + WARN_ONCE(1, "unexpected trampoline jump opcode %x
> > at %ps",
> > + tramp_opcode, (void *)tramp);
> > + continue;
> > + }
> > +
> > + if (tramp_opcode == 0xeb)
> > + func = *(s8 *)(tramp + 1) + (tramp + 2);
>
> I realize you expect some instances of 0xeb due to the assembler
> messing you up (see above), but this seems a bit too permissive, since
> a 0xeb without the appropriate set of NOPs is going to explode. And:
Yep.
> > + else
> > + func = *(s32 *)(tramp + 1) + (tramp + 5);
> > +
> > + call_dest = (long)(func) - (long)(insn + 5);
> > +
> > + printk("static_call_init: poking %lx at %lx\n", (unsigned
> > long)call_dest, (insn+1));
> > +
> > + text_poke_early((void *)(insn + 1), &call_dest, 4);
>
> If you really are going to rewrite an 8-bit jump to a 32-bit jump, I
> think you need to actually patch the opcode byte :)
Oops :-)
--
Josh
