Messages in this thread | | | Date | Wed, 10 Oct 2018 13:16:05 -0500 | From | Josh Poimboeuf <> | Subject | Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function" |
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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
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