Get back to sel_getUid()'s original behaviour

Question

TL;DR: How does one check that selector with given name was registered, without actually registering it?

Thanks!

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Hi, I have an Objective-C application and bunch of NSObjects that are exported into Lua state via simple proxy library written in objc. All Lua-side calls like these:

exported_objc_object:myMethodName(...)

-- same as --
exported_objc_object.myMethodName(exported_objc_object, ...)

-- same as --
key = 'myMethodName'
exported_objc_object[key](exported_objc_object, ...)

are forwarded as if someone called:

[objc_object lua_myMethodName:L];

// declared as
- (int)lua_myMethodName:(lua_State *)L { ... }

Actually, any 'get' opcode on Lua-exported object returns a cached Lua-closure that on invocation will call corresponding Objective-C method via selector constructed with sprintf(s, "lua_%s:", key)) && sel_getUid(s) (all checks included). If produced selector is not implemented in terms of -[respondsToSelector:], then exported_objc_object.myMethodName simply returns nil.

Obviously, proxy library has to make dynamic lookups via sel_getUid() or sel_registerName() (I believe both @selector and NSSelectorFromString() also end up there). The manual states that sel_getUid() was intended to lookup selector names (as opposed to immediately registering them into SEL registry), but it's modern implementation now does the same as sel_registerName() due to bugs in someone's teh codez back then.

I can just stick with sel_registerName() behavior, but that leaves memory-eating attack vector, as some malicious script may start looking up long random/invalid selectors via sml object[makeRandomKey()] in a loop, thus overflowing SEL registry forever. If sel_getUid() worked as planned, proxy lib would be able to test selector presence and only then actually check if object responds to it, without excessive registering. But it doesn't.

Answer 1

Here is a hack that might work, that uses the implementation-dependent fact that a selector is a C string.

sel_isMapped((SEL)(void *)"lua_myMethodName:")