Re:Neat Rust Tricks: Passing Closures to C
by richardwhiuk on Monday November 25, 2019 @04:30PM
(#21633025)
The problems there don't apply I believe because Rust closures don't require an executable stack.
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Re:Neat Rust Tricks: Passing Closures to C
by mmastrac on Monday November 25, 2019 @04:42PM
(#21633130)
That's correct - a Rust closure generally [1] can't be converted to a function pointer as it requires both code and state.
The whole point of jgtrosh's link is that there is a way to hide data behind a function pointer, so Rust could convert any closure to a function pointer. But it requires writable-and-executable memory, so it's a pretty bad idea (in GCC's implementation, that memory is on the stack, which is an extra bad idea, but i don't think it needs to be).
Re:Neat Rust Tricks: Passing Closures to C
by mmastrac on Monday November 25, 2019 @05:02PM
(#21633295)
Definitely.
Technically this can also be done via static code trampolines that are mmap'd as well [1]. That approach has been used on iOS in the past to turn blocks into raw function pointers.
If you have a platform that allows W+X on code (yikes!), you can do [2] as well.
[1] https://github.com/plausiblelabs/plblockimp/blob/master/Sour... [2] https://www.mikeash.com/pyblog/friday-qa-2010-02-12-trampoli...
Re:Neat Rust Tricks: Passing Closures to C
by rabidferret on Monday November 25, 2019 @06:23PM
(#21633988)
Anything that doesn't require W+X would need an entire page allocated per closure, wouldn't it?
Re:Neat Rust Tricks: Passing Closures to C
by devit on Monday November 25, 2019 @07:58PM
(#21634706)
No, you can of course allocate W+X pages from the OS and put multiple closures in them using a standard userspace memory allocator.
Or if the OS doesn't support W+X allocation at all, then you can have a bunch of tightly packed pregenerated trampolines in the binary.
Re:Neat Rust Tricks: Passing Closures to C
by saagarjha on Monday November 25, 2019 @08:26PM
(#21634861)
Right, this is how Objective-C's implementation works, except it keeps around one page of trampolines and remaps that around when necessary to be able to "create" more trampolines on the fly, I believe.
Re:Neat Rust Tricks: Passing Closures to C
by a1369209993 on Monday November 25, 2019 @10:58PM
(#21635551)
Nope! You'd do something to the effect of:
clo_code:
4C8B1501100000 mov r10 [rel clo_code+0x1008]
FF25F30F0000 jmp [rel clo_code+0x1000]
0F1F00 nop3
# one page away...
struct clo_slot {
void (*func)(void* _R10,...);
void* data;
};