Indirect Calling of Nested Functions on GCC Without Executable Stack
uecker.codeberg.page63 points by uecker 9 hours ago
63 points by uecker 9 hours ago
Self-modifying code is cool. It's a shame we had to disable it for security.
Not just security. Instruction caches must also be aware of self-modifying code.
Back in the 1980s, text editors had configuration files. The configuration file would be read every time the editor was loaded. This was very slow on a floppy disk system.
I realized that, instead of a configuration file, I could configure the executable instead! So, any changes in configuration meant the editor would patch its own exe file!
This marvelous technique came to an end when attempts to stop malware got folded into the operating system.
Some early systems, like TeX and I believe some Lisps, took this to the extreme. Instead of patching, they loaded their config once and then dumped the configured process image to a file, which was used in subsequent invocations.
Don’t think TeX ever did that, that was a “simple” Pascal program.
Lisp Machines though.. updating the operating system was by loading bunch of compiled files that replaced currently loaded functions in memory.
Then again, Lisp Machines where very proud of self modification — the CADR had a fun feature where it could modify the next instruction depending on things…
How the world has changed.
Pascal TeX to the best of my knowledge never did it but the web2c version does.
The description is a bit confusing because it can both dump only the warmed up interpreter image or the whole process, I think.
"With the program undump, you can use `core' to reconstitute a preloaded executable, which does not need to read a `.fmt' file to get started. Although preloaded executables save startup time, they have a big disadvantage: neither the disk space to store them nor their code segments (at runtime) can be shared. Therefore, if both tex and latex are running, twice as much memory will be consumed, to the general detriment of performance."
https://mirror.gutenberg-asso.fr/tex.loria.fr/texlive-htmldo...
Emacs does this to create its image with all the added functionality above the minimum required to run elisp, then has a mechanism to pull in text files because elisp is just text anyway.
They just chucked the old system for a portable version of it, but until this last release, they still had to option of doing it the old school way.
On a floppy system, it's much faster to just patch it on disk than rewrite it!
emacs did this too.
And didn't stop doing this until 2020! (Specifically Emacs 27.1, which replaced "unexec", which dumped the process image, with the "portable dumper", which despite the name does not.)
And you could still use the unexec method until this very latest release a few days ago, if you didn't want the pdumper.
IIRC early Turbo Pascal versions worked like that too, there was some "setup" program that let you configure colors, etc, by modifying the COM/EXE file itself.
yes but it's easy enough to issue a cache flush when you modify the code.
The overhead of cache flushing means some old school techniques are no longer viable, like modifying a constant in the next instruction. However it is still interesting to write machine code snippets once and execute them many times, like the nested function trampolines. I had a case where I had RGB masks like R=0x00ff0000 etc (loaded at startup once) and wanted to convert 0x00rrggbb to match the mask (so no-op in the common case but not always) which could have involved setting the shift amounts in a series of shift instructions.
The Linux kernel uses self-modification to change branches depending on whether certain features are on. For example when a user-mode process starts tracing a certain function, it adds code to the beginning of that function to trace the call, otherwise it pads that space with a no-op. JIT compilers also make good use of knowing whether a class has any subclasses, which is statically unknowable in Java but dynamically knowable.
Yep. Even outside of tracing, there are several different ways that the Linux kernel patches itself:
- Static calls: like a call to a global function pointer, except instead of loading a function pointer and doing an indirect call, the code is patched to do a direct call to the destination
- Static keys: like an if statement testing a global boolean, except instead of loading a boolean and doing a conditional branch, the code is patched to do either an unconditional branch or a nop
- Runtime constants: like a load of a global variable, except instead of loading, the value is patched directly into the code
- Alternatives: selects one of multiple possible instruction sequences depending on (usually) whether the CPU supports specific instructions
It's really fascinating to see the kind of fun efficient stuff you can do when you have that level of low-level control. Not just code patching but things like RCU as well.
> However it is still interesting to write machine code snippets once and execute them many times, like the nested function trampolines.
I slightly disagree on this though. In my experience writing code with Clang blocks (which don't use trampolines), they're often useful for code organization even if the callback will only be called once. Therefore, even ignoring security issues, I think GCC choosing a design that required cache flushing was a mistake - certainly in retrospect (as cache flushing has become more expensive over the years), but perhaps even at the time. I did some research, and trampolines were introduced in GCC 2.0, which already included mprotect calls and/or cache flushes on some of the architectures it supported, such as MIPS. However, this was a relatively new development, and on most of the supported architectures it didn't do either of those things. But on MIPS it would do an mprotect every single time a trampoline was created, which can't have been fast.
It's not the cache flush that's expensive, it's loading the new instructions that aren't cached. If you are flushing the very next instruction and then immediately executing it, that's expensive because of the serial dependency, but if you're generating new code, flushing it shouldn't be more expensive than if you were simply accessing new code for the first time. But on old systems you could modify the very next instruction with no penalty because there wasn't a cache. You can still do that and probably faster than those old systems could (they were slower because of not having a cache, everything was an uncached access), it's just a waste of most of the new system's performance.
BTW you can do all of this cool stuff in user mode on Linux too (but not on OpenBSD) - you just have to opt in to executable stack and/or writable .text. I could have written the dynamic shift instruction generator I mentioned, but I didn't want to spend the effort, but I imagined having a language with actual support for something like that (like static keys for variables).
Modifying a constant in code might make sense because it saves several precious bytes of variable storage.
No, it might make sense because it saves precious memory bandwidth as well as uop count. This code wasn't called that often, generally once per drawing operation, but I don't like being uselessly wasteful - that mindset is how software got so bloated.
Your program is either front-end stalled by uop count or instruction cache latency, or back-end bound by memory bandwidth or ALU throughput or a serial dependency chain. It doesn't matter which is the bottleneck for this case, because inlining constants improves most of the above!
s/stalled/bound/
When the back end is the bottleneck, the front end stalls and vice versa, though I'm not sure how all processors report it.
It was maybe cool 50 years ago or so. Nowadays it's no longer needed. Possible performance gains of such code are marginal and modern programming languages allow generating many specialized and optimized code pieces using the same template, so that self-modification is no longer needed.
There is also JIT (like in regexp engines), but it's different story.
It's still beneficial on some x86-64 implementations to rewrite indirect jumps (as used in PLT stubs) to direct jumps when feasible. For example, AMD says this about the Zen 4 architecture:
> Only a limited number of indirect targets that cross a 64MB aligned boundary relative to the branch address can be tracked in the indirect target predictor. Software should limit the number of indirect branch targets that cross such a boundary.
And one way doing this is to replace the indirect branch with a direct branch, which supports a 32-bit signed displacement.
It's pretty much always beneficial to do something more directly. Doing less work is always better than doing more work. The slowness of modern software is the result of a stack of abstractions acting like a stack of interpreters. You write something in React, that manipulates a React object tree and shadows it to a DOM, which gets shadowed to an internal object tree which gets laid out and shadowed to a stack of GPU layers which gets written out as drawing commands... When you want to scroll up there's so much work to do. To make it fast, cut through layers and minimize work. In the 1990s, scrolling up meant calculating how many pixels to scroll, blitting that many pixels in the main framebuffer (usually GPU accelerated) and then rendering the new pixels at the bottom. There wasn't even a double buffer. Very little abstraction there, just the shortest path to achieve the desired result. The GPU driver did abstract the blit and rendering operations, of course, and the mouse driver abstracted the scrollwheel event, and you may have a wrapper component in your GUI tree that manages a viewport over a larger virtual component, but it's all kept as direct as practical. I can't imagine any electron app using the blit-pixels-up approach.
I remember once learning of a runtime environment that would inline class functions. For example they wrote an OS, and if you had an object of a SATA hard drive class, it would copy the function code and inline the drive ID. I don't remember how well it worked for them.
A related idea is the "tracing JIT". You know how you expect a JIT to translate one function at a time? A tracing JIT doesn't - it follows the program logic wherever it goes, through whatever control flow, and compiles all of it until it decides to stop. The most well known implementation is probably LuaJIT.
Related: previous article in the series, https://news.ycombinator.com/item?id=49308685 (103 points, 47 comments)
At first, I was annoyed by having to read AT&T syntax. Then I was disoriented by realizing the next snippet was in AT&T syntax without the '%' sigil for registers. But the technique is cool.
What do you need the executable stack for? You call using a function pointer, there's no executable read/write memory involved in using a function pointer.
A nested function requires an extra parameter for the nested stack pointer, which is passed in a dedicated register on most ABIs. You can't spell this kind of function type in C. To make a C-callable function pointer, the compiler needs to generate a little bit of code (a trampoline) that stuffs the appropriate stack pointer in the appropriate register.
That trampoline needs to live somewhere. Since the function is inherently noncallable after the stack returns, and C programmers hate it when their compiler sneaks in extra malloc calls under the hood, the compiler decides to stick the trampoline on the stack instead of heap-allocating it.
Nested functions may require a context pointer of some kind, which the caller can't supply. One way of doing this: create a thunk on the stack that provides the context pointer, and use that address as the pointer to the nested function.
But now the stack needs to be executable.
Doesn't x86 actually support nested call pointers using enter to natively support this in Pascal?
It’s not a question of ISA support. If you call via a function pointer, how do you supply the pointer to the data? (It has to either be in a separate place from the function code, or the same place. One requires an ABI change, the other an executable, writable section of memory.)
> If you call via a function pointer, how do you supply the pointer to the data?
D has the notion of a "delegate", which is a (function pointer) and (context pointer) pair. This is incredibly useful, because delegates can:
1. call nested functions that need a pointer to the stack frame of the nestee function
2. call member functions that need `this` pointer
3. call lambdas
4. call COM member functions
The neato thing about this is the ABI for delegates is all the same, so a function that gets a delegate parameter will work with any of 1..4. It's one of the most used features of D.
Yes, that’s the “ABI” alternative that I was referring to.
This is why I want to have such a feature in C. It would be extremely useful for language interoperability.
But because ISA was mentioned, x86 does indeed even have native support for this: https://devblogs.microsoft.com/oldnewthing/20231211-00/?p=10... These instructions are not too useful though and I do not think anybody uses them.
This feature would IMO violate the contract that C allows you to specify memory layout of objects, to some level of detail (I am being a little vague about the “level of detail”).
Supporting closures, more or less, requires design decisions that are equivalent to choosing a specific layout for objects in an object-oriented language. C, as it is, makes none of these assumptions and you can translate a lot of different language ABIs into some C code (that may be clumsy). Keeping the abstraction that function pointers = pointers to entry points for functions, well, that’s frustrating for C programmers writing C programs, but extremely useful for interoperability.
At the moment we can not call nested functions or other things from other language from C, which is a pretty big hole in our interoperability story. I do not see why we need to make any design decision to specify object layout, we simply need a code pointer and static chain pair, which would then be sufficient to call arbitrary entities from other languages.
but that can just be executable heap?
Yes, but the advice was W^X. Either a page of memory is executable or writable but not both at the same time.