Is it valid to write below ESP?

Question

For a 32-bit windows application is it valid to use stack memory below ESP for temporary swap space without explicitly decrementing ESP?

Consider a function that returns a floating point value in ST(0). If our value is currently in EAX we would, for example,

PUSH   EAX
FLD    [ESP]
ADD    ESP,4  // or POP EAX, etc
// return...

Or without modifying the ESP register, we could just :

MOV    [ESP-4], EAX
FLD    [ESP-4]
// return...

In both cases the same thing happens except that in the first case we take care to decrement the stack pointer before using the memory, and then to increment it afterwards. In the latter case we do not.

Notwithstanding any real need to persist this value on the stack (reentrancy issues, function calls between PUSHing and reading the value back, etc) is there any fundamental reason why writing to the stack below ESP like this would be invalid?

Census

in windows user mode this valid, because interrupts never use user mode stack, in kernel mode this is not valid - because interrupt can be executed at any time – Reeducate

Why not just use the stack pointer as designed? Kinda the whole point of stacks and stack pointers, so you can have easy to allocate local memory. Why not just do that? My comment has nothing to do with x86, in general use the processor as designed. – Budbudapest

@Budbudapest The reason isn't really relevant - assume it's more performant, for example. Or imagine you found something like this in an existing codebase. The question is about correctness and the documented behaviour (would you 'fix' it? Is it actually broken?). Naturally we would want to be as idiomatic as possible without a reason to do otherwise, but I'd like to distinguish between correct and idiomatic here. – Census

with a list of constraints sure you dont have to move the stack pointer, but that list can collapse, and it is certainly a very bad habit. should be obvious what that list is. – Budbudapest

possible in future windows and add "direct" apc or some "direct" signals - some code will be executed via callback just after thread enter to kernel (during usual hardware interrupt). after this all below esp will be undefined. but until this not exist. until this code will be work always(in current builds) correct. – Reeducate

Answer 1

TL:DR: no, there are some SEH corner cases that can make it unsafe in practice, as well as being documented as unsafe. @Raymond Chen recently wrote a blog post that you should probably read instead of this answer.

His example of a code-fetch page-fault I/O error that can be "fixed" by prompting the user to insert a CD-ROM and retry is also my conclusion for the only practically-recoverable fault if there aren't any other possibly-faulting instructions between store and reload below ESP/RSP.

Or if you ask a debugger to call a function in the program being debugged, it will also use the target process's stack.

This answer has a list of some things you'd think would potentially step on memory below ESP, but actually don't, which might be interesting. It seems to be only SEH and debuggers that can be a problem in practice.

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First of all, if you care about efficiency, can't you avoid x87 in your calling convention? movd xmm0, eax is a more efficient way to return a float that was in an integer register. (And you can often avoid moving FP values to integer registers in the first place, using SSE2 integer instructions to pick apart exponent / mantissa for a log(x), or integer add 1 for nextafter(x).) But if you need to support very old hardware, then you need a 32-bit x87 version of your program as well as an efficient 64-bit version.

But there are other use-cases for small amounts of scratch space on the stack where it would be nice to save a couple instructions that offset ESP/RSP.

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Trying to collect up the combined wisdom of other answers and discussion in comments under them (and on this answer):

It is explicitly documented as being not safe by Microsoft: (for 64-bit code, I didn't find an equivalent statement for 32-bit code but I'm sure there is one)

Stack Usage (for x64)

All memory beyond the current address of RSP is considered volatile: The OS, or a debugger, may overwrite this memory during a user debug session, or an interrupt handler.

So that's the documentation, but the interrupt reason stated doesn't make sense for the user-space stack, only the kernel stack. The important part is that they document it as not guaranteed safe, not the reasons given.

Hardware interrupts can't use the user stack; that would let user-space crash the kernel with mov esp, 0, or worse take over the kernel by having another thread in the user-space process modify return addresses while an interrupt handler was running. This is why kernels always configure things so interrupt context is pushed onto the kernel stack.

Modern debuggers run in a separate process, and are not "intrusive". Back in 16-bit DOS days, without a multi-tasking protected-memory OS to give each task its own address space, debuggers would use the same stack as the program being debugged, between any two instructions while single-stepping.

@RossRidge points out that a debugger might want to let you call a function in the context of the current thread, e.g. with SetThreadContext. This would run with ESP/RSP just below the current value. This could obviously have side-effects for the process being debugged (intentional on the part of the user running the debugger), but clobbering local variables of the current function below ESP/RSP would be an undesirable and unexpected side-effect. (So compilers can't put them there.)

(In a calling convention with a red-zone below ESP/RSP, a debugger could respect that red-zone by decrementing ESP/RSP before making the function call.)

There are existing program that intentionally break when being debugged at all, and consider this a feature (to defend against efforts to reverse-engineer them).

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Related: the x86-64 System V ABI (Linux, OS X, all other non-Windows systems) does define a red-zone for user-space code (64-bit only): 128 bytes below RSP that is guaranteed not to be asynchronously clobbered. Unix signal handlers can run asynchronously between any two user-space instructions, but the kernel respects the red-zone by leaving a 128 byte gap below the old user-space RSP, in case it was in use. With no signal handlers installed, you have an effectively unlimited red-zone even in 32-bit mode (where the ABI does not guarantee a red-zone). Compiler-generated code, or library code, of course can't assume that nothing else in the whole program (or in a library the program called) has installed a signal handler.

So the question becomes: is there anything on Windows that can asynchronously run code using the user-space stack between two arbitrary instructions? (i.e. any equivalent to a Unix signal handler.)

As far as we can tell, SEH (Structured Exception Handling) is the only real obstacle to what you propose for user-space code on current 32 and 64-bit Windows. (But future Windows could include a new feature.) And I guess debugging if you happen ask your debugger to call a function in the target process/thread as mentioned above.

In this specific case, not touching any other memory other than the stack, or doing anything else that could fault, it's probably safe even from SEH.

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SEH (Structured Exception Handling) lets user-space software have hardware exceptions like divide by zero delivered somewhat similarly to C++ exceptions. These are not truly asynchronous: they're for exceptions triggered by instructions you ran, not for events that happened to come after some random instruction.

But unlike normal exceptions, one thing a SEH handler can do is resume from where the exception occurred. (@RossRidge commented: SEH handlers are are initially called in the context of the unwound stack and can choose to ignore the exception and continue executing at the point where the exception occurred.)

So that's a problem even if there's no catch() clause in the current function.

Normally HW exceptions can only be triggered synchronously. e.g. by a div instruction, or by a memory access which could fault with STATUS_ACCESS_VIOLATION (the Windows equivalent of a Linux SIGSEGV segmentation fault). You control what instructions you use, so you can avoid instructions that might fault.

If you limit your code to only accessing stack memory between the store and reload, and you respect the stack-growth guard page, your program won't fault from accessing [esp-4]. (Unless you reached the max stack size (Stack Overflow), in which case push eax would fault, too, and you can't really recover from this situation because there's no stack space for SEH to use.)

So we can rule out STATUS_ACCESS_VIOLATION as a problem, because if we get that on accessing stack memory we're hosed anyway.

An SEH handler for STATUS_IN_PAGE_ERROR could run before any load instruction. Windows can page out any page it wants to, and transparently page it back in if it's needed again (virtual memory paging). But if there's an I/O error, your Windows attempts to let your process handle the failure by delivering a STATUS_IN_PAGE_ERROR

Again, if that happens to the current stack, we're hosed.

But code-fetch could cause STATUS_IN_PAGE_ERROR, and you could plausibly recover from that. But not by resuming execution at the place where the exception occurred (unless we can somehow remap that page to another copy in a highly fault-tolerant system??), so we might still be ok here.

An I/O error paging in the code that wants to read what we stored below ESP rules out any chance of reading it. If you weren't planning to do that anyway, you're fine. A generic SEH handler that doesn't know about this specific piece of code wouldn't be trying to do that anyway. I think usually a STATUS_IN_PAGE_ERROR would at most try to print an error message or maybe log something, not try to carry on whatever computation was happening.

Accessing other memory in between the store and reload to memory below ESP could trigger a STATUS_IN_PAGE_ERROR for that memory. In library code, you probably can't assume that some other pointer you passed isn't going to be weird and the caller is expecting to handle STATUS_ACCESS_VIOLATION or PAGE_ERROR for it.

Current compilers don't take advantage of space below ESP/RSP on Windows, even though they do take advantage of the red-zone in x86-64 System V (in leaf functions that need to spill / reload something, exactly like what you're doing for int -> x87.) That's because MS says it isn't safe, and they don't know whether SEH handlers exist that could try to resume after an SEH.

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Things that you'd think might be a problem in current Windows, and why they're not:

• The guard page stuff below ESP: as long as you don't go too far below the current ESP, you'll be touching the guard page and trigger allocation of more stack space instead of faulting. This is fine as long as the kernel doesn't check user-space ESP and find out that you're touching stack space without having "reserved" it first.

• kernel reclaim of pages below ESP/RSP: apparently Windows doesn't currently do this. So using a lot of stack space once ever will keep those pages allocated for the rest of your process lifetime, unless you manually VirtualAlloc(MEM_RESET) them. (The kernel would be allowed to do this, though, because the docs say memory below RSP is volatile. The kernel could effectively zero it asynchronously if it wants to, copy-on-write mapping it to a zero page instead of writing it to the pagefile under memory pressure.)

• APC (Asynchronous Procedure Calls): They can only be delivered when the process is in an "alertable state", which means only when inside a call to a function like SleepEx(0,1). calling a function already uses an unknown amount of space below E/RSP, so you already have to assume that every call clobbers everything below the stack pointer. Thus these "async" callbacks are not truly asynchronous with respect to normal execution the way Unix signal handlers are. (fun fact: POSIX async io does use signal handlers to run callbacks).

• Console-application callbacks for ctrl-C and other events (SetConsoleCtrlHandler). This looks exactly like registering a Unix signal handler, but in Windows the handler runs in a separate thread with its own stack. (See RbMm's comment)

• SetThreadContext: another thread could change our EIP/RIP asynchronously while this thread is suspended, but the whole program has to be written specially for that to make any sense. Unless it's a debugger using it. Correctness is normally not required when some other thread is messing around with your EIP unless the circumstances are very controlled.

And apparently there are no other ways that another process (or something this thread registered) can trigger execution of anything asynchronously with respect to the execution of user-space code on Windows.

If there are no SEH handlers that could try to resume, Windows more or less has a 4096 byte red-zone below ESP (or maybe more if you touch it incrementally?), but RbMm says nobody takes advantage of it in practice. This is unsurprising because MS says not to, and you can't always know if your callers might have done something with SEH.

Obviously anything that would synchronously clobber it (like a call) must also be avoided, again same as when using the red-zone in the x86-64 System V calling convention. (See https://stackoverflow.com/tags/red-zone/info for more about it.)

Marciano

If a debugger wanted to force an exception or function call in the context of a given thread it would need to be able to push stuff on the stack. SEH handlers are are initially called in the context of the unwound stack and can choose to ignore the exception and continue executing at the point where the exception occurred. Finally another process can trigger asynchronous execution of user-space code through SetThreadContext. – Catachresis

The system could inject a structured exception at any time, such as STATUS_IN_PAGE_ERROR or STATUS_ACCESS_VIOLATION. – Josi

@RaymondChen: interesting, thanks. Working on an update for that and what Ross pointed out. For the specific example in the question, if the memory at [ESP-4] got paged out and couldn't be paged back in, the SEH handler won't be able to run either so your process is pretty much hosed. But if code-fetch caused STATUS_IN_PAGE_ERROR, you could plausibly recover. But not by resuming execution of the original code (unless we can somehow remap that page to another copy in a highly fault-tolerant system??), so we might still be ok here. We're assuming [esp-4] isn't an access violation. – Marciano

@RaymondChen For the specific two instructions the OP used or any similar two instructions that are placed back-to-back, I don't see how that can be a problem. So what if an exception is raised? The second instruction will just not execute. So I don't see a problem. – Pusey

@RossRidge With SetThreadContext, all bets are off anyway. So I don't see how this is relevant. The second case you mentioned has already been discussed in Peter's answer and my answer I think. I need to think more about the debugger thing. – Pusey

@RossRidge Can the debugger inject a call to some function exactly between the two consecutive instructions and still having both of them execute successfully? How does this work exactly? This would indeed be a case where writing below ESP wouldn't work as expected. But I don't know enough about how debuggers work exactly to answer that question. – Pusey

One other case perhaps worth considering is when running the code using an emulator. Maybe a cross-ISA emulator that might not consider the possibly of writing below ESP or assume that this will not happen. I can't think of a specific example though. – Pusey

@RossRidge: Thanks for that summary of SEH, I hadn't realized they could resume. I still think the OP's store/reload without touching any other memory is safe, but I promoted SEH to a whole section in my updated answer, out of the bullet list. – Marciano

@HadiBrais: I think a debugger with a call command would save/restore registers across the call, and print the result interactively (instead of leaving EAX or XMM0 modified. Any side-effects would probably still happen, and it would be up to the user not to call printf while stopped in the middle of another stdio library function, but the debugger would then just restore register state. – Marciano

@RaymondChen: updated with more about SEH. I don't really know Windows low-level stuff beyond what I read on SO, so some of what I said is based on my intuition for how stuff probably works and/or extrapolating from other people's comments, and thus may contain errors. I'm glad I posted the original, though; I think this question needed a summary of all the comments. I know I was curious what exactly (if anything) stopped Windows code from having a red zone in practice, like how i386 Linux essentially has an unlimited red-zone if you don't install any signal handlers. – Marciano

@HadiBrais The exception handler could say "I fixed the problem, resume execution." In that case the second instruction does execute. – Josi

@RaymondChen Oh, I think you are talking about vectored exception handlers where the handler can specify to resume execution from the next instruction. I completely forgot about this possibility. – Pusey

same instruction that raised the exception, not the next instruction. – Pusey

@HadiBrais Structured exception handlers can request resumption as well. Just return EXCEPTION_CONTINUE_EXECUTION to restart the failed instruction (presumably after trying to fix the problem). – Josi

Agner suggests that the system may discard pages below ESP if it is "extremely low on memory". You mention this doesn't happen on current systems. I don't know who is right, but I thought I'd mention it. I found the quote here but perhaps he talks about it more somewhere else. – Implead

Windows 10 version 1809 introduces Special User APCs, which can fire at any moment just like UNIX signals. See repnz.github.io/posts/apc/user-apc/… – Hell

@Hell That's worth adding as an answer. – Census

This is a legendary answer that deserves way more votes. I still come back to re-read this from time to time. @RaymondChen Loved the blog post as well. – Census

Answer 2

in general case (x86/x64 platform) - interrupt can be executed at any time, which overwrite memory bellow stack pointer (if it executed on current stack). because this, even temporary save something bellow stack pointer, not valid in kernel mode - interrupt will be use current kernel stack. but in user mode situation another - windows build interrupt table (IDT) suchwise that when interrupt raised - it will be always executed in kernel mode and in kernel stack. as result user mode stack (below stack pointer) will be not affected. and possible temporary use some stack space bellow it pointer, until you not do any functions calls. if exception will be (say by access invalid address) - also space bellow stack pointer will be overwritten - cpu exception of course begin executed in kernel mode and kernel stack, but than kernel execute callback in user space via ntdll.KiDispatchExecption already on current stack space. so in general this is valid in windows user mode (in current implementation), but you need good understand what you doing. however this is very rarely i think used

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of course, how correct noted in comments that we can, in windows user mode, write below stack pointer - is just the current implementation behavior. this not documented or guaranteed.

but this is very fundamental - unlikely will be changed: interrupts always will be executed in privileged kernel mode only. and kernel mode will be use only kernel mode stack. the user mode context not trusted at all. what will be if user mode program set incorrect stack pointer ? say by mov rsp,1 or mov esp,1 ? and just after this instruction interrupt will be raised. what will be if it begin executed on such invalid esp/rsp ? all operation system just crashed. exactly because this interrupt will be executed only on kernel stack. and not overwrite user stack space.

also need note that stack is limited space (even in user mode), access it bellow 1 page (4Kb)already error (need do stack probing page by page, for move guard page down).

and finally really there is no need usually access [ESP-4], EAX - in what problem decrement ESP first ? even if we need access stack space in loop huge count of time - decrement stack pointer need only once - 1 additional instruction (not in loop) nothing change in performance or code size.

so despite formal this is will be correct work in windows user mode, better (and not need) use this

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of course formal documentation say:

Stack Usage

All memory beyond the current address of RSP is considered volatile

but this is for common case, including kernel mode too. i wrote about user mode and based on current implementation

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possible in future windows and add "direct" apc or some "direct" signals - some code will be executed via callback just after thread enter to kernel (during usual hardware interrupt). after this all below esp will be undefined. but until this not exist. until this code will be work always(in current builds) correct.

Reeducate

Answer 3

In general (not specifically related to any OS); it's not safe to write below ESP if:

• It's possible for the code to be interrupted and the interrupt handler will run at the same privilege level. Note: This is typically very unlikely for "user-space" code, but extremely likely for kernel code.

• You call any other code (where either the call or the stack used by the called routine can trash the data you stored below ESP)

• Something else depends on "normal" stack use. This can include signal handling, (language based) exception unwinding, debuggers, "stack smashing protector"

It's safe to write below ESP if it's not "not safe".

Note that for 64-bit code, writing below RSP is built into the x86-64 ABI ("red zone"); and is made safe by support for it in tool chains/compilers and everything else.

Marr

This question is about Windows. Linux/MacOS have a red zone as it is part of the x86-64 System V ABI. – Fingerprint

Answer 4

When a thread gets created, Windows reserves a contiguous region of virtual memory of a configurable size (the default is 1 MB) for the thread's stack. Initially, the stack looks like this (the stack grows downwards):

--------------
|  committed |
--------------
| guard page |
--------------
|     .      |
| reserved   |
|     .      |
|     .      |
|            |
--------------

ESP will be pointing somewhere inside the committed page. The guard page is used to support automatic stack growth. The reserved pages region ensures that the requested stack size is available in virtual memory.

Consider the two instructions from the question:

MOV    [ESP-4], EAX
FLD    [ESP-4]

There are three possibilities:

• The first instruction executes successfully. There is nothing that uses the user-mode stack that can execute between the two instructions. So the second instruction will use the correct value (@RbMm stated this in the comments under his answer and I agree).
• The first instruction raises an exception and an exception handler does not return EXCEPTION_CONTINUE_EXECUTION. As long as the second instruction is immediately after the first one (it is not in the exception handler or placed after it), then the second instruction will not execute. So you're still safe. Execution continues from stack frame where the exception handler exists.
• The first instruction raises an exception and an exception handler returns EXCEPTION_CONTINUE_EXECUTION. Execution continues from the same instruction that raised the exception (potentially with a context modified by the handler). In this particular example, the first will be re-executed to write a value below ESP. No problem. If the second instruction raised an exception or there are more than two instructions, then the exception might occur a place after a value is written below ESP. When the exception handler gets called, it may overwrite the value and then return EXCEPTION_CONTINUE_EXECUTION. But when execution resumes, the value written is assumed to still be there, but it's not anymore. This is a situation where it's not safe to write below ESP. This applies even if all of the instructions are placed consecutively. Thanks to @RaymondChen for pointing this out.

In general, if the two instructions are not placed back-to-back, if you are writing to locations beyond ESP, there is no guarantee that the written values won't get corrupted or overwritten. One case that I can think of where this might happen is structured exception handling (SEH). If a hardware-defined exception (such as divide by zero) occurs, the kernel exception handler will be invoked (KiUserExceptionDispatcher) in kernel-mode, which will invoke the user-mode side of the handler (RtlDispatchException). When switching from user-mode to kernel-mode and then back to user-mode, whatever value was in ESP will be saved and restored. However, the user-mode handler itself uses the user-mode stack and will iterate over a registered list of exception handlers, each of which uses the user-mode stack. These functions will modify ESP as required. This may lead to losing the values you've written beyond ESP. A similar situation occurs when using software-define exceptions (throw in VC++).

I think you can deal with this by registering your own exception handler before any other exception handlers (so that it is called first). When your handler gets called, you can save your data beyond ESP elsewhere. Later, during unwinding, you get the cleanup opportunity to restore your data to the same location (or any other location) on the stack.

You need also to similarly watch out for asynchronous procedure calls (APCs) and callbacks.

Pusey

Answer 5

Several answers here mention APCs (Asynchronous Procedure Calls), saying that they can only be delivered when the process is in an "alertable state", and are not truly asynchronous with respect to normal execution the way Unix signal handlers are

Windows 10 version 1809 introduces Special User APCs, which can fire at any moment just like Unix signals. See this article for low level details.

The Special User APC is a mechanism that was added in RS5 (and exposed through NtQueueApcThreadEx), but lately (in an insider build) was exposed through a new syscall - NtQueueApcThreadEx2. If this type of APC is used, the thread is signaled in the middle of the execution to execute the special APC.

Hell