[llvm-dev] static stack depth analysis tool

[Annie Cherkaev via llvm-dev]

Hello llvm-dev!

We are currently building a tool using LLVM which statically computes the
worst-case stack depth for programs whose call-graphs are statically
constrained. While this task is undecidable for general programs, we
specifically plan to use it to analyze all entry points into Zircon’s
kernel and the vDSO.

Currently, without such a tool, the best option for allocating kernel
memory in Zircon is taking a best guess of a conservative overestimate of
the memory required. This is problematic, however, because allocating too
much memory wastes valuable, highly limited RAM, and allocating too little
memory can result in stack overflows, which lead to catastrophic system
failure. This tool will eliminate the guesswork by reporting the smallest
amount of memory that is sufficient for all possible executions within the
kernel.

We would appreciate feedback on our approach, and would like to hear if
anyone else has use-cases for such a tool. Below are some implementation
details, followed by some Zircon-specific details.

     Implementation details:

This tool will perform the stack depth analysis by extracting the call
graph and the stack frame sizes from LLVM in two late passes, scheduled
just before codegen. A modulePass will write a dictionary of function names
to the list of called functions, and a machineFunctionPass will write a
dictionary of function names to their stack sizes. A script can then use
these dictionaries to annotate the call graph with the stack sizes by
matching up function names, and use this annotated call graph to find the
execution path with the largest stack.

Backends: We plan to support the x86-64 and aarch64 architectures; adding
other architectures should be straight-forward.

Fancy stacks: We plan to support using SafeStack & ShadowCallStack.

Hand-written assembly: The Zircon codebase contains a nontrivial amount of
hand-written assembly which uses stack space. We will manually audit the
stack usage of assembly code, and provide it to the analysis in a lookup
table.

Indirect Function Calls & Recursion: We hope to not run into too many call
graph complications in our codebase, refactor those which we do run into,
and add this tool into the build system to enforce that future code is
amenable to this analysis. We currently don’t plan on implementing any
additional analysis which LLVM does not already have.

Testing: We’ll validate the correctness of the tool through unit testing,
end-to-end testing and ideally also fuzz testing. We can set up fuzz
testing by instrumenting the source code to dynamically monitor the stack
size-- for instance by inlining assembly to report the stack pointer-- and
comparing that result to the value the tool computes statically. While this
wouldn’t validate that the tool is reporting stack sizes precisely, it
would give us a way to automatically check that the true stack frame size
reported by the dynamic check (which is the stack size for *some* execution
path) is no larger than the worst-case stack size reported by the static
tool.

     Zircon-specific details:

Interrupts:

An executing thread can be temporarily halted by an interrupt, which will
execute code on the stack of the thread it paused. This means to ensure we
find what the true worst-case stack is, we need to model the case where an
interrupt occurs while the function is at its deepest stack depth. Some
interrupts can interrupt other interrupts, therefore to accurately model
worst-case stack depth we need to build a model of how interrupts may
execute in x86-64 and aarch64, and treat them as additional entry points
which are called by every leaf function.

Motivation for analyzing stack usage in the vDSO:

In Zircon, syscalls are exposed through the vDSO (virtual Dynamic Shared
Object). The vDSO is an interface which is like a shared library object,
except it is provided by the OS instead of existing as a literal file. If a
process has permission to make syscalls then the vDSO is mapped into its
memory space, and it can dynamically link against syscalls the same way it
would any other shared library object. The vDSO requires a certain amount
of stack space to safely complete any syscall, and that amount is part of
the system’s ABI contract with user code. Currently, however, that amount
is unknown, undocumented and unenforced; this tool will allow us to analyze
the amount of stack space currently implemented syscalls use, document the
amount that will be contractually required by the ABI, and enforce that
future syscalls do not exceed that amount.

Please let us know if you have any comments or suggestions about the
implementation, and if you have a project that could benefit from such a
tool.

Thanks!

Annie Cherkaev
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