mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-23 03:02:36 +01:00
57 lines
2.3 KiB
ReStructuredText
57 lines
2.3 KiB
ReStructuredText
==================================
|
|
Stack Safety Analysis
|
|
==================================
|
|
|
|
|
|
Introduction
|
|
============
|
|
|
|
The Stack Safety Analysis determines if stack allocated variables can be
|
|
considered 'safe' from memory access bugs.
|
|
|
|
The primary purpose of the analysis is to be used by sanitizers to avoid
|
|
unnecessary instrumentation of 'safe' variables. SafeStack is going to be the
|
|
first user.
|
|
|
|
'safe' variables can be defined as variables that can not be used out-of-scope
|
|
(e.g. use-after-return) or accessed out of bounds. In the future it can be
|
|
extended to track other variable properties. E.g. we plan to extend
|
|
implementation with a check to make sure that variable is always initialized
|
|
before every read to optimize use-of-uninitialized-memory checks.
|
|
|
|
How it works
|
|
============
|
|
|
|
The analysis is implemented in two stages:
|
|
|
|
The intra-procedural, or 'local', stage performs a depth-first search inside
|
|
functions to collect all uses of each alloca, including loads/stores and uses as
|
|
arguments functions. After this stage we know which parts of the alloca are used
|
|
by functions itself but we don't know what happens after it is passed as
|
|
an argument to another function.
|
|
|
|
The inter-procedural, or 'global', stage, resolves what happens to allocas after
|
|
they are passed as function arguments. This stage performs a depth-first search
|
|
on function calls inside a single module and propagates allocas usage through
|
|
functions calls.
|
|
|
|
When used with ThinLTO, the global stage performs a whole program analysis over
|
|
the Module Summary Index.
|
|
|
|
Testing
|
|
=======
|
|
|
|
The analysis is covered with lit tests.
|
|
|
|
We expect that users can tolerate false classification of variables as
|
|
'unsafe' when in-fact it's 'safe'. This may lead to inefficient code. However, we
|
|
can't accept false 'safe' classification which may cause sanitizers to miss actual
|
|
bugs in instrumented code. To avoid that we want additional validation tool.
|
|
|
|
AddressSanitizer may help with this validation. We can instrument all variables
|
|
as usual but additionally store stack-safe information in the
|
|
``ASanStackVariableDescription``. Then if AddressSanitizer detects a bug on
|
|
a 'safe' variable we can produce an additional report to let the user know that
|
|
probably Stack Safety Analysis failed and we should check for a bug in the
|
|
compiler.
|