LLVM: Lib Transforms Instrumentation MemorySanitizer.cpp File Reference

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This file is part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We affiliate a few shadow bits with every byte of the applying memory, poison the shadow of the malloc-ed or alloca-ed memory, load the shadow, bits on each memory read, propagate the shadow bits by a number of the arithmetic instruction (together with MOV), retailer the shadow bits on each memory write, report a bug on another directions (e.g. JMP) if the associated shadow is poisoned. But there are variations too. The first and the key one: compiler instrumentation as an alternative of binary instrumentation. This offers us much better register allocation, Memory Wave doable compiler optimizations and a fast start-up. But this brings the main issue as effectively: msan must see all program events, including system calls and reads/writes in system libraries, Memory Wave clarity support so we both need to compile every little thing with msan or use a binary translation element (e.g. DynamoRIO) to instrument pre-built libraries. One other difference from Memcheck is that we use eight shadow bits per byte of utility memory and use a direct shadow mapping.



This significantly simplifies the instrumentation code and avoids races on shadow updates (Memcheck is single-threaded so races are not a priority there. MemorySanitizer can observe origins (allocation factors) of all uninitialized values. This behavior is managed with a flag (msan-observe-origins) and is disabled by default. Origins are 4-byte values created and interpreted by the runtime library. They're saved in a second shadow mapping, one 4-byte value for four bytes of utility memory. Propagation of origins is basically a bunch of "choose" instructions that pick the origin of a dirty argument, if an instruction has one. Every 4 aligned, consecutive bytes of utility memory have one origin value associated with them. If these bytes comprise uninitialized information coming from 2 completely different allocations, the final store wins. Because of this, MemorySanitizer reviews can present unrelated origins, but this is unlikely in follow. Origins are meaningless for absolutely initialized values, so MemorySanitizer avoids storing origin to memory when a completely initialized worth is stored.



This manner it avoids useless overwriting origin of the 4-byte region on a short (i.e. 1 byte) clean store, and it is also good for performance. Ideally, every atomic store of application worth should replace the corresponding shadow location in an atomic method. Sadly, atomic store of two disjoint areas can't be achieved with out extreme slowdown. Therefore, we implement an approximation that will err on the safe facet. In this implementation, each atomically accessed location in this system could only change from (partially) uninitialized to completely initialized, but not the opposite means round. We load the shadow after the application load, and we store the shadow before the app store. Additionally, we at all times retailer clear shadow (if the applying store is atomic). This manner, if the shop-load pair constitutes a occurs-before arc, shadow store and load are accurately ordered such that the load will get both the value that was saved, or some later value (which is all the time clean).



This doesn't work very nicely with Evaluate-And-Swap (CAS) and read-Modify-Write (RMW) operations. To comply with the above logic, CAS and RMW should retailer the new shadow before the app operation, and load the shadow after the app operation. Computers do not work this way. Present implementation ignores the load aspect of CAS/RMW, at all times returning a clean worth. It implements the shop half as a easy atomic store by storing a clean shadow. For inline assembly code LLVM has little thought about which memory locations turn out to be initialized depending on the arguments. It may be potential to figure out which arguments are meant to point to inputs and outputs, however the precise semantics may be solely visible at runtime. In the Linux kernel it is also potential that the arguments solely indicate the offset for a base taken from a section register, so it's dangerous to deal with any asm() arguments as pointers. The latter can perform extra advanced address checks to determine whether or not it is safe to contact the shadow memory. KMSAN allocates shadow and origin memory for each page individually, so there are no specific accesses to shadow and origin in the instrumentation. X(ptr) features. The corresponding functions test that the X-byte accesses are potential and returns the pointers to shadow and origin memory. Notice that the sanitizer code has to deal with how shadow/origin pairs returned by the these functions are represented in different ABIs. 64 ABI they are returned in RDX:RAX, in PowerPC64 they are returned in r3 and r4, and within the SystemZ ABI they are written to Memory Wave clarity support pointed to by a hidden parameter. TLS variables are saved in a single per-activity struct.



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