LLVM: Lib/Transforms/Instrumentation/MemorySanitizer.cpp File Referenc…

This file is part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We associate a few shadow bits with every byte of the appliance Memory Wave, poison the shadow of the malloc-ed or alloca-ed memory, load the shadow, bits on every memory learn, propagate the shadow bits via a number of the arithmetic instruction (including MOV), retailer the shadow bits on every memory write, report a bug on some other directions (e.g. JMP) if the related shadow is poisoned. But there are differences too. The primary and the major one: compiler instrumentation as a substitute of binary instrumentation. This provides us significantly better register allocation, potential compiler optimizations and a quick begin-up. However this brings the foremost subject as effectively: msan needs to see all program occasions, together with system calls and reads/writes in system libraries, so we either must compile everything with msan or use a binary translation part (e.g. DynamoRIO) to instrument pre-built libraries. Another difference from Memcheck is that we use 8 shadow bits per byte of application memory and use a direct shadow mapping.

This greatly simplifies the instrumentation code and avoids races on shadow updates (Memcheck is single-threaded so races will not be a concern there. MemorySanitizer can monitor origins (allocation points) 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 worth for four bytes of utility memory. Propagation of origins is basically a bunch of "choose" instructions that pick the origin of a soiled argument, if an instruction has one. Each 4 aligned, consecutive bytes of software memory have one origin worth related to them. If these bytes comprise uninitialized information coming from 2 different allocations, the final store wins. Due to this, MemorySanitizer reviews can present unrelated origins, however this is unlikely in apply. 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 unnecessary overwriting origin of the 4-byte area on a short (i.e. 1 byte) clear retailer, and it's also good for efficiency. Ideally, every atomic store of software value should update the corresponding shadow location in an atomic method. Sadly, atomic retailer of two disjoint areas can not be executed without extreme slowdown. Therefore, we implement an approximation which will err on the protected aspect. In this implementation, every atomically accessed location in this system may solely change from (partially) uninitialized to totally initialized, but not the opposite way round. We load the shadow after the appliance load, and we store the shadow before the app retailer. Additionally, we always store clear shadow (if the application store is atomic). This manner, if the store-load pair constitutes a occurs-before arc, shadow retailer and load are appropriately ordered such that the load will get both the value that was saved, or some later worth (which is at all times clear).

This does not work very effectively with Examine-And-Swap (CAS) and browse-Modify-Write (RMW) operations. To comply with the above logic, CAS and RMW must retailer the new shadow before the app operation, and cargo the shadow after the app operation. Computers do not work this way. Present implementation ignores the load facet of CAS/RMW, all the time returning a clean value. It implements the store part as a simple atomic store by storing a clear shadow. For inline assembly code LLVM has little concept about which Memory Wave locations turn out to be initialized depending on the arguments. It may be attainable to figure out which arguments are meant to point to inputs and outputs, but the actual semantics may be solely seen at runtime. In the Linux kernel it is also potential that the arguments solely indicate the offset for a base taken from a segment register, so it is dangerous to deal with any asm() arguments as pointers. The latter can carry out extra advanced deal with checks to figure out whether or not it's safe to contact the shadow memory. KMSAN allocates shadow and origin Memory Wave Workshop for every page separately, so there are not any specific accesses to shadow and origin within the instrumentation. X(ptr) functions. The corresponding capabilities verify that the X-byte accesses are possible 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 features are represented in numerous ABIs. Sixty four ABI they're returned in RDX:RAX, in PowerPC64 they are returned in r3 and r4, and in the SystemZ ABI they're written to memory pointed to by a hidden parameter. TLS variables are saved in a single per-job struct.

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