LLVM: Lib/Transforms/Instrumentation/MemorySanitizer.cpp File Reference > 자유게시판

This file is a part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We affiliate a number of shadow bits with every byte of the appliance memory, poison the shadow of the malloc-ed or alloca-ed memory, load the shadow, bits on each memory learn, propagate the shadow bits via a few of the arithmetic instruction (including MOV), store the shadow bits on every memory write, report a bug on another directions (e.g. JMP) if the associated shadow is poisoned. But there are differences too. The first and the foremost one: compiler instrumentation as a substitute of binary instrumentation. This offers us much better register allocation, potential compiler optimizations and a quick begin-up. But this brings the major problem as nicely: msan must see all program events, together with system calls and reads/writes in system libraries, so we either need to compile the whole lot with msan or use a binary translation component (e.g. DynamoRIO) to instrument pre-built libraries. Another difference from Memcheck is that we use eight shadow bits per byte of utility memory and use a direct shadow mapping.

This drastically simplifies the instrumentation code and avoids races on shadow updates (Memcheck is single-threaded so races are usually not a concern there. MemorySanitizer can track origins (allocation points) of all uninitialized values. This habits is controlled with a flag (msan-track-origins) and is disabled by default. Origins are 4-byte values created and interpreted by the runtime library. They are saved in a second shadow mapping, one 4-byte value for four bytes of utility Memory Wave System. Propagation of origins is mainly a bunch of "choose" instructions that decide the origin of a dirty argument, if an instruction has one. Every four aligned, consecutive bytes of software memory have one origin value related to them. If these bytes include uninitialized information coming from 2 different allocations, the final store wins. Because of this, MemorySanitizer reviews can show unrelated origins, however this is unlikely in practice. Origins are meaningless for fully initialized values, so MemorySanitizer avoids storing origin to memory when a totally initialized value is stored.

This way it avoids needless overwriting origin of the 4-byte area on a short (i.e. 1 byte) clear retailer, and additionally it is good for efficiency. Ideally, every atomic store of software worth ought to update the corresponding shadow location in an atomic way. Unfortunately, atomic store of two disjoint areas cannot be done without severe slowdown. Therefore, we implement an approximation that may err on the safe aspect. In this implementation, each atomically accessed location in this system might solely change from (partially) uninitialized to completely initialized, but not the opposite manner around. We load the shadow after the appliance load, and we store the shadow earlier than the app store. Also, we always store clear shadow (if the applying retailer is atomic). This manner, if the shop-load pair constitutes a happens-before arc, shadow store and cargo are correctly ordered such that the load will get both the worth that was saved, or some later worth (which is all the time clean).

This doesn't work very well with Examine-And-Swap (CAS) and read-Modify-Write (RMW) operations. To comply with the above logic, CAS and RMW should retailer the brand new shadow earlier than the app operation, and load the shadow after the app operation. Computer systems do not work this fashion. Current implementation ignores the load facet of CAS/RMW, always returning a clean worth. It implements the shop part as a easy atomic store by storing a clear shadow. For inline assembly code LLVM has little concept about which memory places change into initialized relying on the arguments. It may be attainable to figure out which arguments are meant to point to inputs and outputs, however the precise semantics may be solely seen at runtime. Within the Linux kernel it's also possible that the arguments only point out the offset for a base taken from a section register, so it's harmful to deal with any asm() arguments as pointers. The latter can perform extra complex address checks to figure out whether it's protected to contact the shadow memory. KMSAN allocates shadow and origin memory for every page individually, so there are not any explicit accesses to shadow and origin in the instrumentation. X(ptr) capabilities. The corresponding features 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 capabilities are represented in numerous ABIs. 64 ABI they are returned in RDX:RAX, in PowerPC64 they're returned in r3 and r4, and within the SystemZ ABI they're written to memory pointed to by a hidden parameter. TLS variables are saved in a single per-activity struct.

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