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

This file is a part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We associate a couple of shadow bits with every byte of the application memory, poison the shadow of the malloc-ed or alloca-ed memory, load the shadow, bits on each memory read, propagate the shadow bits via some of the arithmetic instruction (including MOV), retailer the shadow bits on each memory write, report a bug on another instructions (e.g. JMP) if the associated shadow is poisoned. But there are differences too. The first and the most important one: compiler instrumentation as an alternative of binary instrumentation. This gives us much better register allocation, doable compiler optimizations and a quick begin-up. But this brings the major situation as properly: msan needs to see all program events, together with system calls and reads/writes in system libraries, so we both need to compile all the things with msan or use a binary translation element (e.g. DynamoRIO) to instrument pre-constructed libraries. One other distinction from Memcheck is that we use eight shadow bits per byte of software memory and use a direct shadow mapping.

This enormously simplifies the instrumentation code and avoids races on shadow updates (Memcheck is single-threaded so races are usually not a concern there. MemorySanitizer can observe origins (allocation factors) of all uninitialized values. This behavior is managed with a flag (msan-monitor-origins) and is disabled by default. Origins are 4-byte values created and interpreted by the runtime library. They are stored 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 choose the origin of a dirty argument, if an instruction has one. Every four aligned, consecutive bytes of utility memory have one origin value related to them. If these bytes contain uninitialized information coming from 2 totally different allocations, the last retailer wins. Due to this, MemorySanitizer studies can show unrelated origins, however this is unlikely in follow. Origins are meaningless for totally initialized values, so MemorySanitizer avoids storing origin to memory when a fully initialized worth is saved.

This way it avoids needless overwriting origin of the 4-byte region on a brief (i.e. 1 byte) clean retailer, and additionally it is good for performance. Ideally, every atomic retailer of application worth should update the corresponding shadow location in an atomic manner. Unfortunately, atomic retailer of two disjoint areas cannot be accomplished with out severe slowdown. Therefore, we implement an approximation that may err on the secure side. On this implementation, each atomically accessed location in this system may solely change from (partially) uninitialized to fully initialized, however not the opposite approach around. We load the shadow after the applying load, and we store the shadow earlier than the app store. Also, we all the time store clear shadow (if the appliance retailer is atomic). This manner, if the shop-load pair constitutes a happens-earlier than arc, shadow store and load are accurately ordered such that the load will get both the value that was saved, or some later worth (which is at all times clean).

This doesn't work very effectively with Examine-And-Swap (CAS) and browse-Modify-Write (RMW) operations. To observe the above logic, CAS and RMW must store the new shadow before the app operation, and load the shadow after the app operation. Computers do not work this way. Current implementation ignores the load side of CAS/RMW, at all times returning a clean worth. It implements the store half as a easy atomic store by storing a clean shadow. For inline meeting code LLVM has little concept about which memory locations change into initialized relying on the arguments. It may be potential to figure out which arguments are meant to level to inputs and outputs, but the precise semantics could be solely seen at runtime. Within the Linux kernel it is also possible that the arguments only point out the offset for a base taken from a section register, so it's dangerous to treat any asm() arguments as pointers. The latter can carry out more complicated deal with checks to figure out whether it's safe to touch the shadow memory. KMSAN allocates shadow and origin memory for every web page separately, so there aren't any explicit accesses to shadow and origin within the instrumentation. X(ptr) features. The corresponding functions test that the X-byte accesses are possible and returns the pointers to shadow and origin Memory Wave Protocol. Observe 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'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-job struct.

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