This file is part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We affiliate just a few 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 every memory learn, propagate the shadow bits through a number of the arithmetic instruction (including MOV), store the shadow bits on each Memory Wave Workshop write, report a bug on another directions (e.g. JMP) if the associated shadow is poisoned. But there are differences too. The primary and the main one: compiler instrumentation as an alternative of binary instrumentation. This gives us much better register allocation, possible compiler optimizations and a quick start-up. But this brings the key concern as nicely: msan must see all program events, including system calls and reads/writes in system libraries, so we both must compile every thing with msan or use a binary translation component (e.g. DynamoRIO) to instrument pre-built libraries. Another difference from Memcheck is that we use 8 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 not a priority there. MemorySanitizer can observe origins (allocation points) of all uninitialized values. This habits 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 4 bytes of application memory. Propagation of origins is mainly a bunch of “choose” instructions that pick the origin of a soiled argument, if an instruction has one. Every four aligned, consecutive bytes of utility memory have one origin worth associated with them. If these bytes comprise uninitialized information coming from 2 different allocations, the last store wins. Due to this, MemorySanitizer studies can show unrelated origins, however that is unlikely in practice. Origins are meaningless for absolutely initialized values, so MemorySanitizer avoids storing origin to memory when a totally initialized value is stored.
This manner it avoids useless overwriting origin of the 4-byte area on a short (i.e. 1 byte) clear store, and additionally it is good for performance. Ideally, each atomic store of application value should update the corresponding shadow location in an atomic method. Sadly, atomic store of two disjoint areas cannot be done without extreme slowdown. Subsequently, we implement an approximation which will err on the safe side. In this implementation, each atomically accessed location in this system could solely change from (partially) uninitialized to completely initialized, however not the other approach round. We load the shadow after the applying load, and we store the shadow earlier than the app store. Additionally, we always store clear shadow (if the application store is atomic). This way, if the store-load pair constitutes a occurs-before arc, shadow store and load are correctly ordered such that the load will get either the value that was stored, or some later value (which is at all times clear).
This does not work very effectively with Compare-And-Swap (CAS) and browse-Modify-Write (RMW) operations. To follow the above logic, CAS and RMW should retailer the brand new shadow before the app operation, and cargo the shadow after the app operation. Computers don’t work this manner. Present implementation ignores the load facet of CAS/RMW, always returning a clear value. It implements the shop part as a simple atomic store by storing a clean shadow. For inline assembly code LLVM has little idea about which memory locations change into initialized depending on the arguments. It can be doable to determine which arguments are meant to level to inputs and outputs, however the precise semantics can be solely seen at runtime. Within the Linux kernel it’s also attainable that the arguments solely indicate the offset for a base taken from a segment register, so it is harmful to deal with any asm() arguments as pointers. The latter can perform extra complicated tackle checks to determine whether it’s secure to contact the shadow memory. KMSAN allocates shadow and origin memory for every web page individually, so there are no specific accesses to shadow and origin within the instrumentation. X(ptr) capabilities. The corresponding capabilities verify that the X-byte accesses are possible and returns the pointers to shadow and origin memory. Be aware that the sanitizer code has to deal with how shadow/origin pairs returned by the these capabilities are represented in numerous ABIs. Sixty four ABI they are returned in RDX:RAX, in PowerPC64 they’re returned in r3 and r4, and in the SystemZ ABI they are written to memory pointed to by a hidden parameter. TLS variables are stored in a single per-task struct.
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