This file is part of MemorySanitizer, Memory Wave a detector of uninitialized reads. 2005.html) We affiliate a number of shadow bits with each byte of the applying memory, poison the shadow of the malloc-ed or alloca-ed memory, load the shadow, bits on every memory read, propagate the shadow bits through a number of the arithmetic instruction (including MOV), store the shadow bits on each memory write, report a bug on some other directions (e.g. JMP) if the associated shadow is poisoned. However there are variations too. The first and the main one: compiler instrumentation instead of binary instrumentation. This offers us much better register allocation, potential compiler optimizations and a quick start-up. However this brings the main challenge as well: msan needs to see all program occasions, together with system calls and reads/writes in system libraries, so we either must compile every thing with msan or use a binary translation element (e.g. DynamoRIO) to instrument pre-constructed libraries. Another difference from Memcheck is that we use 8 shadow bits per byte of application 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 will not be a priority there. MemorySanitizer can monitor origins (allocation factors) of all uninitialized values. This habits is controlled with a flag (msan-observe-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 worth for four bytes of application memory. Propagation of origins is mainly a bunch of "choose" directions that choose the origin of a soiled argument, if an instruction has one. Every 4 aligned, consecutive bytes of software memory have one origin worth related to them. If these bytes contain uninitialized data coming from 2 different allocations, the last store wins. Because of this, MemorySanitizer experiences can show unrelated origins, however this is unlikely in observe. Origins are meaningless for totally initialized values, so MemorySanitizer avoids storing origin to memory when a totally initialized value is saved.
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This fashion it avoids unnecessary overwriting origin of the 4-byte region on a short (i.e. 1 byte) clean store, and it's also good for efficiency. Ideally, every atomic retailer of software value should update the corresponding shadow location in an atomic method. Unfortunately, atomic store of two disjoint locations can not be finished with out extreme slowdown. Due to this fact, we implement an approximation that will err on the protected facet. In this implementation, every atomically accessed location in this system could solely change from (partially) uninitialized to completely initialized, but not the opposite way round. We load the shadow after the applying load, and we store the shadow before the app store. Also, we always store clean shadow (if the applying retailer is atomic). This manner, if the shop-load pair constitutes a occurs-before arc, shadow retailer and cargo are accurately ordered such that the load will get either the value that was saved, or some later value (which is all the time clear).

This does not work very properly with Evaluate-And-Swap (CAS) and read-Modify-Write (RMW) operations. To observe the above logic, CAS and RMW must store the brand new shadow earlier than the app operation, and cargo the shadow after the app operation. Computer systems don't work this way. Current implementation ignores the load facet of CAS/RMW, all the time returning a clear value. It implements the shop half as a simple atomic retailer by storing a clear shadow. For inline assembly code LLVM has little thought about which memory places change into initialized depending on the arguments. It may be potential to determine which arguments are meant to point to inputs and outputs, however the precise semantics might be solely seen at runtime. Within the Linux kernel it is also doable 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 perform more complex tackle checks to figure out whether or not it's protected to contact the shadow memory. KMSAN allocates shadow and origin memory for every web page individually, so there are not any express 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 Wave Routine. Notice that the sanitizer code has to deal with how shadow/origin pairs returned by the these features are represented in several ABIs. Sixty four 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 stored in a single per-task struct.

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