This file is part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We associate a number of shadow bits with each 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 via a number of the arithmetic instruction (together with MOV), store the shadow bits on each memory write, report a bug on another directions (e.g. JMP) if the related shadow is poisoned. But there are variations too. The primary and the foremost one: compiler instrumentation instead of binary instrumentation. This provides us a lot better register allocation, attainable compiler optimizations and a fast begin-up. But this brings the most important issue as properly: msan must see all program events, together with system calls and reads/writes in system libraries, so we both must compile all the pieces with msan or use a binary translation part (e.g. DynamoRIO) to instrument pre-built libraries. Another distinction from Memcheck is that we use eight shadow bits per byte of utility Memory Wave Program and use a direct shadow mapping.

This vastly simplifies the instrumentation code and avoids races on shadow updates (Memcheck is single-threaded so races are usually not a priority there. MemorySanitizer can track origins (allocation factors) of all uninitialized values. This behavior Memory Wave Program 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're stored in a second shadow mapping, one 4-byte value for four bytes of software memory. Propagation of origins is mainly a bunch of "select" directions that decide the origin of a dirty argument, if an instruction has one. Each 4 aligned, consecutive bytes of application memory have one origin worth associated with them. If these bytes include uninitialized data coming from 2 different allocations, the final store wins. Because of this, MemorySanitizer reports can show unrelated origins, but that is unlikely in observe. Origins are meaningless for absolutely initialized values, so MemorySanitizer avoids storing origin to memory when a completely initialized value is saved.

This manner it avoids pointless overwriting origin of the 4-byte region on a short (i.e. 1 byte) clean retailer, and it is also good for efficiency. Ideally, every atomic retailer of application value should update the corresponding shadow location in an atomic way. Unfortunately, atomic retailer of two disjoint places cannot be performed with out extreme slowdown. Subsequently, we implement an approximation which will err on the safe facet. On this implementation, every atomically accessed location in the program might solely change from (partially) uninitialized to totally initialized, however not the other method round. We load the shadow after the applying load, and we retailer the shadow before the app retailer. Also, we at all times retailer clear shadow (if the application retailer is atomic). This way, if the shop-load pair constitutes a happens-before arc, shadow retailer and load are appropriately ordered such that the load will get both the worth that was stored, or some later value (which is always clean).

This does not work very nicely with Examine-And-Swap (CAS) and browse-Modify-Write (RMW) operations. To observe the above logic, CAS and RMW should store the brand new shadow before the app operation, and load the shadow after the app operation. Computer systems do not 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 easy atomic retailer by storing a clean shadow. For inline meeting code LLVM has little thought about which memory areas turn out to be initialized relying on the arguments. It may be potential to determine which arguments are meant to level to inputs and outputs, however the precise semantics will be solely seen at runtime. In the Linux kernel it is also doable that the arguments only point out the offset for a base taken from a phase register, so it's dangerous to deal with any asm() arguments as pointers. The latter can perform more complex handle checks to determine whether or not it is protected to touch the shadow memory. KMSAN allocates shadow and origin memory for every web page separately, so there are no express accesses to shadow and origin within the instrumentation. X(ptr) functions. The corresponding functions examine 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 functions are represented in different 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-job struct.

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