This file is part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We associate just a few 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 among the arithmetic instruction (together with 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. But there are variations too. The first and the key one: compiler instrumentation as an alternative of binary instrumentation. This provides us much better register allocation, potential compiler optimizations and a quick begin-up. But this brings the key situation as properly: msan needs to see all program occasions, including system calls and reads/writes in system libraries, so we both need to compile the whole lot with msan or use a binary translation component (e.g. DynamoRIO) to instrument pre-built libraries. Another distinction from Memcheck is that we use eight shadow bits per byte of software memory and use a direct shadow mapping.
This vastly simplifies the instrumentation code and avoids races on shadow updates (Memcheck is single-threaded so races should not a concern there. MemorySanitizer can monitor origins (allocation points) of all uninitialized values. This behavior is managed 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 saved in a second shadow mapping, one 4-byte worth for four bytes of utility memory. Propagation of origins is mainly a bunch of "select" directions that pick the origin of a soiled argument, if an instruction has one. Every 4 aligned, consecutive bytes of utility memory have one origin worth associated with them. If these bytes include uninitialized data coming from 2 totally different allocations, the last retailer wins. Because of this, Memory Wave MemorySanitizer reports can present unrelated origins, however this is unlikely in follow. Origins are meaningless for absolutely initialized values, so MemorySanitizer avoids storing origin to memory when a completely initialized value is stored.
This manner it avoids unnecessary overwriting origin of the 4-byte region on a brief (i.e. 1 byte) clear store, and it is also good for efficiency. Ideally, each atomic retailer of utility worth should update the corresponding shadow location in an atomic method. Unfortunately, atomic store of two disjoint areas can not be performed with out extreme slowdown. Subsequently, we implement an approximation that will err on the secure side. In this implementation, every atomically accessed location in this system might only change from (partially) uninitialized to fully initialized, however not the opposite means round. We load the shadow after the applying load, and we retailer the shadow earlier than the app store. Additionally, we all the time retailer clear shadow (if the applying store is atomic). This way, if the store-load pair constitutes a happens-earlier than 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 doesn't work very nicely with Compare-And-Swap (CAS) and browse-Modify-Write (RMW) operations. To follow the above logic, CAS and RMW must store the 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 value. It implements the store part as a simple atomic retailer by storing a clear shadow. For inline meeting code LLVM has little concept about which memory areas grow to be initialized depending on the arguments. It can be doable to figure out which arguments are meant to point to inputs and outputs, however the actual semantics could be solely seen at runtime. In the Linux kernel it is also attainable that the arguments solely point out the offset for a base taken from a phase register, so it's harmful to treat any asm() arguments as pointers. The latter can carry out extra advanced address checks to figure out whether or not it's safe to touch the shadow Memory Wave memory booster. KMSAN allocates shadow and origin memory for each page separately, so there are no express accesses to shadow and origin within the instrumentation. X(ptr) functions. The corresponding capabilities test that the X-byte accesses are attainable 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 several ABIs. Sixty four ABI they're 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 saved in a single per-job struct.
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