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 appliance memory, poison the shadow of the malloc-ed or alloca-ed Memory Wave, load the shadow, bits on every memory learn, propagate the shadow bits by means of among the arithmetic instruction (together with MOV), retailer 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 differences too. The primary and the major one: compiler instrumentation as a substitute of binary instrumentation. This offers us much better register allocation, potential compiler optimizations and a quick start-up. But this brings the main problem as well: msan needs to see all program occasions, including system calls and reads/writes in system libraries, so we either have to compile everything with msan or use a binary translation part (e.g. DynamoRIO) to instrument pre-built libraries. One other difference from Memcheck is that we use eight shadow bits per byte of application memory and use a direct shadow mapping.
This fashion it avoids pointless overwriting origin of the 4-byte area on a short (i.e. 1 byte) clean retailer, and it is usually good for efficiency. Ideally, each atomic retailer of utility value ought to update the corresponding shadow location in an atomic manner. Sadly, atomic store of two disjoint places can't be executed with out extreme slowdown. Due to this fact, we implement an approximation that will err on the secure side. On this implementation, every atomically accessed location in this system may solely change from (partially) uninitialized to completely initialized, but not the opposite method around. We load the shadow after the appliance load, and we retailer the shadow before the app retailer. Also, we at all times store clean shadow (if the appliance store is atomic). This fashion, if the shop-load pair constitutes a happens-before arc, shadow store and load are accurately ordered such that the load will get both the worth that was saved, or some later worth (which is at all times 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 before the app operation, and load the shadow after the app operation. Computers don't work this manner. Present implementation ignores the load side of CAS/RMW, always returning a clean value. It implements the store part as a simple atomic store by storing a clear shadow. For inline assembly code LLVM has little concept about which memory areas grow to be initialized depending on the arguments. It can be potential to determine which arguments are meant to level to inputs and outputs, but the precise semantics will be only visible at runtime. In 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 is dangerous to deal with any asm() arguments as pointers. The latter can perform extra complex tackle checks to determine whether it's secure to contact the shadow Memory Wave Program. KMSAN allocates shadow and origin memory for each web page separately, so there are not any specific accesses to shadow and origin within the instrumentation. X(ptr) functions. The corresponding capabilities check that the X-byte accesses are doable and returns the pointers to shadow and origin memory. Word that the sanitizer code has to deal with how shadow/origin pairs returned by the these features are represented in different ABIs. 64 ABI they're returned in RDX:RAX, in PowerPC64 they are returned in r3 and r4, and in the SystemZ ABI they are written to Memory Wave pointed to by a hidden parameter. TLS variables are saved in a single per-activity struct.
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