This file is part of MemorySanitizer, a detector of uninitialized reads. 2005.html) We affiliate a couple of shadow bits with every byte of the applying memory, poison the shadow of the malloc-ed or alloca-ed memory, load the shadow, bits on each memory learn, propagate the shadow bits by means of a few of the arithmetic instruction (together with MOV), store the shadow bits on each memory write, report a bug on some other instructions (e.g. JMP) if the associated shadow is poisoned. However there are variations too. The first and the key one: compiler instrumentation instead of binary instrumentation. This gives us significantly better register allocation, attainable compiler optimizations and a fast start-up. But this brings the most important challenge as properly: Memory Wave Routine msan must see all program events, together with system calls and reads/writes in system libraries, so we both have to compile every thing 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 8 shadow bits per byte of utility 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 usually not a concern there. MemorySanitizer can observe origins (allocation points) of all uninitialized values. This behavior 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 worth for four bytes of software memory. Propagation of origins is basically a bunch of "select" directions that pick the origin of a dirty argument, if an instruction has one. Each 4 aligned, consecutive bytes of application Memory Wave Routine have one origin value related to them. If these bytes comprise uninitialized data coming from 2 completely different allocations, the last retailer wins. Due to this, MemorySanitizer experiences can show unrelated origins, however that is unlikely in apply. Origins are meaningless for totally initialized values, so MemorySanitizer avoids storing origin to memory when a completely initialized value is saved.

This way it avoids useless overwriting origin of the 4-byte region on a brief (i.e. 1 byte) clean retailer, and it is also good for efficiency. Ideally, each atomic store of application worth should replace the corresponding shadow location in an atomic manner. Sadly, atomic store of two disjoint places can't be carried out without extreme slowdown. Due to this fact, we implement an approximation that will err on the protected aspect. In this implementation, every atomically accessed location in the program may solely change from (partially) uninitialized to fully initialized, however not the other means round. We load the shadow after the applying load, and we retailer the shadow before the app retailer. Also, we at all times store clean shadow (if the applying store is atomic). This way, if the store-load pair constitutes a happens-before arc, shadow store and load are appropriately ordered such that the load will get both the value that was stored, or some later worth (which is all the time clear).

This does not work very properly with Evaluate-And-Swap (CAS) and read-Modify-Write (RMW) operations. To follow the above logic, CAS and RMW should retailer the brand new shadow before the app operation, and load the shadow after the app operation. Computers don't work this way. Current implementation ignores the load aspect of CAS/RMW, at all times returning a clean worth. It implements the store half as a simple atomic store by storing a clean shadow. For inline meeting code LLVM has little thought about which memory places turn into initialized depending on the arguments. It may be possible to determine which arguments are meant to level to inputs and outputs, however the precise semantics will be only visible at runtime. Within the Linux kernel it is also potential that the arguments only point out the offset for a base taken from a phase register, so it is harmful to treat any asm() arguments as pointers. The latter can carry out extra advanced tackle checks to figure out whether it is safe to contact the shadow memory. KMSAN allocates shadow and origin memory for every page individually, so there are no express accesses to shadow and origin in the instrumentation. X(ptr) functions. The corresponding functions verify that the X-byte accesses are potential 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 functions are represented in several ABIs. Sixty four ABI they are returned in RDX:RAX, in PowerPC64 they are returned in r3 and r4, and in the SystemZ ABI they're written to memory pointed to by a hidden parameter. TLS variables are saved in a single per-job struct.

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Pub: 14 Aug 2025 06:51 UTC

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