Linux Kernel /proc/*/mem Pseudofile: Understanding Punch Through Semantics for Unwritable Memory
By
medbar
A baker's-dozen of insight crammed into one ring.
Summary
This article explores the Linux kernel's /proc/*/mem pseudofile and its unique 'punch through' semantics that allow writes to succeed even when destination virtual memory is marked unwritable. The behavior is intentional and used by projects like Julia JIT compiler and rr debugger. The article examines the relationship between kernel memory access and hardware virtual memory permissions, questioning to what degree hardware can inhibit kernel memory access.
Key quotes
· 3 pulledWrites performed through this file will succeed even if the destination virtual memory is marked unwritable.
This behavior is intentional and actively used by projects such as the Julia JIT compiler and rr debugger.
Is privileged code subject to virtual memory permissions? In general, to what degree can the hardware inhibit kernel memory access?
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