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Monkeying with Memory

🙊 🙈 🙉

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Motivation

• I ran across this monkey patching library the other day:

  https://github.com/bouk/monkey

• I realized this library must be modifying function bodies to achieve this.

• That got me thinking that I could use arbitrary read/write access to memory to better understand Go's implementation and some of its quirks.

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Demo - The Quirks

• typed nil interfaces
  • nil != nil ?
• struct field alignment
  • memory consumption
  • atomic panics

Keep these in mind as we learn about accessing raw memory.

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Disclaimer

Before we get into it:

• These techniques for accessing raw memory are unsafe.
• They should not be used in real code.
• This is for educational purposes only.
• Remember...

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unsafe Pacakge

• The unsafe package contains operations that step around the type safety of Go programs.

• Programs that use unsafe may be non-portable and might break with newer compilers.

• unsafe has magical behaviour based on how you use it. For instance:

  syscall.Syscall(SYS_READ, uintptr(fd), u, uintptr(n))
  

  is not the same as:

  ptr := uintptr(fd)
  syscall.Syscall(SYS_READ, ptr, u, uintptr(n))
  

  The first invocation will ensure the memory pointed at by fd is not moved or garbage collected until the syscall completes. The second invocation is invalid.

• Make sure to read the docs and run go vet!

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Pointer Types

• unsafe.Pointer

  • A type representing a pointer to any value.

  • Any pointer type can be converted into unsafe.Pointer:

    unsafe.Pointer(&i)
    

  • unsafe.Pointer can be cast into any pointer type:

    (*int)(p)
    

  • A reference to an unsafe.Pointer will keep the underlying object from being garbage collected.

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Pointer Types

• uintptr

  • An integer type that is large enough to contain a pointer.

  • Any unsafe.Pointer can be converted into a uintptr:

    uintptr(p)
    

  • Any uintptr can be converted into an unsafe.Pointer:

    unsafe.Pointer(p)
    

  • References contained inside uintptr are not considered by the garbage collector.

  • The memory pointed at by the uintptr can be moved or reclaimed.

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Endianess

Endianess with memory deals with the ordering of bytes.

    address 00 01 02 03
    byte    16 a5 7d 6f

• Little endian:
  • 00 (16) is least significant
  • 03 (6f) is most significant
• Big endian:
  • 03 (6f) is least significant
  • 00 (16) is most significant

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Demo - Primitive Types

• int
• numeric types
• array
• string

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Demo - Slices

• slice header
  • pointer to array
  • length
  • capacity

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Demo - Struct Field Alignment

• x86 vs amd64

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Demo - Typed nil Interfaces

• nil
• typed nil
• error value

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Overtime

• mutating write protected values

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Thanks!

Thanks to Bouke van der Bijl for inspiring this talk.

Library: https://github.com/bouk/monkey
Blog Post: https://bou.ke/blog/monkey-patching-in-go/