How to get func code size

[涛叔]

Hi, guys,

While I can get a pointer to func's code  memory block by the following go code

code := **(**uintptr)(unsafe.Pointer(&someFunc))

Is there any way to detect this code block's size(in bytes)?

Thank you.

[Ian Lance Taylor]

Not at the language level, no.

You can get this information, though not completely reliably, by
opening the executable as a file and looking at the symbol table. In
ELF the answer should be precise, but it may not be on systems that
use other object file formats (namely Windows and macOS).

Ian

[Josh Bleecher Snyder]

For fun, some other ways:

Another approach to this is parsing the first few instructions; Go functions have a common prologue, one of which is a jump to a standard epilogue that handles stack growth. The details will vary by architecture and Go version, and there are several flavors of prologues, including some functions with no prologues, but it’ll work for most functions.

Also, on some architectures, gaps between functions are filled with special break/interrupt instructions. In those cases you can probably read forward until you encounter such instructions. (This will be wrong in the rare case in which someone has written an assembly routine containing such an instruction.)

Closures may complicate some of these approaches as well.

-josh

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[Jan Mercl]

On Tue, Aug 24, 2021 at 5:56 AM 涛叔 <haita...@gmail.com> wrote:

> While I can get a pointer to func's code memory block by the following go code
>
> code := **(**uintptr)(unsafe.Pointer(&someFunc))

You cannot take an address of a function using &:
https://play.golang.org/p/AaxLM8Ju0d1

[Jeremy Faller]

NB: The "walk till you find a break," method that Josh suggests won't always work. If the functions are packed perfectly, no break instructions will be inserted by the linker, and you're stuck with falling back to prologue inspection.

To view this discussion on the web visit https://groups.google.com/d/msgid/golang-dev/CAFAcib8vhno025L0M4ymCRY85mjbKkai_djQ8P7kEMxvNUNW1w%40mail.gmail.com.

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Warmest Regards,
Jeremy Faller