Re: Fw: Re: Why do these mprotect always fail?

[Oskar Skog <oskar@oskog97.com>]

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On 2023-02-15 14:40, w6b7rk5yu4mt25v3 via Cygwin wrote:
> You misunderstood what I said.
> 
> It's really just a naming conflict and a coincident. On the context of the source code (it's an interpreter), PAGE_SIZE is indeed JIT_PAGE_SIZE (not the system page size, but the page size defined internally by the interpreter). On Linux, the name doesn't conflict. On Cygwin, I found on limits.h and cygwin/limits.h already defined PAGE_SIZE so it caused a naming conflict:
> 
> #define __PAGESIZE 65536
> #define PAGESIZE __PAGESIZE
> #define PAGE_SIZE PAGESIZE
> 
> But the problem not related to the naming conflict. If I renamed PAGE_SIZE to JIT_PAGE_SIZE, the problem is still there. The problem is Cygwin will not work with JIT_PAGE_SIZE = 4096. Please have a look at the code I posted. It will always error with "Unable to mprotect".

The Linux man page for mprotect says that the address MUST be aligned
(with the systems PAGE_SIZE).  Your call to posix_memalign is NOT
aligning the allocation with the systems PAGE_SIZE (65536), but with
your PAGE_SIZE (4096).

You need to allocate memory that is aligned with the systems PAGE_SIZE.

Eg.
if(posix_memalign((void**)&buffer, (PAGE_SIZE>JIT_PAGE_SIZE) ? PAGE_SIZE 
: JIT_PAGE_SIZE, available)) { ... }

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